const ArrayIncludes = Array.prototype.includes;
const ArrayPrototypePush = Array.prototype.push;
const IntlDateTimeFormat = globalThis.Intl.DateTimeFormat;
const MathMin = Math.min;
const MathMax = Math.max;
const MathAbs = Math.abs;
const MathFloor = Math.floor;
const MathSign = Math.sign;
const MathTrunc = Math.trunc;
const NumberIsNaN = Number.isNaN;
const NumberIsFinite = Number.isFinite;
const NumberCtor = Number;
const StringCtor = String;
const NumberMaxSafeInteger = Number.MAX_SAFE_INTEGER;
const ObjectAssign = Object.assign;
const ObjectCreate = Object.create;
const ObjectDefineProperty = Object.defineProperty;
const ObjectGetOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
const ObjectIs = Object.is;
const ReflectApply = Reflect.apply;

import { DEBUG } from './debug';
import JSBI from 'jsbi';

import type { Temporal } from '..';
import type {
  AnyTemporalLikeType,
  UnitSmallerThanOrEqualTo,
  CalendarProtocolParams,
  TimeZoneProtocolParams,
  InstantParams,
  PlainMonthDayParams,
  ZonedDateTimeParams,
  CalendarParams,
  TimeZoneParams,
  PlainDateParams,
  PlainTimeParams,
  DurationParams,
  PlainDateTimeParams,
  PlainYearMonthParams,
  PrimitiveFieldsOf,
  BuiltinCalendarId
} from './internaltypes';
import { GetIntrinsic } from './intrinsicclass';
import {
  CreateSlots,
  GetSlot,
  HasSlot,
  SetSlot,
  EPOCHNANOSECONDS,
  TIMEZONE_ID,
  CALENDAR_ID,
  INSTANT,
  ISO_YEAR,
  ISO_MONTH,
  ISO_DAY,
  ISO_HOUR,
  ISO_MINUTE,
  ISO_SECOND,
  ISO_MILLISECOND,
  ISO_MICROSECOND,
  ISO_NANOSECOND,
  DATE_BRAND,
  YEAR_MONTH_BRAND,
  MONTH_DAY_BRAND,
  TIME_ZONE,
  CALENDAR,
  YEARS,
  MONTHS,
  WEEKS,
  DAYS,
  HOURS,
  MINUTES,
  SECONDS,
  MILLISECONDS,
  MICROSECONDS,
  NANOSECONDS
} from './slots';

export const ZERO = JSBI.BigInt(0);
const ONE = JSBI.BigInt(1);
const SIXTY = JSBI.BigInt(60);
export const THOUSAND = JSBI.BigInt(1e3);
export const MILLION = JSBI.BigInt(1e6);
export const BILLION = JSBI.BigInt(1e9);
const NEGATIVE_ONE = JSBI.BigInt(-1);
const DAY_SECONDS = 86400;
const DAY_NANOS = JSBI.multiply(JSBI.BigInt(DAY_SECONDS), BILLION);
const NS_MIN = JSBI.multiply(JSBI.BigInt(-86400), JSBI.BigInt(1e17));
const NS_MAX = JSBI.multiply(JSBI.BigInt(86400), JSBI.BigInt(1e17));
const YEAR_MIN = -271821;
const YEAR_MAX = 275760;
const BEFORE_FIRST_OFFSET_TRANSITION = JSBI.multiply(JSBI.BigInt(-388152), JSBI.BigInt(1e13)); // 1847-01-01T00:00:00Z
const ABOUT_TEN_YEARS_NANOS = JSBI.multiply(DAY_NANOS, JSBI.BigInt(366 * 10));
const ABOUT_ONE_YEAR_NANOS = JSBI.multiply(DAY_NANOS, JSBI.BigInt(366 * 1));
const TWO_WEEKS_NANOS = JSBI.multiply(DAY_NANOS, JSBI.BigInt(2 * 7));

const BUILTIN_CALENDAR_IDS = [
  'iso8601',
  'hebrew',
  'islamic',
  'islamic-umalqura',
  'islamic-tbla',
  'islamic-civil',
  'islamic-rgsa',
  'islamicc',
  'persian',
  'ethiopic',
  'ethioaa',
  'coptic',
  'chinese',
  'dangi',
  'roc',
  'indian',
  'buddhist',
  'japanese',
  'gregory'
];

function IsInteger(value: unknown): value is number {
  if (typeof value !== 'number' || !NumberIsFinite(value)) return false;
  const abs = MathAbs(value);
  return MathFloor(abs) === abs;
}

// For unknown values, this narrows the result to a Record. But for union types
// like `Temporal.DurationLike | string`, it'll strip the primitive types while
// leaving the object type(s) unchanged.
export function IsObject<T>(
  value: T
): value is Exclude<T, string | null | undefined | number | bigint | symbol | boolean>;
export function IsObject(value: unknown): value is Record<string | number | symbol, unknown> {
  return (typeof value === 'object' && value !== null) || typeof value === 'function';
}

export function ToNumber(value: unknown): number {
  if (typeof value === 'bigint') throw new TypeError('Cannot convert BigInt to number');
  return NumberCtor(value);
}

function ToInteger(value: unknown): number {
  const num = ToNumber(value);
  if (NumberIsNaN(num)) return 0;
  const integer = MathTrunc(num);
  if (num === 0) return 0;
  return integer;
}

export function ToString(value: unknown): string {
  if (typeof value === 'symbol') {
    throw new TypeError('Cannot convert a Symbol value to a String');
  }
  return StringCtor(value);
}

export function ToIntegerThrowOnInfinity(value: unknown): number {
  const integer = ToInteger(value);
  if (!NumberIsFinite(integer)) {
    throw new RangeError('infinity is out of range');
  }
  return integer;
}

function ToPositiveInteger(valueParam: unknown, property?: string): number {
  const value = ToInteger(valueParam);
  if (!NumberIsFinite(value)) {
    throw new RangeError('infinity is out of range');
  }
  if (value < 1) {
    if (property !== undefined) {
      throw new RangeError(`property '${property}' cannot be a a number less than one`);
    }
    throw new RangeError('Cannot convert a number less than one to a positive integer');
  }
  return value;
}

export function ToIntegerWithoutRounding(valueParam: unknown): number {
  const value = ToNumber(valueParam);
  if (NumberIsNaN(value)) return 0;
  if (!NumberIsFinite(value)) {
    throw new RangeError('infinity is out of range');
  }
  if (!IsInteger(value)) {
    throw new RangeError(`unsupported fractional value ${value}`);
  }
  return ToInteger(value); // ℝ(value) in spec text; converts -0 to 0
}
function divmod(x: JSBI, y: JSBI): { quotient: JSBI; remainder: JSBI } {
  const quotient = JSBI.divide(x, y);
  const remainder = JSBI.remainder(x, y);
  return { quotient, remainder };
}

function abs(x: JSBI): JSBI {
  if (JSBI.lessThan(x, ZERO)) return JSBI.multiply(x, NEGATIVE_ONE);
  return x;
}

export function ArrayPush<NewValue>(
  arr: ReadonlyArray<NewValue>,
  ...newItem: readonly NewValue[]
): ReadonlyArray<NewValue> {
  ArrayPrototypePush.apply(arr, newItem as any[]);
  return arr;
}

type BuiltinCastFunction = (v: unknown) => string | number;
const BUILTIN_CASTS = new Map<AnyTemporalKey, BuiltinCastFunction>([
  ['year', ToIntegerThrowOnInfinity],
  ['month', ToPositiveInteger],
  ['monthCode', ToString],
  ['day', ToPositiveInteger],
  ['hour', ToIntegerThrowOnInfinity],
  ['minute', ToIntegerThrowOnInfinity],
  ['second', ToIntegerThrowOnInfinity],
  ['millisecond', ToIntegerThrowOnInfinity],
  ['microsecond', ToIntegerThrowOnInfinity],
  ['nanosecond', ToIntegerThrowOnInfinity],
  ['years', ToIntegerWithoutRounding],
  ['months', ToIntegerWithoutRounding],
  ['weeks', ToIntegerWithoutRounding],
  ['days', ToIntegerWithoutRounding],
  ['hours', ToIntegerWithoutRounding],
  ['minutes', ToIntegerWithoutRounding],
  ['seconds', ToIntegerWithoutRounding],
  ['milliseconds', ToIntegerWithoutRounding],
  ['microseconds', ToIntegerWithoutRounding],
  ['nanoseconds', ToIntegerWithoutRounding],
  ['era', ToString],
  ['eraYear', ToInteger],
  ['offset', ToString]
]);

const BUILTIN_DEFAULTS = new Map([
  ['hour', 0],
  ['minute', 0],
  ['second', 0],
  ['millisecond', 0],
  ['microsecond', 0],
  ['nanosecond', 0]
]);

// each item is [plural, singular, category]
const SINGULAR_PLURAL_UNITS = [
  ['years', 'year', 'date'],
  ['months', 'month', 'date'],
  ['weeks', 'week', 'date'],
  ['days', 'day', 'date'],
  ['hours', 'hour', 'time'],
  ['minutes', 'minute', 'time'],
  ['seconds', 'second', 'time'],
  ['milliseconds', 'millisecond', 'time'],
  ['microseconds', 'microsecond', 'time'],
  ['nanoseconds', 'nanosecond', 'time']
] as const;
const SINGULAR_FOR = new Map(SINGULAR_PLURAL_UNITS.map((e) => [e[0], e[1]] as const));
const PLURAL_FOR = new Map(SINGULAR_PLURAL_UNITS.map(([p, s]) => [s, p]));
const UNITS_DESCENDING = SINGULAR_PLURAL_UNITS.map(([, s]) => s);

const DURATION_FIELDS = Array.from(SINGULAR_FOR.keys()).sort();

import * as PARSE from './regex';

const IntlDateTimeFormatEnUsCache = new Map<string, Intl.DateTimeFormat>();

function getIntlDateTimeFormatEnUsForTimeZone(timeZoneIdentifier: string) {
  let instance = IntlDateTimeFormatEnUsCache.get(timeZoneIdentifier);
  if (instance === undefined) {
    instance = new IntlDateTimeFormat('en-us', {
      timeZone: StringCtor(timeZoneIdentifier),
      hour12: false,
      era: 'short',
      year: 'numeric',
      month: 'numeric',
      day: 'numeric',
      hour: 'numeric',
      minute: 'numeric',
      second: 'numeric'
    });
    IntlDateTimeFormatEnUsCache.set(timeZoneIdentifier, instance);
  }
  return instance;
}

export function IsTemporalInstant(item: unknown): item is Temporal.Instant {
  return HasSlot(item, EPOCHNANOSECONDS) && !HasSlot(item, TIME_ZONE, CALENDAR);
}

export function IsTemporalTimeZone(item: unknown): item is Temporal.TimeZone {
  return HasSlot(item, TIMEZONE_ID);
}
export function IsTemporalCalendar(item: unknown): item is Temporal.Calendar {
  return HasSlot(item, CALENDAR_ID);
}
export function IsTemporalDuration(item: unknown): item is Temporal.Duration {
  return HasSlot(item, YEARS, MONTHS, DAYS, HOURS, MINUTES, SECONDS, MILLISECONDS, MICROSECONDS, NANOSECONDS);
}
export function IsTemporalDate(item: unknown): item is Temporal.PlainDate {
  return HasSlot(item, DATE_BRAND);
}
export function IsTemporalTime(item: unknown): item is Temporal.PlainTime {
  return (
    HasSlot(item, ISO_HOUR, ISO_MINUTE, ISO_SECOND, ISO_MILLISECOND, ISO_MICROSECOND, ISO_NANOSECOND) &&
    !HasSlot(item, ISO_YEAR, ISO_MONTH, ISO_DAY)
  );
}
export function IsTemporalDateTime(item: unknown): item is Temporal.PlainDateTime {
  return HasSlot(
    item,
    ISO_YEAR,
    ISO_MONTH,
    ISO_DAY,
    ISO_HOUR,
    ISO_MINUTE,
    ISO_SECOND,
    ISO_MILLISECOND,
    ISO_MICROSECOND,
    ISO_NANOSECOND
  );
}
export function IsTemporalYearMonth(item: unknown): item is Temporal.PlainYearMonth {
  return HasSlot(item, YEAR_MONTH_BRAND);
}
export function IsTemporalMonthDay(item: unknown): item is Temporal.PlainMonthDay {
  return HasSlot(item, MONTH_DAY_BRAND);
}
export function IsTemporalZonedDateTime(item: unknown): item is Temporal.ZonedDateTime {
  return HasSlot(item, EPOCHNANOSECONDS, TIME_ZONE, CALENDAR);
}
export function RejectObjectWithCalendarOrTimeZone(item: AnyTemporalLikeType) {
  if (HasSlot(item, CALENDAR) || HasSlot(item, TIME_ZONE)) {
    throw new TypeError('with() does not support a calendar or timeZone property');
  }
  if ((item as { calendar: unknown }).calendar !== undefined) {
    throw new TypeError('with() does not support a calendar property');
  }
  if ((item as { timeZone: unknown }).timeZone !== undefined) {
    throw new TypeError('with() does not support a timeZone property');
  }
}
function ParseTemporalTimeZone(stringIdent: string) {
  let { ianaName, offset, z } = ParseTemporalTimeZoneString(stringIdent);
  if (ianaName) return ianaName;
  if (z) return 'UTC';
  return offset as string; // if !ianaName && !z then offset must be present
}

function FormatCalendarAnnotation(id: string, showCalendar: Temporal.ShowCalendarOption['calendarName']) {
  if (showCalendar === 'never') return '';
  if (showCalendar === 'auto' && id === 'iso8601') return '';
  return `[u-ca=${id}]`;
}

function ParseISODateTime(isoString: string) {
  // ZDT is the superset of fields for every other Temporal type
  const match = PARSE.zoneddatetime.exec(isoString);
  if (!match) throw new RangeError(`invalid ISO 8601 string: ${isoString}`);
  let yearString = match[1];
  if (yearString[0] === '\u2212') yearString = `-${yearString.slice(1)}`;
  if (yearString === '-000000') throw new RangeError(`invalid ISO 8601 string: ${isoString}`);
  const year = ToInteger(yearString);
  const month = ToInteger(match[2] || match[4]);
  const day = ToInteger(match[3] || match[5]);
  const hour = ToInteger(match[6]);
  const hasTime = match[6] !== undefined;
  const minute = ToInteger(match[7] || match[10]);
  let second = ToInteger(match[8] || match[11]);
  if (second === 60) second = 59;
  const fraction = (match[9] || match[12]) + '000000000';
  const millisecond = ToInteger(fraction.slice(0, 3));
  const microsecond = ToInteger(fraction.slice(3, 6));
  const nanosecond = ToInteger(fraction.slice(6, 9));
  let offset;
  let z = false;
  if (match[13]) {
    offset = undefined;
    z = true;
  } else if (match[14] && match[15]) {
    const offsetSign = match[14] === '-' || match[14] === '\u2212' ? '-' : '+';
    const offsetHours = match[15] || '00';
    const offsetMinutes = match[16] || '00';
    const offsetSeconds = match[17] || '00';
    let offsetFraction = match[18] || '0';
    offset = `${offsetSign}${offsetHours}:${offsetMinutes}`;
    if (+offsetFraction) {
      while (offsetFraction.endsWith('0')) offsetFraction = offsetFraction.slice(0, -1);
      offset += `:${offsetSeconds}.${offsetFraction}`;
    } else if (+offsetSeconds) {
      offset += `:${offsetSeconds}`;
    }
    if (offset === '-00:00') offset = '+00:00';
  }
  let ianaName = match[19];
  if (ianaName) {
    try {
      // Canonicalize name if it is an IANA link name or is capitalized wrong
      ianaName = GetCanonicalTimeZoneIdentifier(ianaName).toString();
    } catch {
      // Not an IANA name, may be a custom ID, pass through unchanged
    }
  }
  const calendar = match[20];
  RejectDateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
  return {
    year,
    month,
    day,
    hasTime,
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond,
    ianaName,
    offset,
    z,
    calendar
  };
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalInstantString(isoString: string) {
  const result = ParseISODateTime(isoString);
  if (!result.z && !result.offset) throw new RangeError('Temporal.Instant requires a time zone offset');
  return result;
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalZonedDateTimeString(isoString: string) {
  const result = ParseISODateTime(isoString);
  if (!result.ianaName) throw new RangeError('Temporal.ZonedDateTime requires a time zone ID in brackets');
  return result;
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalDateTimeString(isoString: string) {
  return ParseISODateTime(isoString);
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalDateString(isoString: string) {
  return ParseISODateTime(isoString);
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalTimeString(isoString: string) {
  const match = PARSE.time.exec(isoString);
  let hour, minute, second, millisecond, microsecond, nanosecond, calendar;
  if (match) {
    hour = ToInteger(match[1]);
    minute = ToInteger(match[2] || match[5]);
    second = ToInteger(match[3] || match[6]);
    if (second === 60) second = 59;
    const fraction = (match[4] || match[7]) + '000000000';
    millisecond = ToInteger(fraction.slice(0, 3));
    microsecond = ToInteger(fraction.slice(3, 6));
    nanosecond = ToInteger(fraction.slice(6, 9));
    calendar = match[15];
  } else {
    let z, hasTime;
    ({ hasTime, hour, minute, second, millisecond, microsecond, nanosecond, calendar, z } =
      ParseISODateTime(isoString));
    if (!hasTime) throw new RangeError(`time is missing in string: ${isoString}`);
    if (z) throw new RangeError('Z designator not supported for PlainTime');
  }
  // if it's a date-time string, OK
  if (/[tT ][0-9][0-9]/.test(isoString)) {
    return { hour, minute, second, millisecond, microsecond, nanosecond, calendar };
  }
  // slow but non-grammar-dependent way to ensure that time-only strings that
  // are also valid PlainMonthDay and PlainYearMonth throw. corresponds to
  // assertion in spec text
  try {
    const { month, day } = ParseTemporalMonthDayString(isoString);
    RejectISODate(1972, month, day);
  } catch {
    try {
      const { year, month } = ParseTemporalYearMonthString(isoString);
      RejectISODate(year, month, 1);
    } catch {
      return { hour, minute, second, millisecond, microsecond, nanosecond, calendar };
    }
  }
  throw new RangeError(`invalid ISO 8601 time-only string ${isoString}; may need a T prefix`);
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalYearMonthString(isoString: string) {
  const match = PARSE.yearmonth.exec(isoString);
  let year, month, calendar, referenceISODay;
  if (match) {
    let yearString = match[1];
    if (yearString[0] === '\u2212') yearString = `-${yearString.slice(1)}`;
    if (yearString === '-000000') throw new RangeError(`invalid ISO 8601 string: ${isoString}`);
    year = ToInteger(yearString);
    month = ToInteger(match[2]);
    calendar = match[3];
  } else {
    let z;
    ({ year, month, calendar, day: referenceISODay, z } = ParseISODateTime(isoString));
    if (z) throw new RangeError('Z designator not supported for PlainYearMonth');
  }
  return { year, month, calendar, referenceISODay };
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalMonthDayString(isoString: string) {
  const match = PARSE.monthday.exec(isoString);
  let month, day, calendar, referenceISOYear;
  if (match) {
    month = ToInteger(match[1]);
    day = ToInteger(match[2]);
  } else {
    let z;
    ({ month, day, calendar, year: referenceISOYear, z } = ParseISODateTime(isoString));
    if (z) throw new RangeError('Z designator not supported for PlainMonthDay');
  }
  return { month, day, calendar, referenceISOYear };
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalTimeZoneString(stringIdent: string): Partial<{
  ianaName: string | undefined;
  offset: string | undefined;
  z: boolean | undefined;
}> {
  try {
    let canonicalIdent = GetCanonicalTimeZoneIdentifier(stringIdent);
    if (canonicalIdent) return { ianaName: canonicalIdent.toString() };
  } catch {
    // fall through
  }
  try {
    // Try parsing ISO string instead
    const result = ParseISODateTime(stringIdent);
    if (result.z || result.offset || result.ianaName) {
      return result;
    }
  } catch {
    // fall through
  }
  throw new RangeError(`Invalid time zone: ${stringIdent}`);
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalDurationString(isoString: string) {
  const match = PARSE.duration.exec(isoString);
  if (!match) throw new RangeError(`invalid duration: ${isoString}`);
  if (match.slice(2).every((element) => element === undefined)) {
    throw new RangeError(`invalid duration: ${isoString}`);
  }
  const sign = match[1] === '-' || match[1] === '\u2212' ? -1 : 1;
  const years = ToInteger(match[2]) * sign;
  const months = ToInteger(match[3]) * sign;
  const weeks = ToInteger(match[4]) * sign;
  const days = ToInteger(match[5]) * sign;
  const hours = ToInteger(match[6]) * sign;
  let fHours: number | string = match[7];
  let minutes = ToInteger(match[8]) * sign;
  let fMinutes: number | string = match[9];
  let seconds = ToInteger(match[10]) * sign;
  const fSeconds = match[11] + '000000000';
  let milliseconds = ToInteger(fSeconds.slice(0, 3)) * sign;
  let microseconds = ToInteger(fSeconds.slice(3, 6)) * sign;
  let nanoseconds = ToInteger(fSeconds.slice(6, 9)) * sign;

  fHours = fHours ? (sign * ToInteger(fHours)) / 10 ** fHours.length : 0;
  fMinutes = fMinutes ? (sign * ToInteger(fMinutes)) / 10 ** fMinutes.length : 0;

  ({ minutes, seconds, milliseconds, microseconds, nanoseconds } = DurationHandleFractions(
    fHours,
    minutes,
    fMinutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds
  ));
  RejectDuration(years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
  return { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

// ts-prune-ignore-next TODO: remove if test/validStrings is converted to TS.
export function ParseTemporalInstant(isoString: string) {
  let { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, offset, z } =
    ParseTemporalInstantString(isoString);

  if (!z && !offset) throw new RangeError('Temporal.Instant requires a time zone offset');
  // At least one of z or offset is defined, but TS doesn't seem to understand
  // that we only use offset if z is not defined (and thus offset must be defined).
  // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
  const offsetNs = z ? 0 : ParseTimeZoneOffsetString(offset!);
  ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = BalanceISODateTime(
    year,
    month,
    day,
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond - offsetNs
  ));

  const epochNs = GetEpochFromISOParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
  if (epochNs === null) throw new RangeError('DateTime outside of supported range');
  return epochNs;
}

export function RegulateISODate(
  yearParam: number,
  monthParam: number,
  dayParam: number,
  overflow: Temporal.ArithmeticOptions['overflow']
) {
  let year = yearParam;
  let month = monthParam;
  let day = dayParam;
  switch (overflow) {
    case 'reject':
      RejectISODate(year, month, day);
      break;
    case 'constrain':
      ({ year, month, day } = ConstrainISODate(year, month, day));
      break;
  }
  return { year, month, day };
}

export function RegulateTime(
  hourParam: number,
  minuteParam: number,
  secondParam: number,
  millisecondParam: number,
  microsecondParam: number,
  nanosecondParam: number,
  overflow: Temporal.ArithmeticOptions['overflow']
) {
  let hour = hourParam;
  let minute = minuteParam;
  let second = secondParam;
  let millisecond = millisecondParam;
  let microsecond = microsecondParam;
  let nanosecond = nanosecondParam;

  switch (overflow) {
    case 'reject':
      RejectTime(hour, minute, second, millisecond, microsecond, nanosecond);
      break;
    case 'constrain':
      ({ hour, minute, second, millisecond, microsecond, nanosecond } = ConstrainTime(
        hour,
        minute,
        second,
        millisecond,
        microsecond,
        nanosecond
      ));
      break;
  }
  return { hour, minute, second, millisecond, microsecond, nanosecond };
}

export function RegulateISOYearMonth(
  yearParam: number,
  monthParam: number,
  overflow: Temporal.ArithmeticOptions['overflow']
) {
  let year = yearParam;
  let month = monthParam;
  const referenceISODay = 1;
  switch (overflow) {
    case 'reject':
      RejectISODate(year, month, referenceISODay);
      break;
    case 'constrain':
      ({ year, month } = ConstrainISODate(year, month));
      break;
  }
  return { year, month };
}

function DurationHandleFractions(
  fHoursParam: number,
  minutesParam: number,
  fMinutesParam: number,
  secondsParam: number,
  millisecondsParam: number,
  microsecondsParam: number,
  nanosecondsParam: number
) {
  let fHours = fHoursParam;
  let minutes = minutesParam;
  let fMinutes = fMinutesParam;
  let seconds = secondsParam;
  let milliseconds = millisecondsParam;
  let microseconds = microsecondsParam;
  let nanoseconds = nanosecondsParam;

  if (fHours !== 0) {
    [minutes, fMinutes, seconds, milliseconds, microseconds, nanoseconds].forEach((val) => {
      if (val !== 0) throw new RangeError('only the smallest unit can be fractional');
    });
    const mins = fHours * 60;
    minutes = MathTrunc(mins);
    fMinutes = mins % 1;
  }

  if (fMinutes !== 0) {
    [seconds, milliseconds, microseconds, nanoseconds].forEach((val) => {
      if (val !== 0) throw new RangeError('only the smallest unit can be fractional');
    });
    const secs = fMinutes * 60;
    seconds = MathTrunc(secs);
    const fSeconds = secs % 1;

    if (fSeconds !== 0) {
      const mils = fSeconds * 1000;
      milliseconds = MathTrunc(mils);
      const fMilliseconds = mils % 1;

      if (fMilliseconds !== 0) {
        const mics = fMilliseconds * 1000;
        microseconds = MathTrunc(mics);
        const fMicroseconds = mics % 1;

        if (fMicroseconds !== 0) {
          const nans = fMicroseconds * 1000;
          nanoseconds = MathTrunc(nans);
        }
      }
    }
  }

  return { minutes, seconds, milliseconds, microseconds, nanoseconds };
}

function ToTemporalDurationRecord(item: Temporal.DurationLike | string) {
  if (!IsObject(item)) {
    return ParseTemporalDurationString(ToString(item));
  }
  if (IsTemporalDuration(item)) {
    return {
      years: GetSlot(item, YEARS),
      months: GetSlot(item, MONTHS),
      weeks: GetSlot(item, WEEKS),
      days: GetSlot(item, DAYS),
      hours: GetSlot(item, HOURS),
      minutes: GetSlot(item, MINUTES),
      seconds: GetSlot(item, SECONDS),
      milliseconds: GetSlot(item, MILLISECONDS),
      microseconds: GetSlot(item, MICROSECONDS),
      nanoseconds: GetSlot(item, NANOSECONDS)
    };
  }
  const result = {
    years: 0,
    months: 0,
    weeks: 0,
    days: 0,
    hours: 0,
    minutes: 0,
    seconds: 0,
    milliseconds: 0,
    microseconds: 0,
    nanoseconds: 0
  };
  let partial = ToTemporalPartialDurationRecord(item);
  for (const property of DURATION_FIELDS) {
    const value = partial[property];
    if (value !== undefined) {
      result[property] = value;
    }
  }
  let { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = result;
  RejectDuration(years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
  return { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

function ToTemporalPartialDurationRecord(temporalDurationLike: Temporal.DurationLike | string) {
  if (!IsObject(temporalDurationLike)) {
    throw new TypeError('invalid duration-like');
  }
  const result: Record<typeof DURATION_FIELDS[number], number | undefined> = {
    years: undefined,
    months: undefined,
    weeks: undefined,
    days: undefined,
    hours: undefined,
    minutes: undefined,
    seconds: undefined,
    milliseconds: undefined,
    microseconds: undefined,
    nanoseconds: undefined
  };
  let any = false;
  for (const property of DURATION_FIELDS) {
    const value = temporalDurationLike[property];
    if (value !== undefined) {
      any = true;
      result[property] = ToIntegerWithoutRounding(value);
    }
  }
  if (!any) {
    throw new TypeError('invalid duration-like');
  }
  return result;
}

function ToLimitedTemporalDuration(
  item: Temporal.DurationLike | string,
  disallowedProperties: (keyof Temporal.DurationLike)[]
) {
  let record = ToTemporalDurationRecord(item);
  for (const property of disallowedProperties) {
    if (record[property] !== 0) {
      throw new RangeError(
        `Duration field ${property} not supported by Temporal.Instant. Try Temporal.ZonedDateTime instead.`
      );
    }
  }
  return record;
}

export function ToTemporalOverflow(options: Temporal.AssignmentOptions | undefined) {
  if (options === undefined) return 'constrain';
  return GetOption(options, 'overflow', ['constrain', 'reject'], 'constrain');
}

export function ToTemporalDisambiguation(options: Temporal.ToInstantOptions | undefined) {
  if (options === undefined) return 'compatible';
  return GetOption(options, 'disambiguation', ['compatible', 'earlier', 'later', 'reject'], 'compatible');
}

export function ToTemporalRoundingMode(
  options: { roundingMode?: Temporal.RoundingMode },
  fallback: Temporal.RoundingMode
) {
  return GetOption(options, 'roundingMode', ['ceil', 'floor', 'trunc', 'halfExpand'], fallback);
}

function NegateTemporalRoundingMode(roundingMode: Temporal.RoundingMode) {
  switch (roundingMode) {
    case 'ceil':
      return 'floor';
    case 'floor':
      return 'ceil';
    default:
      return roundingMode;
  }
}

export function ToTemporalOffset(
  options: Temporal.OffsetDisambiguationOptions | undefined,
  fallback: Required<Temporal.OffsetDisambiguationOptions>['offset']
) {
  if (options === undefined) return fallback;
  return GetOption(options, 'offset', ['prefer', 'use', 'ignore', 'reject'], fallback);
}

export function ToShowCalendarOption(options: Temporal.ShowCalendarOption) {
  return GetOption(options, 'calendarName', ['auto', 'always', 'never'], 'auto');
}

export function ToShowTimeZoneNameOption(options: Temporal.ZonedDateTimeToStringOptions) {
  return GetOption(options, 'timeZoneName', ['auto', 'never'], 'auto');
}

export function ToShowOffsetOption(options: Temporal.ZonedDateTimeToStringOptions) {
  return GetOption(options, 'offset', ['auto', 'never'], 'auto');
}

export function ToTemporalRoundingIncrement(
  options: { roundingIncrement?: number },
  dividend: number | undefined,
  inclusive?: boolean
) {
  let maximum = Infinity;
  if (dividend !== undefined) maximum = dividend;
  if (!inclusive && dividend !== undefined) maximum = dividend > 1 ? dividend - 1 : 1;
  const increment = GetNumberOption(options, 'roundingIncrement', 1, maximum, 1);
  if (dividend !== undefined && dividend % increment !== 0) {
    throw new RangeError(`Rounding increment must divide evenly into ${dividend}`);
  }
  return increment;
}

type TemporalDateTimeRoundingMaximumIncrements = {
  year?: number;
  month?: number;
  week?: number;
  day?: number;
  hour?: number;
  minute: number;
  second: number;
  millisecond: number;
  microsecond: number;
  nanosecond: number;
};

export function ToTemporalDateTimeRoundingIncrement(
  options: { roundingIncrement?: number },
  smallestUnit: keyof TemporalDateTimeRoundingMaximumIncrements
) {
  const maximumIncrements: TemporalDateTimeRoundingMaximumIncrements = {
    year: undefined,
    month: undefined,
    week: undefined,
    day: undefined,
    hour: 24,
    minute: 60,
    second: 60,
    millisecond: 1000,
    microsecond: 1000,
    nanosecond: 1000
  };
  return ToTemporalRoundingIncrement(options, maximumIncrements[smallestUnit], false);
}

export function ToSecondsStringPrecision(options: Temporal.ToStringPrecisionOptions): {
  precision: 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 'auto' | 'minute';
  unit: UnitSmallerThanOrEqualTo<'minute'>;
  increment: number;
} {
  const smallestUnit = GetTemporalUnit(options, 'smallestUnit', 'time', undefined);
  if (smallestUnit === 'hour') {
    const ALLOWED_UNITS = SINGULAR_PLURAL_UNITS.reduce((allowed, [p, s, c]) => {
      // Weirdly, local type inference seems to understand the types of s and p, but tsc still complains.
      // Maybe this is fixed in later TS versions?
      if (c === 'time' && s !== 'hour') {
        allowed.push(s as Temporal.TimeUnit, p as Temporal.PluralUnit<Temporal.TimeUnit>);
      }
      return allowed;
    }, [] as Array<Temporal.TimeUnit | Temporal.PluralUnit<Temporal.TimeUnit>>);
    throw new RangeError(`smallestUnit must be one of ${ALLOWED_UNITS.join(', ')}, not ${smallestUnit}`);
  }
  switch (smallestUnit) {
    case 'minute':
      return { precision: 'minute', unit: 'minute', increment: 1 };
    case 'second':
      return { precision: 0, unit: 'second', increment: 1 };
    case 'millisecond':
      return { precision: 3, unit: 'millisecond', increment: 1 };
    case 'microsecond':
      return { precision: 6, unit: 'microsecond', increment: 1 };
    case 'nanosecond':
      return { precision: 9, unit: 'nanosecond', increment: 1 };
    default: // fall through if option not given
  }
  let digits = options.fractionalSecondDigits;
  if (digits === undefined) digits = 'auto';
  if (typeof digits !== 'number') {
    const stringDigits = ToString(digits);
    if (stringDigits === 'auto') return { precision: 'auto', unit: 'nanosecond', increment: 1 };
    throw new RangeError(`fractionalSecondDigits must be 'auto' or 0 through 9, not ${stringDigits}`);
  }
  if (NumberIsNaN(digits) || digits < 0 || digits > 9) {
    throw new RangeError(`fractionalSecondDigits must be 'auto' or 0 through 9, not ${digits}`);
  }
  const precision = MathFloor(digits);
  switch (precision) {
    case 0:
      return { precision, unit: 'second', increment: 1 };
    case 1:
    case 2:
    case 3:
      return { precision, unit: 'millisecond', increment: 10 ** (3 - precision) };
    case 4:
    case 5:
    case 6:
      return { precision, unit: 'microsecond', increment: 10 ** (6 - precision) };
    case 7:
    case 8:
    case 9:
      return { precision, unit: 'nanosecond', increment: 10 ** (9 - precision) };
    default:
      throw new RangeError(`fractionalSecondDigits must be 'auto' or 0 through 9, not ${digits}`);
  }
}

export const REQUIRED = Symbol('~required~');

interface TemporalUnitOptionsBag {
  smallestUnit?: Temporal.PluralUnit<Temporal.DateTimeUnit> | Temporal.DateTimeUnit;
  largestUnit?: Temporal.PluralUnit<Temporal.DateTimeUnit> | Temporal.DateTimeUnit | 'auto';
  unit?: Temporal.PluralUnit<Temporal.DateTimeUnit> | Temporal.DateTimeUnit;
}
type UnitTypeMapping = {
  date: Temporal.DateUnit;
  time: Temporal.TimeUnit;
  datetime: Temporal.DateTimeUnit;
};
// This type specifies the allowed defaults for each unit key type.
type AllowedGetTemporalUnitDefaultValues = {
  smallestUnit: undefined;
  largestUnit: 'auto' | undefined;
  unit: undefined;
};

export function GetTemporalUnit<
  U extends keyof TemporalUnitOptionsBag,
  T extends keyof UnitTypeMapping,
  D extends typeof REQUIRED | UnitTypeMapping[T] | AllowedGetTemporalUnitDefaultValues[U],
  R extends Exclude<D, typeof REQUIRED> | UnitTypeMapping[T]
>(options: TemporalUnitOptionsBag, key: U, unitGroup: T, requiredOrDefault: D): R;
export function GetTemporalUnit<
  U extends keyof TemporalUnitOptionsBag,
  T extends keyof UnitTypeMapping,
  D extends typeof REQUIRED | UnitTypeMapping[T] | AllowedGetTemporalUnitDefaultValues[U],
  E extends 'auto' | Temporal.DateTimeUnit,
  R extends UnitTypeMapping[T] | Exclude<D, typeof REQUIRED> | E
>(options: TemporalUnitOptionsBag, key: U, unitGroup: T, requiredOrDefault: D, extraValues: ReadonlyArray<E>): R;
// This signature of the function is NOT used in type-checking, so restricting
// the default value via generic binding like the other overloads isn't
// necessary.
export function GetTemporalUnit<
  T extends keyof UnitTypeMapping,
  D extends typeof REQUIRED | UnitTypeMapping[T] | 'auto' | undefined,
  E extends 'auto' | Temporal.DateTimeUnit,
  R extends UnitTypeMapping[T] | Exclude<D, typeof REQUIRED> | E
>(
  options: TemporalUnitOptionsBag,
  key: keyof typeof options,
  unitGroup: T,
  requiredOrDefault: D,
  extraValues: ReadonlyArray<E> | never[] = []
): R {
  const allowedSingular: Array<Temporal.DateTimeUnit | 'auto'> = [];
  for (const [, singular, category] of SINGULAR_PLURAL_UNITS) {
    if (unitGroup === 'datetime' || unitGroup === category) {
      allowedSingular.push(singular);
    }
  }
  allowedSingular.push(...extraValues);
  let defaultVal: typeof REQUIRED | Temporal.DateTimeUnit | 'auto' | undefined = requiredOrDefault;
  if (defaultVal === REQUIRED) {
    defaultVal = undefined;
  } else if (defaultVal !== undefined) {
    allowedSingular.push(defaultVal);
  }
  const allowedValues: Array<Temporal.DateTimeUnit | Temporal.PluralUnit<Temporal.DateTimeUnit> | 'auto'> = [
    ...allowedSingular
  ];
  for (const singular of allowedSingular) {
    const plural = PLURAL_FOR.get(singular as Parameters<typeof PLURAL_FOR.get>[0]);
    if (plural !== undefined) allowedValues.push(plural);
  }
  let retval = GetOption(options, key, allowedValues, defaultVal);
  if (retval === undefined && requiredOrDefault === REQUIRED) {
    throw new RangeError(`${key} is required`);
  }
  // Coerce any plural units into their singular form
  if (SINGULAR_FOR.has(retval as Temporal.PluralUnit<Temporal.DateTimeUnit>)) {
    // We just has-checked this, but tsc doesn't understand that.
    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
    return SINGULAR_FOR.get(retval as Temporal.PluralUnit<Temporal.DateTimeUnit>)! as R;
  }
  return retval as R;
}

export function ToRelativeTemporalObject(options: {
  relativeTo?:
    | Temporal.ZonedDateTime
    | Temporal.PlainDateTime
    | Temporal.ZonedDateTimeLike
    | Temporal.PlainDateTimeLike
    | string
    | undefined;
}): Temporal.ZonedDateTime | Temporal.PlainDate | undefined {
  const relativeTo = options.relativeTo;
  if (relativeTo === undefined) return relativeTo;

  let offsetBehaviour: OffsetBehaviour = 'option';
  let matchMinutes = false;
  let year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar, timeZone, offset;
  if (IsObject(relativeTo)) {
    if (IsTemporalZonedDateTime(relativeTo) || IsTemporalDate(relativeTo)) return relativeTo;
    if (IsTemporalDateTime(relativeTo)) return TemporalDateTimeToDate(relativeTo);
    calendar = GetTemporalCalendarWithISODefault(relativeTo);
    const fieldNames = CalendarFields(calendar, [
      'day',
      'hour',
      'microsecond',
      'millisecond',
      'minute',
      'month',
      'monthCode',
      'nanosecond',
      'second',
      'year'
    ] as const);
    const fields = PrepareTemporalFields(relativeTo, fieldNames, []);
    const dateOptions = ObjectCreate(null);
    dateOptions.overflow = 'constrain';
    ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = InterpretTemporalDateTimeFields(
      calendar,
      fields,
      dateOptions
    ));
    // The `offset` and `timeZone` properties only exist on ZonedDateTime (or
    // ZonedDateTimeLike-property bags). The assertions below are used to avoid
    // TS errors while not diverging runtime code from proposal-temporal.
    offset = (relativeTo as Temporal.ZonedDateTimeLike).offset;
    if (offset === undefined) offsetBehaviour = 'wall';
    timeZone = (relativeTo as Temporal.ZonedDateTimeLike).timeZone;
  } else {
    let ianaName, z;
    ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar, ianaName, offset, z } =
      ParseISODateTime(ToString(relativeTo)));
    if (ianaName) timeZone = ianaName;
    if (z) {
      offsetBehaviour = 'exact';
    } else if (!offset) {
      offsetBehaviour = 'wall';
    }
    if (!calendar) calendar = GetISO8601Calendar();
    calendar = ToTemporalCalendar(calendar);
    matchMinutes = true;
  }
  if (timeZone !== undefined) {
    timeZone = ToTemporalTimeZone(timeZone);
    let offsetNs = 0;
    if (offsetBehaviour === 'option') offsetNs = ParseTimeZoneOffsetString(ToString(offset));
    const epochNanoseconds = InterpretISODateTimeOffset(
      year,
      month,
      day,
      hour,
      minute,
      second,
      millisecond,
      microsecond,
      nanosecond,
      offsetBehaviour,
      offsetNs,
      timeZone,
      'compatible',
      'reject',
      matchMinutes
    );
    return CreateTemporalZonedDateTime(epochNanoseconds, timeZone, calendar);
  }
  return CreateTemporalDate(year, month, day, calendar);
}

export function DefaultTemporalLargestUnit(
  years: number,
  months: number,
  weeks: number,
  days: number,
  hours: number,
  minutes: number,
  seconds: number,
  milliseconds: number,
  microseconds: number,
  nanoseconds: number
): Temporal.DateTimeUnit {
  for (const [prop, v] of [
    ['years', years],
    ['months', months],
    ['weeks', weeks],
    ['days', days],
    ['hours', hours],
    ['minutes', minutes],
    ['seconds', seconds],
    ['milliseconds', milliseconds],
    ['microseconds', microseconds],
    ['nanoseconds', nanoseconds]
  ] as const) {
    if (v !== 0) {
      // All the above keys are definitely in SINGULAR_FOR
      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
      return SINGULAR_FOR.get(prop)!;
    }
  }
  return 'nanosecond';
}

export function LargerOfTwoTemporalUnits<T1 extends Temporal.DateTimeUnit, T2 extends Temporal.DateTimeUnit>(
  unit1: T1,
  unit2: T2
) {
  if (UNITS_DESCENDING.indexOf(unit1) > UNITS_DESCENDING.indexOf(unit2)) return unit2;
  return unit1;
}

function MergeLargestUnitOption<T extends Temporal.DateTimeUnit>(
  optionsParam: Temporal.DifferenceOptions<T> | undefined,
  largestUnit: T
): Temporal.DifferenceOptions<T> {
  let options = optionsParam;
  if (options === undefined) options = ObjectCreate(null);
  return ObjectAssign(ObjectCreate(null), options, { largestUnit });
}

type FieldCompleteness = 'complete' | 'partial';
interface FieldPrepareOptions {
  emptySourceErrorMessage: string;
}

type AnyTemporalKey = Exclude<Keys<AnyTemporalLikeType>, symbol>;

// Keys is a conditionally-mapped version of keyof
type Keys<T> = T extends Record<string, unknown> ? keyof T : never;

// Returns all potential owners from all Temporal Like-types for a given union
// of keys in K.
// e.g.
// Owner<'nanosecond'> => PlainDateTimeLike | ZonedDateTimeLike | PlainDateTimeLike | ZonedDateTimeLike
// Owner<'nanoseconds'> => Duration (the only type with plural keys)
type Owner<K extends AnyTemporalKey> =
  // Conditional typing maps over all of the types given in AnyTemporalLikeType
  // union
  K extends unknown ? OwnerOf<K, AnyTemporalLikeType> : 'ThisShouldNeverHappen';

// Returns T iff T has K as all of the key(s) (even if those keys are optional
// in T), never otherwise. This is a private type for use only in the Owner type
// above.
type OwnerOf<K extends AnyTemporalKey, T> =
  // Distribute the union before passing to Required
  // Without distributing, this is
  // Required<ZonedDateTimeLike | DurationLike> extends Record
  // vs (with distribution)
  // Required<ZonedDateTimeLike> extends Record<....> | Required<DurationLike> extends Record<....>
  T extends unknown
    ? // All the keys in the Like-types are optional, so in order for them to
      // 'extend Record<K,...>', where K indicates the required fields, we pass T
      // through Required to make all the keys non-optional.
      // Note this doesn't work the other way around: using Partial<Record<K, ..>>
      // will always be extended by any object (as all the keys are optional).
      Required<T> extends Record<K, unknown>
      ? T
      : // never is the 'identity' type for unions - nothing will be added or
        // removed from the union.
        never
    : 'ThisShouldNeverHappen';

type Prop<T, K> = T extends unknown ? (K extends keyof T ? T[K] : undefined) : 'ThisShouldNeverHappen';

// Resolve copies the keys and values of a given object type so that TS will
// stop using type names in error messages / autocomplete. Generally, those
// names can be more useful, but sometimes having the primitive object shape is
// significantly easier to reason about (e.g. deeply-nested types).
// Resolve is an identity function for function types.
type Resolve<T> =
  // Re-mapping doesn't work very well for functions, so exclude them
  T extends (...args: never[]) => unknown
    ? T
    : // Re-map all the keys in T to the same value. This forces TS into no longer
      // using type aliases, etc.
      { [K in keyof T]: T[K] };

type FieldObjectFromOwners<OwnerT, FieldKeys extends AnyTemporalKey> = Resolve<
  // The resulting object type contains:
  // - All keys in FieldKeys, which are required properties and their values
  //   don't include undefined.
  // - All the other keys in OwnerT that aren't in FieldKeys, which are optional
  //   properties and their value types explicitly include undefined.
  {
    -readonly [k in FieldKeys]: Exclude<Prop<OwnerT, k>, undefined>;
  } & {
    -readonly [k in Exclude<Keys<OwnerT>, FieldKeys>]?: Prop<OwnerT, k> | undefined;
  }
>;

type PrepareTemporalFieldsReturn<
  FieldKeys extends AnyTemporalKey,
  RequiredFieldsOpt extends ReadonlyArray<FieldKeys> | FieldCompleteness,
  OwnerT extends Owner<FieldKeys>
> = RequiredFieldsOpt extends 'partial' ? Partial<OwnerT> : FieldObjectFromOwners<OwnerT, FieldKeys>;
export function PrepareTemporalFields<
  FieldKeys extends AnyTemporalKey,
  // Constrains the Required keys to be a subset of the given field keys
  // This could have been written directly into the parameter type, but that
  // causes an unintended effect where the required fields are added to the list
  // of field keys, even if that key isn't present in 'fields'.
  // RequiredFieldKeys extends FieldKeys,
  RequiredFields extends ReadonlyArray<FieldKeys> | FieldCompleteness
>(
  bag: Partial<Record<FieldKeys, unknown>>,
  fields: ReadonlyArray<FieldKeys>,
  requiredFields: RequiredFields,
  { emptySourceErrorMessage }: FieldPrepareOptions = { emptySourceErrorMessage: 'no supported properties found' }
): PrepareTemporalFieldsReturn<FieldKeys, RequiredFields, Owner<FieldKeys>> {
  const result: Partial<Record<AnyTemporalKey, unknown>> = ObjectCreate(null);
  let any = false;
  for (const property of fields) {
    let value = bag[property];
    if (value !== undefined) {
      any = true;
      if (BUILTIN_CASTS.has(property)) {
        // We just has-checked this map access, so there will definitely be a
        // value.
        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
        value = BUILTIN_CASTS.get(property)!(value);
      }
      result[property] = value;
    } else if (requiredFields !== 'partial') {
      // TODO: using .call in this way is not correctly type-checked by tsc.
      // We might need a type-safe Call wrapper?
      if (ArrayIncludes.call(requiredFields, property)) {
        throw new TypeError(`required property '${property}' missing or undefined`);
      }
      value = BUILTIN_DEFAULTS.get(property);
      result[property] = value;
    }
  }
  if (requiredFields === 'partial' && !any) {
    throw new TypeError(emptySourceErrorMessage);
  }
  if ((result.era === undefined) !== (result.eraYear === undefined)) {
    throw new RangeError("properties 'era' and 'eraYear' must be provided together");
  }
  return result as unknown as PrepareTemporalFieldsReturn<FieldKeys, RequiredFields, Owner<FieldKeys>>;
}

interface TimeRecord {
  hour?: number;
  minute?: number;
  second?: number;
  microsecond?: number;
  millisecond?: number;
  nanosecond?: number;
}
export function ToTemporalTimeRecord(bag: Partial<Record<keyof TimeRecord, string | number>>): Required<TimeRecord>;
export function ToTemporalTimeRecord(
  bag: Partial<Record<keyof TimeRecord, string | number | undefined>>,
  completeness: 'partial'
): Partial<TimeRecord>;
export function ToTemporalTimeRecord(
  bag: Partial<Record<keyof TimeRecord, string | number>>,
  completeness: 'complete'
): Required<TimeRecord>;
export function ToTemporalTimeRecord(
  bag: Partial<Record<keyof TimeRecord, string | number | undefined>>,
  completeness: FieldCompleteness = 'complete'
): Partial<TimeRecord> {
  // NOTE: Field order here is important.
  const fields = ['hour', 'microsecond', 'millisecond', 'minute', 'nanosecond', 'second'] as const;
  const partial = PrepareTemporalFields(bag, fields, 'partial', { emptySourceErrorMessage: 'invalid time-like' });
  const result: Partial<TimeRecord> = {};
  for (const field of fields) {
    const valueDesc = ObjectGetOwnPropertyDescriptor(partial, field);
    if (valueDesc !== undefined) {
      result[field] = valueDesc.value;
    } else if (completeness === 'complete') {
      result[field] = 0;
    }
  }
  return result;
}

export function ToTemporalDate(
  itemParam: PlainDateParams['from'][0],
  options?: PlainDateParams['from'][1]
): Temporal.PlainDate {
  let item = itemParam;
  if (IsObject(item)) {
    if (IsTemporalDate(item)) return item;
    if (IsTemporalZonedDateTime(item)) {
      ToTemporalOverflow(options); // validate and ignore
      item = BuiltinTimeZoneGetPlainDateTimeFor(
        GetSlot(item, TIME_ZONE),
        GetSlot(item, INSTANT),
        GetSlot(item, CALENDAR)
      );
    }
    if (IsTemporalDateTime(item)) {
      ToTemporalOverflow(options); // validate and ignore
      return CreateTemporalDate(
        GetSlot(item, ISO_YEAR),
        GetSlot(item, ISO_MONTH),
        GetSlot(item, ISO_DAY),
        GetSlot(item, CALENDAR)
      );
    }
    const calendar = GetTemporalCalendarWithISODefault(item);
    const fieldNames = CalendarFields(calendar, ['day', 'month', 'monthCode', 'year'] as const);
    const fields = PrepareTemporalFields(item, fieldNames, []);
    return CalendarDateFromFields(calendar, fields, options);
  }
  ToTemporalOverflow(options); // validate and ignore
  const { year, month, day, calendar, z } = ParseTemporalDateString(ToString(item));
  if (z) throw new RangeError('Z designator not supported for PlainDate');
  const TemporalPlainDate = GetIntrinsic('%Temporal.PlainDate%');
  return new TemporalPlainDate(year, month, day, calendar); // include validation
}

export function InterpretTemporalDateTimeFields(
  calendar: Temporal.CalendarProtocol,
  fields: PrimitiveFieldsOf<Temporal.PlainDateTimeLike> & Parameters<typeof CalendarDateFromFields>[1],
  options?: Temporal.AssignmentOptions
) {
  let { hour, minute, second, millisecond, microsecond, nanosecond } = ToTemporalTimeRecord(fields);
  const overflow = ToTemporalOverflow(options);
  const date = CalendarDateFromFields(calendar, fields, options);
  const year = GetSlot(date, ISO_YEAR);
  const month = GetSlot(date, ISO_MONTH);
  const day = GetSlot(date, ISO_DAY);
  ({ hour, minute, second, millisecond, microsecond, nanosecond } = RegulateTime(
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond,
    overflow
  ));
  return { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond };
}

export function ToTemporalDateTime(item: PlainDateTimeParams['from'][0], options?: PlainDateTimeParams['from'][1]) {
  let year: number,
    month: number,
    day: number,
    hour: number,
    minute: number,
    second: number,
    millisecond: number,
    microsecond: number,
    nanosecond: number,
    calendar;
  if (IsObject(item)) {
    if (IsTemporalDateTime(item)) return item;
    if (IsTemporalZonedDateTime(item)) {
      ToTemporalOverflow(options); // validate and ignore
      return BuiltinTimeZoneGetPlainDateTimeFor(
        GetSlot(item, TIME_ZONE),
        GetSlot(item, INSTANT),
        GetSlot(item, CALENDAR)
      );
    }
    if (IsTemporalDate(item)) {
      ToTemporalOverflow(options); // validate and ignore
      return CreateTemporalDateTime(
        GetSlot(item, ISO_YEAR),
        GetSlot(item, ISO_MONTH),
        GetSlot(item, ISO_DAY),
        0,
        0,
        0,
        0,
        0,
        0,
        GetSlot(item, CALENDAR)
      );
    }

    calendar = GetTemporalCalendarWithISODefault(item);
    const fieldNames = CalendarFields(calendar, [
      'day',
      'hour',
      'microsecond',
      'millisecond',
      'minute',
      'month',
      'monthCode',
      'nanosecond',
      'second',
      'year'
    ] as const);
    const fields = PrepareTemporalFields(item, fieldNames, []);
    ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = InterpretTemporalDateTimeFields(
      calendar,
      fields,
      options
    ));
  } else {
    ToTemporalOverflow(options); // validate and ignore
    let z;
    ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar, z } =
      ParseTemporalDateTimeString(ToString(item)));
    if (z) throw new RangeError('Z designator not supported for PlainDateTime');
    RejectDateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
    if (calendar === undefined) calendar = GetISO8601Calendar();
    calendar = ToTemporalCalendar(calendar);
  }
  return CreateTemporalDateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar);
}

export function ToTemporalDuration(item: DurationParams['from'][0]) {
  if (IsTemporalDuration(item)) return item;
  let { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
    ToTemporalDurationRecord(item);
  const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
  return new TemporalDuration(
    years,
    months,
    weeks,
    days,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds
  );
}

export function ToTemporalInstant(item: InstantParams['from'][0]) {
  if (IsTemporalInstant(item)) return item;
  if (IsTemporalZonedDateTime(item)) {
    const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
    return new TemporalInstant(GetSlot(item, EPOCHNANOSECONDS));
  }
  const ns = ParseTemporalInstant(ToString(item));
  const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
  return new TemporalInstant(ns);
}

export function ToTemporalMonthDay(
  itemParam: PlainMonthDayParams['from'][0],
  options?: PlainMonthDayParams['from'][1]
) {
  let item = itemParam;
  if (IsObject(item)) {
    if (IsTemporalMonthDay(item)) return item;
    let calendar: Temporal.CalendarProtocol, calendarAbsent: boolean;
    if (HasSlot(item, CALENDAR)) {
      calendar = GetSlot(item, CALENDAR);
      calendarAbsent = false;
    } else {
      let maybeStringCalendar = item.calendar;
      calendarAbsent = maybeStringCalendar === undefined;
      if (maybeStringCalendar === undefined) maybeStringCalendar = GetISO8601Calendar();
      calendar = ToTemporalCalendar(maybeStringCalendar);
    }
    // HasSlot above adjusts the type of 'item' to include
    // TypesWithCalendarUnits, which causes type-inference failures below.
    // This is probably indicative of problems with HasSlot's typing.
    item = item as Temporal.PlainMonthDayLike;
    const fieldNames = CalendarFields(calendar, ['day', 'month', 'monthCode', 'year'] as const);
    const fields = PrepareTemporalFields(item, fieldNames, []);
    // Callers who omit the calendar are not writing calendar-independent
    // code. In that case, `monthCode`/`year` can be omitted; `month` and
    // `day` are sufficient. Add a `year` to satisfy calendar validation.
    if (calendarAbsent && fields.month !== undefined && fields.monthCode === undefined && fields.year === undefined) {
      fields.year = 1972;
    }
    return CalendarMonthDayFromFields(calendar, fields, options);
  }

  ToTemporalOverflow(options); // validate and ignore
  let { month, day, referenceISOYear, calendar: maybeStringCalendar } = ParseTemporalMonthDayString(ToString(item));
  let calendar: Temporal.CalendarProtocol | string | undefined = maybeStringCalendar;
  if (calendar === undefined) calendar = GetISO8601Calendar();
  calendar = ToTemporalCalendar(calendar);

  if (referenceISOYear === undefined) {
    RejectISODate(1972, month, day);
    return CreateTemporalMonthDay(month, day, calendar);
  }
  const result = CreateTemporalMonthDay(month, day, calendar, referenceISOYear);
  return CalendarMonthDayFromFields(calendar, result);
}

export function ToTemporalTime(
  itemParam: PlainTimeParams['from'][0],
  overflow: NonNullable<PlainTimeParams['from'][1]>['overflow'] = 'constrain'
) {
  let item = itemParam;
  let hour, minute, second, millisecond, microsecond, nanosecond, calendar;
  if (IsObject(item)) {
    if (IsTemporalTime(item)) return item;
    if (IsTemporalZonedDateTime(item)) {
      item = BuiltinTimeZoneGetPlainDateTimeFor(
        GetSlot(item, TIME_ZONE),
        GetSlot(item, INSTANT),
        GetSlot(item, CALENDAR)
      );
    }
    if (IsTemporalDateTime(item)) {
      const TemporalPlainTime = GetIntrinsic('%Temporal.PlainTime%');
      return new TemporalPlainTime(
        GetSlot(item, ISO_HOUR),
        GetSlot(item, ISO_MINUTE),
        GetSlot(item, ISO_SECOND),
        GetSlot(item, ISO_MILLISECOND),
        GetSlot(item, ISO_MICROSECOND),
        GetSlot(item, ISO_NANOSECOND)
      );
    }
    calendar = GetTemporalCalendarWithISODefault(item);
    if (ToString(calendar) !== 'iso8601') {
      throw new RangeError('PlainTime can only have iso8601 calendar');
    }
    ({ hour, minute, second, millisecond, microsecond, nanosecond } = ToTemporalTimeRecord(item));
    ({ hour, minute, second, millisecond, microsecond, nanosecond } = RegulateTime(
      hour,
      minute,
      second,
      millisecond,
      microsecond,
      nanosecond,
      overflow
    ));
  } else {
    ({ hour, minute, second, millisecond, microsecond, nanosecond, calendar } = ParseTemporalTimeString(
      ToString(item)
    ));
    RejectTime(hour, minute, second, millisecond, microsecond, nanosecond);
    if (calendar !== undefined && calendar !== 'iso8601') {
      throw new RangeError('PlainTime can only have iso8601 calendar');
    }
  }
  const TemporalPlainTime = GetIntrinsic('%Temporal.PlainTime%');
  return new TemporalPlainTime(hour, minute, second, millisecond, microsecond, nanosecond);
}

export function ToTemporalYearMonth(
  item: PlainYearMonthParams['from'][0],
  options?: PlainYearMonthParams['from'][1]
): Temporal.PlainYearMonth {
  if (IsObject(item)) {
    if (IsTemporalYearMonth(item)) return item;
    const calendar = GetTemporalCalendarWithISODefault(item);
    const fieldNames = CalendarFields(calendar, ['month', 'monthCode', 'year'] as const);
    const fields = PrepareTemporalFields(item, fieldNames, []);
    return CalendarYearMonthFromFields(calendar, fields, options);
  }

  ToTemporalOverflow(options); // validate and ignore
  let { year, month, referenceISODay, calendar: maybeStringCalendar } = ParseTemporalYearMonthString(ToString(item));
  // TODO: replace with ternary?
  let calendar: Temporal.CalendarProtocol | string = maybeStringCalendar;
  if (calendar === undefined) calendar = GetISO8601Calendar();
  calendar = ToTemporalCalendar(calendar);

  if (referenceISODay === undefined) {
    RejectISODate(year, month, 1);
    return CreateTemporalYearMonth(year, month, calendar);
  }
  const result = CreateTemporalYearMonth(year, month, calendar, referenceISODay);
  return CalendarYearMonthFromFields(calendar, result);
}

type OffsetBehaviour = 'wall' | 'exact' | 'option';

export function InterpretISODateTimeOffset(
  year: number,
  month: number,
  day: number,
  hour: number,
  minute: number,
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number,
  offsetBehaviour: OffsetBehaviour,
  offsetNs: number,
  timeZone: Temporal.TimeZoneProtocol,
  disambiguation: NonNullable<Temporal.ToInstantOptions['disambiguation']>,
  offsetOpt: Temporal.OffsetDisambiguationOptions['offset'],
  matchMinute: boolean
) {
  const DateTime = GetIntrinsic('%Temporal.PlainDateTime%');
  const dt = new DateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);

  if (offsetBehaviour === 'wall' || offsetOpt === 'ignore') {
    // Simple case: ISO string without a TZ offset (or caller wants to ignore
    // the offset), so just convert DateTime to Instant in the given time zone
    const instant = BuiltinTimeZoneGetInstantFor(timeZone, dt, disambiguation);
    return GetSlot(instant, EPOCHNANOSECONDS);
  }

  // The caller wants the offset to always win ('use') OR the caller is OK
  // with the offset winning ('prefer' or 'reject') as long as it's valid
  // for this timezone and date/time.
  if (offsetBehaviour === 'exact' || offsetOpt === 'use') {
    // Calculate the instant for the input's date/time and offset
    const epochNs = GetEpochFromISOParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
    if (epochNs === null) throw new RangeError('ZonedDateTime outside of supported range');
    return JSBI.subtract(epochNs, JSBI.BigInt(offsetNs));
  }

  // "prefer" or "reject"
  const possibleInstants = GetPossibleInstantsFor(timeZone, dt);
  for (const candidate of possibleInstants) {
    const candidateOffset = GetOffsetNanosecondsFor(timeZone, candidate);
    const roundedCandidateOffset = JSBI.toNumber(
      RoundNumberToIncrement(JSBI.BigInt(candidateOffset), 60e9, 'halfExpand')
    );
    if (candidateOffset === offsetNs || (matchMinute && roundedCandidateOffset === offsetNs)) {
      return GetSlot(candidate, EPOCHNANOSECONDS);
    }
  }

  // the user-provided offset doesn't match any instants for this time
  // zone and date/time.
  if (offsetOpt === 'reject') {
    const offsetStr = FormatTimeZoneOffsetString(offsetNs);
    const timeZoneString = IsTemporalTimeZone(timeZone) ? GetSlot(timeZone, TIMEZONE_ID) : 'time zone';
    // The tsc emit for this line rewrites to invoke the PlainDateTime's valueOf method, NOT
    // toString (which is invoked by Node when using template literals directly).
    // See https://github.com/microsoft/TypeScript/issues/39744 for the proposed fix in tsc emit
    throw new RangeError(`Offset ${offsetStr} is invalid for ${dt.toString()} in ${timeZoneString}`);
  }
  // fall through: offsetOpt === 'prefer', but the offset doesn't match
  // so fall back to use the time zone instead.
  const instant = DisambiguatePossibleInstants(possibleInstants, timeZone, dt, disambiguation);
  return GetSlot(instant, EPOCHNANOSECONDS);
}

export function ToTemporalZonedDateTime(
  item: ZonedDateTimeParams['from'][0],
  options?: ZonedDateTimeParams['from'][1]
) {
  let year: number,
    month: number,
    day: number,
    hour: number,
    minute: number,
    second: number,
    millisecond: number,
    microsecond: number,
    nanosecond: number,
    timeZone,
    offset: string | undefined,
    calendar: string | Temporal.CalendarProtocol;
  let matchMinute = false;
  let offsetBehaviour: OffsetBehaviour = 'option';
  if (IsObject(item)) {
    if (IsTemporalZonedDateTime(item)) return item;
    calendar = GetTemporalCalendarWithISODefault(item);
    const fieldNames = CalendarFields(calendar, [
      'day',
      'hour',
      'microsecond',
      'millisecond',
      'minute',
      'month',
      'monthCode',
      'nanosecond',
      'second',
      'year'
    ] as const);
    const fieldNamesWithTzAndOffset = ArrayPush(fieldNames, 'timeZone', 'offset');
    const fields = PrepareTemporalFields(item, fieldNamesWithTzAndOffset, ['timeZone']);
    ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = InterpretTemporalDateTimeFields(
      calendar,
      fields,
      options
    ));
    timeZone = ToTemporalTimeZone(fields.timeZone);
    offset = fields.offset;
    if (offset === undefined) {
      offsetBehaviour = 'wall';
    } else {
      offset = ToString(offset);
    }
  } else {
    ToTemporalOverflow(options); // validate and ignore
    let ianaName, z;
    ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, ianaName, offset, z, calendar } =
      ParseTemporalZonedDateTimeString(ToString(item)));
    if (!ianaName) throw new RangeError('time zone ID required in brackets');
    if (z) {
      offsetBehaviour = 'exact';
    } else if (!offset) {
      offsetBehaviour = 'wall';
    }
    const TemporalTimeZone = GetIntrinsic('%Temporal.TimeZone%');
    timeZone = new TemporalTimeZone(ianaName);
    if (!calendar) calendar = GetISO8601Calendar();
    calendar = ToTemporalCalendar(calendar);
    matchMinute = true; // ISO strings may specify offset with less precision
  }
  let offsetNs = 0;
  // The code above guarantees that if offsetBehaviour === 'option', then
  // `offset` is not undefined.
  if (offsetBehaviour === 'option') offsetNs = ParseTimeZoneOffsetString(offset as string);
  const disambiguation = ToTemporalDisambiguation(options);
  const offsetOpt = ToTemporalOffset(options, 'reject');
  const epochNanoseconds = InterpretISODateTimeOffset(
    year,
    month,
    day,
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond,
    offsetBehaviour,
    offsetNs,
    timeZone,
    disambiguation,
    offsetOpt,
    matchMinute
  );
  return CreateTemporalZonedDateTime(epochNanoseconds, timeZone, calendar);
}

export function CreateTemporalDateSlots(
  result: Temporal.PlainDate,
  isoYear: number,
  isoMonth: number,
  isoDay: number,
  calendar: Temporal.CalendarProtocol
) {
  RejectISODate(isoYear, isoMonth, isoDay);
  RejectDateRange(isoYear, isoMonth, isoDay);

  CreateSlots(result);
  SetSlot(result, ISO_YEAR, isoYear);
  SetSlot(result, ISO_MONTH, isoMonth);
  SetSlot(result, ISO_DAY, isoDay);
  SetSlot(result, CALENDAR, calendar);
  SetSlot(result, DATE_BRAND, true);

  if (DEBUG) {
    ObjectDefineProperty(result, '_repr_', {
      value: `${result[Symbol.toStringTag]} <${TemporalDateToString(result)}>`,
      writable: false,
      enumerable: false,
      configurable: false
    });
  }
}

export function CreateTemporalDate(
  isoYear: number,
  isoMonth: number,
  isoDay: number,
  calendar: Temporal.CalendarProtocol = GetISO8601Calendar()
) {
  const TemporalPlainDate = GetIntrinsic('%Temporal.PlainDate%');
  const result = ObjectCreate(TemporalPlainDate.prototype);
  CreateTemporalDateSlots(result, isoYear, isoMonth, isoDay, calendar);
  return result;
}

export function CreateTemporalDateTimeSlots(
  result: Temporal.PlainDateTime,
  isoYear: number,
  isoMonth: number,
  isoDay: number,
  h: number,
  min: number,
  s: number,
  ms: number,
  µs: number,
  ns: number,
  calendar: Temporal.CalendarProtocol
) {
  RejectDateTime(isoYear, isoMonth, isoDay, h, min, s, ms, µs, ns);
  RejectDateTimeRange(isoYear, isoMonth, isoDay, h, min, s, ms, µs, ns);

  CreateSlots(result);
  SetSlot(result, ISO_YEAR, isoYear);
  SetSlot(result, ISO_MONTH, isoMonth);
  SetSlot(result, ISO_DAY, isoDay);
  SetSlot(result, ISO_HOUR, h);
  SetSlot(result, ISO_MINUTE, min);
  SetSlot(result, ISO_SECOND, s);
  SetSlot(result, ISO_MILLISECOND, ms);
  SetSlot(result, ISO_MICROSECOND, µs);
  SetSlot(result, ISO_NANOSECOND, ns);
  SetSlot(result, CALENDAR, calendar);

  if (DEBUG) {
    Object.defineProperty(result, '_repr_', {
      value: `${result[Symbol.toStringTag]} <${TemporalDateTimeToString(result, 'auto')}>`,
      writable: false,
      enumerable: false,
      configurable: false
    });
  }
}

export function CreateTemporalDateTime(
  isoYear: number,
  isoMonth: number,
  isoDay: number,
  h: number,
  min: number,
  s: number,
  ms: number,
  µs: number,
  ns: number,
  calendar: Temporal.CalendarProtocol = GetISO8601Calendar()
) {
  const TemporalPlainDateTime = GetIntrinsic('%Temporal.PlainDateTime%');
  const result = ObjectCreate(TemporalPlainDateTime.prototype);
  CreateTemporalDateTimeSlots(result, isoYear, isoMonth, isoDay, h, min, s, ms, µs, ns, calendar);
  return result as Temporal.PlainDateTime;
}

export function CreateTemporalMonthDaySlots(
  result: Temporal.PlainMonthDay,
  isoMonth: number,
  isoDay: number,
  calendar: Temporal.CalendarProtocol,
  referenceISOYear: number
) {
  RejectISODate(referenceISOYear, isoMonth, isoDay);
  RejectDateRange(referenceISOYear, isoMonth, isoDay);

  CreateSlots(result);
  SetSlot(result, ISO_MONTH, isoMonth);
  SetSlot(result, ISO_DAY, isoDay);
  SetSlot(result, ISO_YEAR, referenceISOYear);
  SetSlot(result, CALENDAR, calendar);
  SetSlot(result, MONTH_DAY_BRAND, true);

  if (DEBUG) {
    Object.defineProperty(result, '_repr_', {
      value: `${result[Symbol.toStringTag]} <${TemporalMonthDayToString(result)}>`,
      writable: false,
      enumerable: false,
      configurable: false
    });
  }
}

export function CreateTemporalMonthDay(
  isoMonth: number,
  isoDay: number,
  calendar: Temporal.CalendarProtocol = GetISO8601Calendar(),
  referenceISOYear = 1972
) {
  const TemporalPlainMonthDay = GetIntrinsic('%Temporal.PlainMonthDay%');
  const result = ObjectCreate(TemporalPlainMonthDay.prototype);
  CreateTemporalMonthDaySlots(result, isoMonth, isoDay, calendar, referenceISOYear);
  return result;
}

export function CreateTemporalYearMonthSlots(
  result: Temporal.PlainYearMonth,
  isoYear: number,
  isoMonth: number,
  calendar: Temporal.CalendarProtocol,
  referenceISODay: number
) {
  RejectISODate(isoYear, isoMonth, referenceISODay);
  RejectYearMonthRange(isoYear, isoMonth);

  CreateSlots(result);
  SetSlot(result, ISO_YEAR, isoYear);
  SetSlot(result, ISO_MONTH, isoMonth);
  SetSlot(result, ISO_DAY, referenceISODay);
  SetSlot(result, CALENDAR, calendar);
  SetSlot(result, YEAR_MONTH_BRAND, true);

  if (DEBUG) {
    Object.defineProperty(result, '_repr_', {
      value: `${result[Symbol.toStringTag]} <${TemporalYearMonthToString(result)}>`,
      writable: false,
      enumerable: false,
      configurable: false
    });
  }
}

export function CreateTemporalYearMonth(
  isoYear: number,
  isoMonth: number,
  calendar: Temporal.CalendarProtocol = GetISO8601Calendar(),
  referenceISODay = 1
) {
  const TemporalPlainYearMonth = GetIntrinsic('%Temporal.PlainYearMonth%');
  const result = ObjectCreate(TemporalPlainYearMonth.prototype);
  CreateTemporalYearMonthSlots(result, isoYear, isoMonth, calendar, referenceISODay);
  return result;
}

export function CreateTemporalZonedDateTimeSlots(
  result: Temporal.ZonedDateTime,
  epochNanoseconds: JSBI,
  timeZone: Temporal.TimeZoneProtocol,
  calendar: Temporal.CalendarProtocol
) {
  ValidateEpochNanoseconds(epochNanoseconds);

  CreateSlots(result);
  SetSlot(result, EPOCHNANOSECONDS, epochNanoseconds);
  SetSlot(result, TIME_ZONE, timeZone);
  SetSlot(result, CALENDAR, calendar);

  const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
  const instant = new TemporalInstant(GetSlot(result, EPOCHNANOSECONDS));
  SetSlot(result, INSTANT, instant);

  if (DEBUG) {
    Object.defineProperty(result, '_repr_', {
      value: `${result[Symbol.toStringTag]} <${TemporalZonedDateTimeToString(result, 'auto')}>`,
      writable: false,
      enumerable: false,
      configurable: false
    });
  }
}

export function CreateTemporalZonedDateTime(
  epochNanoseconds: JSBI,
  timeZone: Temporal.TimeZoneProtocol,
  calendar: Temporal.CalendarProtocol = GetISO8601Calendar()
) {
  const TemporalZonedDateTime = GetIntrinsic('%Temporal.ZonedDateTime%');
  const result = ObjectCreate(TemporalZonedDateTime.prototype);
  CreateTemporalZonedDateTimeSlots(result, epochNanoseconds, timeZone, calendar);
  return result;
}

export function GetISO8601Calendar() {
  const TemporalCalendar = GetIntrinsic('%Temporal.Calendar%');
  return new TemporalCalendar('iso8601');
}

// TODO: should (can?) we make this generic so the field names are checked
// against the type that the calendar is a property of?
export function CalendarFields<F extends Iterable<string>>(calendar: Temporal.CalendarProtocol, fieldNamesParam: F) {
  let fieldNames = fieldNamesParam;
  if (calendar.fields) {
    fieldNames = calendar.fields(fieldNames) as F;
  }
  const result: string[] = [];
  for (const name of fieldNames) {
    if (typeof name !== 'string') throw new TypeError('bad return from calendar.fields()');
    ArrayPrototypePush.call(result, name);
  }
  return result as unknown as F;
}

export function CalendarMergeFields(
  calendar: Temporal.CalendarProtocol,
  fields: Record<string, unknown>,
  additionalFields: Record<string, unknown>
) {
  const calMergeFields = calendar.mergeFields;
  if (!calMergeFields) {
    return { ...fields, ...additionalFields };
  }
  const result = Reflect.apply(calMergeFields, calendar, [fields, additionalFields]);
  if (!IsObject(result)) throw new TypeError('bad return from calendar.mergeFields()');
  return result;
}

export function CalendarDateAdd(
  calendar: Temporal.CalendarProtocol,
  date: CalendarProtocolParams['dateAdd'][0],
  duration: CalendarProtocolParams['dateAdd'][1],
  options: CalendarProtocolParams['dateAdd'][2],
  dateAddParam?: Temporal.CalendarProtocol['dateAdd']
) {
  let dateAdd = dateAddParam;
  if (dateAdd === undefined) {
    dateAdd = calendar.dateAdd;
  }
  const result = ReflectApply(dateAdd, calendar, [date, duration, options]);
  if (!IsTemporalDate(result)) throw new TypeError('invalid result');
  return result;
}

function CalendarDateUntil(
  calendar: Temporal.CalendarProtocol,
  date: CalendarProtocolParams['dateUntil'][0],
  otherDate: CalendarProtocolParams['dateUntil'][1],
  options: CalendarProtocolParams['dateUntil'][2],
  dateUntilParam?: Temporal.CalendarProtocol['dateUntil']
) {
  let dateUntil = dateUntilParam;
  if (dateUntil === undefined) {
    dateUntil = calendar.dateUntil;
  }
  const result = ReflectApply(dateUntil, calendar, [date, otherDate, options]);
  if (!IsTemporalDuration(result)) throw new TypeError('invalid result');
  return result;
}

export function CalendarYear(calendar: Temporal.CalendarProtocol, dateLike: CalendarProtocolParams['year'][0]) {
  const result = calendar.year(dateLike);
  if (result === undefined) {
    throw new RangeError('calendar year result must be an integer');
  }
  return ToIntegerThrowOnInfinity(result);
}

export function CalendarMonth(calendar: Temporal.CalendarProtocol, dateLike: CalendarProtocolParams['month'][0]) {
  const result = calendar.month(dateLike);
  if (result === undefined) {
    throw new RangeError('calendar month result must be a positive integer');
  }
  return ToPositiveInteger(result);
}

export function CalendarMonthCode(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['monthCode'][0]
) {
  const result = calendar.monthCode(dateLike);
  if (result === undefined) {
    throw new RangeError('calendar monthCode result must be a string');
  }
  return ToString(result);
}

export function CalendarDay(calendar: Temporal.CalendarProtocol, dateLike: CalendarProtocolParams['day'][0]) {
  const result = calendar.day(dateLike);
  if (result === undefined) {
    throw new RangeError('calendar day result must be a positive integer');
  }
  return ToPositiveInteger(result);
}

export function CalendarEra(calendar: Temporal.CalendarProtocol, dateLike: CalendarProtocolParams['era'][0]) {
  let result = calendar.era(dateLike);
  if (result !== undefined) {
    result = ToString(result);
  }
  return result;
}

export function CalendarEraYear(calendar: Temporal.CalendarProtocol, dateLike: CalendarProtocolParams['eraYear'][0]) {
  let result = calendar.eraYear(dateLike);
  if (result !== undefined) {
    result = ToIntegerThrowOnInfinity(result);
  }
  return result;
}

export function CalendarDayOfWeek(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['dayOfWeek'][0]
) {
  return calendar.dayOfWeek(dateLike);
}

export function CalendarDayOfYear(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['dayOfYear'][0]
) {
  return calendar.dayOfYear(dateLike);
}

export function CalendarWeekOfYear(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['weekOfYear'][0]
) {
  return calendar.weekOfYear(dateLike);
}

export function CalendarDaysInWeek(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['daysInWeek'][0]
) {
  return calendar.daysInWeek(dateLike);
}

export function CalendarDaysInMonth(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['daysInMonth'][0]
) {
  return calendar.daysInMonth(dateLike);
}

export function CalendarDaysInYear(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['daysInYear'][0]
) {
  return calendar.daysInYear(dateLike);
}

export function CalendarMonthsInYear(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['monthsInYear'][0]
) {
  return calendar.monthsInYear(dateLike);
}

export function CalendarInLeapYear(
  calendar: Temporal.CalendarProtocol,
  dateLike: CalendarProtocolParams['inLeapYear'][0]
) {
  return calendar.inLeapYear(dateLike);
}

export function ToTemporalCalendar(calendarLikeParam: CalendarParams['from'][0]) {
  let calendarLike = calendarLikeParam;
  if (IsObject(calendarLike)) {
    if (HasSlot(calendarLike, CALENDAR)) return GetSlot(calendarLike, CALENDAR);
    if (!('calendar' in calendarLike)) return calendarLike;
    calendarLike = (calendarLike as { calendar: Temporal.CalendarProtocol | string }).calendar;
    if (IsObject(calendarLike) && !('calendar' in calendarLike)) return calendarLike;
  }
  const identifier = ToString(calendarLike);
  const TemporalCalendar = GetIntrinsic('%Temporal.Calendar%');
  if (IsBuiltinCalendar(identifier)) return new TemporalCalendar(identifier);
  let calendar;
  try {
    ({ calendar } = ParseISODateTime(identifier));
  } catch {
    throw new RangeError(`Invalid calendar: ${identifier}`);
  }
  if (!calendar) calendar = 'iso8601';
  return new TemporalCalendar(calendar);
}

function GetTemporalCalendarWithISODefault(
  item: Temporal.CalendarProtocol | { calendar?: Temporal.PlainDateLike['calendar'] }
): Temporal.Calendar | Temporal.CalendarProtocol {
  if (HasSlot(item, CALENDAR)) return GetSlot(item, CALENDAR);
  const { calendar } = item as Exclude<typeof item, Temporal.CalendarProtocol>;
  if (calendar === undefined) return GetISO8601Calendar();
  return ToTemporalCalendar(calendar);
}

export function CalendarEquals(one: Temporal.CalendarProtocol, two: Temporal.CalendarProtocol) {
  if (one === two) return true;
  const cal1 = ToString(one);
  const cal2 = ToString(two);
  return cal1 === cal2;
}

export function ConsolidateCalendars(one: Temporal.CalendarProtocol, two: Temporal.CalendarProtocol) {
  if (one === two) return two;
  const sOne = ToString(one);
  const sTwo = ToString(two);
  if (sOne === sTwo || sOne === 'iso8601') {
    return two;
  } else if (sTwo === 'iso8601') {
    return one;
  } else {
    throw new RangeError('irreconcilable calendars');
  }
}

export function CalendarDateFromFields(
  calendar: Temporal.CalendarProtocol,
  fields: CalendarProtocolParams['dateFromFields'][0],
  options?: Partial<CalendarProtocolParams['dateFromFields'][1]>
) {
  const result = calendar.dateFromFields(fields, options);
  if (!IsTemporalDate(result)) throw new TypeError('invalid result');
  return result;
}

export function CalendarYearMonthFromFields(
  calendar: Temporal.CalendarProtocol,
  fields: CalendarProtocolParams['yearMonthFromFields'][0],
  options?: CalendarProtocolParams['yearMonthFromFields'][1]
) {
  const result = calendar.yearMonthFromFields(fields, options);
  if (!IsTemporalYearMonth(result)) throw new TypeError('invalid result');
  return result;
}

export function CalendarMonthDayFromFields(
  calendar: Temporal.CalendarProtocol,
  fields: CalendarProtocolParams['monthDayFromFields'][0],
  options?: CalendarProtocolParams['monthDayFromFields'][1]
) {
  const result = calendar.monthDayFromFields(fields, options);
  if (!IsTemporalMonthDay(result)) throw new TypeError('invalid result');
  return result;
}

export function ToTemporalTimeZone(temporalTimeZoneLikeParam: TimeZoneParams['from'][0]) {
  let temporalTimeZoneLike = temporalTimeZoneLikeParam;
  if (IsObject(temporalTimeZoneLike)) {
    if (IsTemporalZonedDateTime(temporalTimeZoneLike)) return GetSlot(temporalTimeZoneLike, TIME_ZONE);
    if (!('timeZone' in temporalTimeZoneLike)) return temporalTimeZoneLike;
    temporalTimeZoneLike = (temporalTimeZoneLike as { timeZone: Temporal.TimeZoneProtocol | string }).timeZone;
    if (IsObject(temporalTimeZoneLike) && !('timeZone' in temporalTimeZoneLike)) {
      return temporalTimeZoneLike;
    }
  }
  const identifier = ToString(temporalTimeZoneLike);
  const timeZone = ParseTemporalTimeZone(identifier);
  const TemporalTimeZone = GetIntrinsic('%Temporal.TimeZone%');
  return new TemporalTimeZone(timeZone);
}

export function TimeZoneEquals(one: Temporal.TimeZoneProtocol, two: Temporal.TimeZoneProtocol) {
  if (one === two) return true;
  const tz1 = ToString(one);
  const tz2 = ToString(two);
  return tz1 === tz2;
}

export function TemporalDateTimeToDate(dateTime: Temporal.PlainDateTime) {
  return CreateTemporalDate(
    GetSlot(dateTime, ISO_YEAR),
    GetSlot(dateTime, ISO_MONTH),
    GetSlot(dateTime, ISO_DAY),
    GetSlot(dateTime, CALENDAR)
  );
}

export function TemporalDateTimeToTime(dateTime: Temporal.PlainDateTime) {
  const Time = GetIntrinsic('%Temporal.PlainTime%');
  return new Time(
    GetSlot(dateTime, ISO_HOUR),
    GetSlot(dateTime, ISO_MINUTE),
    GetSlot(dateTime, ISO_SECOND),
    GetSlot(dateTime, ISO_MILLISECOND),
    GetSlot(dateTime, ISO_MICROSECOND),
    GetSlot(dateTime, ISO_NANOSECOND)
  );
}

export function GetOffsetNanosecondsFor(
  timeZone: Temporal.TimeZoneProtocol,
  instant: TimeZoneProtocolParams['getOffsetNanosecondsFor'][0]
) {
  let getOffsetNanosecondsFor = timeZone.getOffsetNanosecondsFor;
  if (typeof getOffsetNanosecondsFor !== 'function') {
    throw new TypeError('getOffsetNanosecondsFor not callable');
  }
  const offsetNs = Reflect.apply(getOffsetNanosecondsFor, timeZone, [instant]);
  if (typeof offsetNs !== 'number') {
    throw new TypeError('bad return from getOffsetNanosecondsFor');
  }
  if (!IsInteger(offsetNs) || MathAbs(offsetNs) > 86400e9) {
    throw new RangeError('out-of-range return from getOffsetNanosecondsFor');
  }
  return offsetNs;
}

export function BuiltinTimeZoneGetOffsetStringFor(timeZone: Temporal.TimeZoneProtocol, instant: Temporal.Instant) {
  const offsetNs = GetOffsetNanosecondsFor(timeZone, instant);
  return FormatTimeZoneOffsetString(offsetNs);
}

export function BuiltinTimeZoneGetPlainDateTimeFor(
  timeZone: Temporal.TimeZoneProtocol,
  instant: Temporal.Instant,
  calendar: Temporal.CalendarProtocol
) {
  const ns = GetSlot(instant, EPOCHNANOSECONDS);
  const offsetNs = GetOffsetNanosecondsFor(timeZone, instant);
  let { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = GetISOPartsFromEpoch(ns);
  ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = BalanceISODateTime(
    year,
    month,
    day,
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond + offsetNs
  ));
  return CreateTemporalDateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar);
}

export function BuiltinTimeZoneGetInstantFor(
  timeZone: Temporal.TimeZoneProtocol,
  dateTime: Temporal.PlainDateTime,
  disambiguation: NonNullable<Temporal.ToInstantOptions['disambiguation']>
) {
  const possibleInstants = GetPossibleInstantsFor(timeZone, dateTime);
  return DisambiguatePossibleInstants(possibleInstants, timeZone, dateTime, disambiguation);
}

function DisambiguatePossibleInstants(
  possibleInstants: Temporal.Instant[],
  timeZone: Temporal.TimeZoneProtocol,
  dateTime: Temporal.PlainDateTime,
  disambiguation: NonNullable<Temporal.ToInstantOptions['disambiguation']>
) {
  const Instant = GetIntrinsic('%Temporal.Instant%');
  const numInstants = possibleInstants.length;

  if (numInstants === 1) return possibleInstants[0];
  if (numInstants) {
    switch (disambiguation) {
      case 'compatible':
      // fall through because 'compatible' means 'earlier' for "fall back" transitions
      case 'earlier':
        return possibleInstants[0];
      case 'later':
        return possibleInstants[numInstants - 1];
      case 'reject': {
        throw new RangeError('multiple instants found');
      }
    }
  }

  const year = GetSlot(dateTime, ISO_YEAR);
  const month = GetSlot(dateTime, ISO_MONTH);
  const day = GetSlot(dateTime, ISO_DAY);
  const hour = GetSlot(dateTime, ISO_HOUR);
  const minute = GetSlot(dateTime, ISO_MINUTE);
  const second = GetSlot(dateTime, ISO_SECOND);
  const millisecond = GetSlot(dateTime, ISO_MILLISECOND);
  const microsecond = GetSlot(dateTime, ISO_MICROSECOND);
  const nanosecond = GetSlot(dateTime, ISO_NANOSECOND);
  const utcns = GetEpochFromISOParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
  if (utcns === null) throw new RangeError('DateTime outside of supported range');
  const dayBefore = new Instant(JSBI.subtract(utcns, DAY_NANOS));
  const dayAfter = new Instant(JSBI.add(utcns, DAY_NANOS));
  const offsetBefore = GetOffsetNanosecondsFor(timeZone, dayBefore);
  const offsetAfter = GetOffsetNanosecondsFor(timeZone, dayAfter);
  const nanoseconds = offsetAfter - offsetBefore;
  switch (disambiguation) {
    case 'earlier': {
      const calendar = GetSlot(dateTime, CALENDAR);
      const PlainDateTime = GetIntrinsic('%Temporal.PlainDateTime%');
      const earlier = AddDateTime(
        year,
        month,
        day,
        hour,
        minute,
        second,
        millisecond,
        microsecond,
        nanosecond,
        calendar,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        -nanoseconds,
        undefined
      );
      const earlierPlainDateTime = new PlainDateTime(
        earlier.year,
        earlier.month,
        earlier.day,
        earlier.hour,
        earlier.minute,
        earlier.second,
        earlier.millisecond,
        earlier.microsecond,
        earlier.nanosecond,
        calendar
      );
      return GetPossibleInstantsFor(timeZone, earlierPlainDateTime)[0];
    }
    case 'compatible':
    // fall through because 'compatible' means 'later' for "spring forward" transitions
    case 'later': {
      const calendar = GetSlot(dateTime, CALENDAR);
      const PlainDateTime = GetIntrinsic('%Temporal.PlainDateTime%');
      const later = AddDateTime(
        year,
        month,
        day,
        hour,
        minute,
        second,
        millisecond,
        microsecond,
        nanosecond,
        calendar,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        nanoseconds,
        undefined
      );
      const laterPlainDateTime = new PlainDateTime(
        later.year,
        later.month,
        later.day,
        later.hour,
        later.minute,
        later.second,
        later.millisecond,
        later.microsecond,
        later.nanosecond,
        calendar
      );
      const possible = GetPossibleInstantsFor(timeZone, laterPlainDateTime);
      return possible[possible.length - 1];
    }
    case 'reject': {
      throw new RangeError('no such instant found');
    }
  }
}

function GetPossibleInstantsFor(
  timeZone: Temporal.TimeZoneProtocol,
  dateTime: TimeZoneProtocolParams['getPossibleInstantsFor'][0]
) {
  const possibleInstants = timeZone.getPossibleInstantsFor(dateTime);
  const result: Temporal.Instant[] = [];
  for (const instant of possibleInstants) {
    if (!IsTemporalInstant(instant)) {
      throw new TypeError('bad return from getPossibleInstantsFor');
    }
    ArrayPrototypePush.call(result, instant);
  }
  return result;
}

export function ISOYearString(year: number) {
  let yearString;
  if (year < 0 || year > 9999) {
    const sign = year < 0 ? '-' : '+';
    const yearNumber = MathAbs(year);
    yearString = sign + `000000${yearNumber}`.slice(-6);
  } else {
    yearString = `0000${year}`.slice(-4);
  }
  return yearString;
}

export function ISODateTimePartString(part: number) {
  return `00${part}`.slice(-2);
}
export function FormatSecondsStringPart(
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number,
  precision: ReturnType<typeof ToSecondsStringPrecision>['precision']
) {
  if (precision === 'minute') return '';

  const secs = `:${ISODateTimePartString(second)}`;
  let fractionNumber = millisecond * 1e6 + microsecond * 1e3 + nanosecond;
  let fraction: string;

  if (precision === 'auto') {
    if (fractionNumber === 0) return secs;
    fraction = `${fractionNumber}`.padStart(9, '0');
    while (fraction[fraction.length - 1] === '0') fraction = fraction.slice(0, -1);
  } else {
    if (precision === 0) return secs;
    fraction = `${fractionNumber}`.padStart(9, '0').slice(0, precision);
  }
  return `${secs}.${fraction}`;
}

export function TemporalInstantToString(
  instant: Temporal.Instant,
  timeZone: Temporal.TimeZoneProtocol | undefined,
  precision: ReturnType<typeof ToSecondsStringPrecision>['precision']
) {
  let outputTimeZone = timeZone;
  if (outputTimeZone === undefined) {
    const TemporalTimeZone = GetIntrinsic('%Temporal.TimeZone%');
    outputTimeZone = new TemporalTimeZone('UTC');
  }
  const iso = GetISO8601Calendar();
  const dateTime = BuiltinTimeZoneGetPlainDateTimeFor(outputTimeZone, instant, iso);
  const year = ISOYearString(GetSlot(dateTime, ISO_YEAR));
  const month = ISODateTimePartString(GetSlot(dateTime, ISO_MONTH));
  const day = ISODateTimePartString(GetSlot(dateTime, ISO_DAY));
  const hour = ISODateTimePartString(GetSlot(dateTime, ISO_HOUR));
  const minute = ISODateTimePartString(GetSlot(dateTime, ISO_MINUTE));
  const seconds = FormatSecondsStringPart(
    GetSlot(dateTime, ISO_SECOND),
    GetSlot(dateTime, ISO_MILLISECOND),
    GetSlot(dateTime, ISO_MICROSECOND),
    GetSlot(dateTime, ISO_NANOSECOND),
    precision
  );
  let timeZoneString = 'Z';
  if (timeZone !== undefined) {
    const offsetNs = GetOffsetNanosecondsFor(outputTimeZone, instant);
    timeZoneString = FormatISOTimeZoneOffsetString(offsetNs);
  }
  return `${year}-${month}-${day}T${hour}:${minute}${seconds}${timeZoneString}`;
}

interface ToStringOptions {
  unit: ReturnType<typeof ToSecondsStringPrecision>['unit'];
  increment: number;
  roundingMode: ReturnType<typeof ToTemporalRoundingMode>;
}

export function TemporalDurationToString(
  duration: Temporal.Duration,
  precision: Temporal.ToStringPrecisionOptions['fractionalSecondDigits'] = 'auto',
  options: ToStringOptions | undefined = undefined
) {
  function formatNumber(num: number) {
    if (num <= NumberMaxSafeInteger) return num.toString(10);
    return JSBI.BigInt(num).toString(10);
  }

  const years = GetSlot(duration, YEARS);
  const months = GetSlot(duration, MONTHS);
  const weeks = GetSlot(duration, WEEKS);
  const days = GetSlot(duration, DAYS);
  const hours = GetSlot(duration, HOURS);
  const minutes = GetSlot(duration, MINUTES);
  let seconds = GetSlot(duration, SECONDS);
  let ms = GetSlot(duration, MILLISECONDS);
  let µs = GetSlot(duration, MICROSECONDS);
  let ns = GetSlot(duration, NANOSECONDS);
  const sign = DurationSign(years, months, weeks, days, hours, minutes, seconds, ms, µs, ns);

  if (options) {
    const { unit, increment, roundingMode } = options;
    ({
      seconds,
      milliseconds: ms,
      microseconds: µs,
      nanoseconds: ns
    } = RoundDuration(0, 0, 0, 0, 0, 0, seconds, ms, µs, ns, increment, unit, roundingMode));
  }

  const dateParts = [];
  if (years) dateParts.push(`${formatNumber(MathAbs(years))}Y`);
  if (months) dateParts.push(`${formatNumber(MathAbs(months))}M`);
  if (weeks) dateParts.push(`${formatNumber(MathAbs(weeks))}W`);
  if (days) dateParts.push(`${formatNumber(MathAbs(days))}D`);

  const timeParts = [];
  if (hours) timeParts.push(`${formatNumber(MathAbs(hours))}H`);
  if (minutes) timeParts.push(`${formatNumber(MathAbs(minutes))}M`);

  const secondParts = [];
  let total = TotalDurationNanoseconds(0, 0, 0, seconds, ms, µs, ns, 0);
  let nsBigInt: JSBI, µsBigInt: JSBI, msBigInt: JSBI, secondsBigInt: JSBI;
  ({ quotient: total, remainder: nsBigInt } = divmod(total, THOUSAND));
  ({ quotient: total, remainder: µsBigInt } = divmod(total, THOUSAND));
  ({ quotient: secondsBigInt, remainder: msBigInt } = divmod(total, THOUSAND));
  const fraction =
    MathAbs(JSBI.toNumber(msBigInt)) * 1e6 + MathAbs(JSBI.toNumber(µsBigInt)) * 1e3 + MathAbs(JSBI.toNumber(nsBigInt));
  let decimalPart;
  if (precision === 'auto') {
    if (fraction !== 0) {
      decimalPart = `${fraction}`.padStart(9, '0');
      while (decimalPart[decimalPart.length - 1] === '0') {
        decimalPart = decimalPart.slice(0, -1);
      }
    }
  } else if (precision !== 0) {
    decimalPart = `${fraction}`.padStart(9, '0').slice(0, precision);
  }
  if (decimalPart) secondParts.unshift('.', decimalPart);
  if (!JSBI.equal(secondsBigInt, ZERO) || secondParts.length || precision !== 'auto') {
    secondParts.unshift(abs(secondsBigInt).toString());
  }
  if (secondParts.length) timeParts.push(`${secondParts.join('')}S`);
  if (timeParts.length) timeParts.unshift('T');
  if (!dateParts.length && !timeParts.length) return 'PT0S';
  return `${sign < 0 ? '-' : ''}P${dateParts.join('')}${timeParts.join('')}`;
}

export function TemporalDateToString(
  date: Temporal.PlainDate,
  showCalendar: Temporal.ShowCalendarOption['calendarName'] = 'auto'
) {
  const year = ISOYearString(GetSlot(date, ISO_YEAR));
  const month = ISODateTimePartString(GetSlot(date, ISO_MONTH));
  const day = ISODateTimePartString(GetSlot(date, ISO_DAY));
  const calendarID = ToString(GetSlot(date, CALENDAR));
  const calendar = FormatCalendarAnnotation(calendarID, showCalendar);
  return `${year}-${month}-${day}${calendar}`;
}

export function TemporalDateTimeToString(
  dateTime: Temporal.PlainDateTime,
  precision: ReturnType<typeof ToSecondsStringPrecision>['precision'],
  showCalendar: ReturnType<typeof ToShowCalendarOption> = 'auto',
  options: ToStringOptions | undefined = undefined
) {
  let year = GetSlot(dateTime, ISO_YEAR);
  let month = GetSlot(dateTime, ISO_MONTH);
  let day = GetSlot(dateTime, ISO_DAY);
  let hour = GetSlot(dateTime, ISO_HOUR);
  let minute = GetSlot(dateTime, ISO_MINUTE);
  let second = GetSlot(dateTime, ISO_SECOND);
  let millisecond = GetSlot(dateTime, ISO_MILLISECOND);
  let microsecond = GetSlot(dateTime, ISO_MICROSECOND);
  let nanosecond = GetSlot(dateTime, ISO_NANOSECOND);

  if (options) {
    const { unit, increment, roundingMode } = options;
    ({ year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = RoundISODateTime(
      year,
      month,
      day,
      hour,
      minute,
      second,
      millisecond,
      microsecond,
      nanosecond,
      increment,
      unit,
      roundingMode
    ));
  }

  const yearString = ISOYearString(year);
  const monthString = ISODateTimePartString(month);
  const dayString = ISODateTimePartString(day);
  const hourString = ISODateTimePartString(hour);
  const minuteString = ISODateTimePartString(minute);
  const secondsString = FormatSecondsStringPart(second, millisecond, microsecond, nanosecond, precision);
  const calendarID = ToString(GetSlot(dateTime, CALENDAR));
  const calendar = FormatCalendarAnnotation(calendarID, showCalendar);
  return `${yearString}-${monthString}-${dayString}T${hourString}:${minuteString}${secondsString}${calendar}`;
}

export function TemporalMonthDayToString(
  monthDay: Temporal.PlainMonthDay,
  showCalendar: Temporal.ShowCalendarOption['calendarName'] = 'auto'
) {
  const month = ISODateTimePartString(GetSlot(monthDay, ISO_MONTH));
  const day = ISODateTimePartString(GetSlot(monthDay, ISO_DAY));
  let resultString = `${month}-${day}`;
  const calendar = GetSlot(monthDay, CALENDAR);
  const calendarID = ToString(calendar);
  if (showCalendar === 'always' || calendarID !== 'iso8601') {
    const year = ISOYearString(GetSlot(monthDay, ISO_YEAR));
    resultString = `${year}-${resultString}`;
  }
  const calendarString = FormatCalendarAnnotation(calendarID, showCalendar);
  if (calendarString) resultString += calendarString;
  return resultString;
}

export function TemporalYearMonthToString(
  yearMonth: Temporal.PlainYearMonth,
  showCalendar: Temporal.ShowCalendarOption['calendarName'] = 'auto'
) {
  const year = ISOYearString(GetSlot(yearMonth, ISO_YEAR));
  const month = ISODateTimePartString(GetSlot(yearMonth, ISO_MONTH));
  let resultString = `${year}-${month}`;
  const calendar = GetSlot(yearMonth, CALENDAR);
  const calendarID = ToString(calendar);
  if (showCalendar === 'always' || calendarID !== 'iso8601') {
    const day = ISODateTimePartString(GetSlot(yearMonth, ISO_DAY));
    resultString += `-${day}`;
  }
  const calendarString = FormatCalendarAnnotation(calendarID, showCalendar);
  if (calendarString) resultString += calendarString;
  return resultString;
}

export function TemporalZonedDateTimeToString(
  zdt: Temporal.ZonedDateTime,
  precision: ReturnType<typeof ToSecondsStringPrecision>['precision'],
  showCalendar: ReturnType<typeof ToShowCalendarOption> = 'auto',
  showTimeZone: ReturnType<typeof ToShowTimeZoneNameOption> = 'auto',
  showOffset: ReturnType<typeof ToShowOffsetOption> = 'auto',
  options: ToStringOptions | undefined = undefined
) {
  let instant = GetSlot(zdt, INSTANT);

  if (options) {
    const { unit, increment, roundingMode } = options;
    const ns = RoundInstant(GetSlot(zdt, EPOCHNANOSECONDS), increment, unit, roundingMode);
    const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
    instant = new TemporalInstant(ns);
  }

  const tz = GetSlot(zdt, TIME_ZONE);
  const iso = GetISO8601Calendar();
  const dateTime = BuiltinTimeZoneGetPlainDateTimeFor(tz, instant, iso);

  const year = ISOYearString(GetSlot(dateTime, ISO_YEAR));
  const month = ISODateTimePartString(GetSlot(dateTime, ISO_MONTH));
  const day = ISODateTimePartString(GetSlot(dateTime, ISO_DAY));
  const hour = ISODateTimePartString(GetSlot(dateTime, ISO_HOUR));
  const minute = ISODateTimePartString(GetSlot(dateTime, ISO_MINUTE));
  const seconds = FormatSecondsStringPart(
    GetSlot(dateTime, ISO_SECOND),
    GetSlot(dateTime, ISO_MILLISECOND),
    GetSlot(dateTime, ISO_MICROSECOND),
    GetSlot(dateTime, ISO_NANOSECOND),
    precision
  );
  let result = `${year}-${month}-${day}T${hour}:${minute}${seconds}`;
  if (showOffset !== 'never') {
    const offsetNs = GetOffsetNanosecondsFor(tz, instant);
    result += FormatISOTimeZoneOffsetString(offsetNs);
  }
  if (showTimeZone !== 'never') result += `[${tz}]`;
  const calendarID = ToString(GetSlot(zdt, CALENDAR));
  result += FormatCalendarAnnotation(calendarID, showCalendar);
  return result;
}

export function TestTimeZoneOffsetString(string: string) {
  return OFFSET.test(StringCtor(string));
}

export function ParseTimeZoneOffsetString(string: string): number {
  const match = OFFSET.exec(StringCtor(string));
  if (!match) {
    throw new RangeError(`invalid time zone offset: ${string}`);
  }
  const sign = match[1] === '-' || match[1] === '\u2212' ? -1 : +1;
  const hours = +match[2];
  const minutes = +(match[3] || 0);
  const seconds = +(match[4] || 0);
  const nanoseconds = +((match[5] || 0) + '000000000').slice(0, 9);
  return sign * (((hours * 60 + minutes) * 60 + seconds) * 1e9 + nanoseconds);
}

export function GetCanonicalTimeZoneIdentifier(timeZoneIdentifier: string): string {
  if (TestTimeZoneOffsetString(timeZoneIdentifier)) {
    const offsetNs = ParseTimeZoneOffsetString(timeZoneIdentifier);
    return FormatTimeZoneOffsetString(offsetNs);
  }
  const formatter = getIntlDateTimeFormatEnUsForTimeZone(StringCtor(timeZoneIdentifier));
  return formatter.resolvedOptions().timeZone;
}

export function GetIANATimeZoneOffsetNanoseconds(epochNanoseconds: JSBI, id: string) {
  const { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = GetIANATimeZoneDateTimeParts(
    epochNanoseconds,
    id
  );
  const utc = GetEpochFromISOParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
  if (utc === null) throw new RangeError('Date outside of supported range');
  return JSBI.toNumber(JSBI.subtract(utc, epochNanoseconds));
}

function FormatTimeZoneOffsetString(offsetNanosecondsParam: number): string {
  const sign = offsetNanosecondsParam < 0 ? '-' : '+';
  const offsetNanoseconds = MathAbs(offsetNanosecondsParam);
  const nanoseconds = offsetNanoseconds % 1e9;
  const seconds = MathFloor(offsetNanoseconds / 1e9) % 60;
  const minutes = MathFloor(offsetNanoseconds / 60e9) % 60;
  const hours = MathFloor(offsetNanoseconds / 3600e9);

  const hourString = ISODateTimePartString(hours);
  const minuteString = ISODateTimePartString(minutes);
  const secondString = ISODateTimePartString(seconds);
  let post = '';
  if (nanoseconds) {
    let fraction = `${nanoseconds}`.padStart(9, '0');
    while (fraction[fraction.length - 1] === '0') fraction = fraction.slice(0, -1);
    post = `:${secondString}.${fraction}`;
  } else if (seconds) {
    post = `:${secondString}`;
  }
  return `${sign}${hourString}:${minuteString}${post}`;
}

function FormatISOTimeZoneOffsetString(offsetNanosecondsParam: number): string {
  let offsetNanoseconds = JSBI.toNumber(
    RoundNumberToIncrement(JSBI.BigInt(offsetNanosecondsParam), 60e9, 'halfExpand')
  );
  const sign = offsetNanoseconds < 0 ? '-' : '+';
  offsetNanoseconds = MathAbs(offsetNanoseconds);
  const minutes = (offsetNanoseconds / 60e9) % 60;
  const hours = MathFloor(offsetNanoseconds / 3600e9);

  const hourString = ISODateTimePartString(hours);
  const minuteString = ISODateTimePartString(minutes);
  return `${sign}${hourString}:${minuteString}`;
}
export function GetEpochFromISOParts(
  year: number,
  month: number,
  day: number,
  hour: number,
  minute: number,
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number
) {
  // Note: Date.UTC() interprets one and two-digit years as being in the
  // 20th century, so don't use it
  const legacyDate = new Date();
  legacyDate.setUTCHours(hour, minute, second, millisecond);
  legacyDate.setUTCFullYear(year, month - 1, day);
  const ms = legacyDate.getTime();
  if (NumberIsNaN(ms)) return null;
  let ns = JSBI.multiply(JSBI.BigInt(ms), MILLION);
  ns = JSBI.add(ns, JSBI.multiply(JSBI.BigInt(microsecond), THOUSAND));
  ns = JSBI.add(ns, JSBI.BigInt(nanosecond));
  if (JSBI.lessThan(ns, NS_MIN) || JSBI.greaterThan(ns, NS_MAX)) return null;
  return ns;
}

function GetISOPartsFromEpoch(epochNanoseconds: JSBI) {
  const { quotient, remainder } = divmod(epochNanoseconds, MILLION);
  let epochMilliseconds = JSBI.toNumber(quotient);
  let nanos = JSBI.toNumber(remainder);
  if (nanos < 0) {
    nanos += 1e6;
    epochMilliseconds -= 1;
  }
  const microsecond = MathFloor(nanos / 1e3) % 1e3;
  const nanosecond = nanos % 1e3;

  const item = new Date(epochMilliseconds);
  const year = item.getUTCFullYear();
  const month = item.getUTCMonth() + 1;
  const day = item.getUTCDate();
  const hour = item.getUTCHours();
  const minute = item.getUTCMinutes();
  const second = item.getUTCSeconds();
  const millisecond = item.getUTCMilliseconds();

  return { epochMilliseconds, year, month, day, hour, minute, second, millisecond, microsecond, nanosecond };
}

// ts-prune-ignore-next TODO: remove this after tests are converted to TS
export function GetIANATimeZoneDateTimeParts(epochNanoseconds: JSBI, id: string) {
  const { epochMilliseconds, millisecond, microsecond, nanosecond } = GetISOPartsFromEpoch(epochNanoseconds);
  const { year, month, day, hour, minute, second } = GetFormatterParts(id, epochMilliseconds);
  return BalanceISODateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
}

function maxJSBI(one: JSBI, two: JSBI) {
  return JSBI.lessThan(one, two) ? two : one;
}

/**
 * Our best guess at how far in advance new rules will be put into the TZDB for
 * future offset transitions. We'll pick 10 years but can always revise it if
 * we find that countries are being unusually proactive in their announcing
 * of offset changes.
 */
function afterLatestPossibleTzdbRuleChange() {
  return JSBI.add(SystemUTCEpochNanoSeconds(), ABOUT_TEN_YEARS_NANOS);
}

export function GetIANATimeZoneNextTransition(epochNanoseconds: JSBI, id: string): JSBI | null {
  // Decide how far in the future after `epochNanoseconds` we'll look for an
  // offset change. There are two cases:
  // 1. If it's a past date (or a date in the near future) then it's possible
  //    that the time zone may have newly added DST in the next few years. So
  //    we'll have to look from the provided time until a few years after the
  //    current system time. (Changes to DST policy are usually announced a few
  //    years in the future.) Note that the first DST anywhere started in 1847,
  //    so we'll start checks in 1847 instead of wasting cycles on years where
  //    there will never be transitions.
  // 2. If it's a future date beyond the next few years, then we'll just assume
  //    that the latest DST policy in TZDB will still be in effect.  In this
  //    case, we only need to look one year in the future to see if there are
  //    any DST transitions.  We actually only need to look 9-10 months because
  //    DST has two transitions per year, but we'll use a year just to be safe.
  const oneYearLater = JSBI.add(epochNanoseconds, ABOUT_ONE_YEAR_NANOS);
  const uppercap = maxJSBI(afterLatestPossibleTzdbRuleChange(), oneYearLater);
  // The first transition (in any timezone) recorded in the TZDB was in 1847, so
  // start there if an earlier date is supplied.
  let leftNanos = maxJSBI(BEFORE_FIRST_OFFSET_TRANSITION, epochNanoseconds);
  const leftOffsetNs = GetIANATimeZoneOffsetNanoseconds(leftNanos, id);
  let rightNanos = leftNanos;
  let rightOffsetNs = leftOffsetNs;
  while (leftOffsetNs === rightOffsetNs && JSBI.lessThan(JSBI.BigInt(leftNanos), uppercap)) {
    rightNanos = JSBI.add(leftNanos, TWO_WEEKS_NANOS);
    rightOffsetNs = GetIANATimeZoneOffsetNanoseconds(rightNanos, id);
    if (leftOffsetNs === rightOffsetNs) {
      leftNanos = rightNanos;
    }
  }
  if (leftOffsetNs === rightOffsetNs) return null;
  const result = bisect(
    (epochNs: JSBI) => GetIANATimeZoneOffsetNanoseconds(epochNs, id),
    leftNanos,
    rightNanos,
    leftOffsetNs,
    rightOffsetNs
  );
  return result;
}

export function GetIANATimeZonePreviousTransition(epochNanoseconds: JSBI, id: string): JSBI | null {
  // If a time zone uses DST (at the time of `epochNanoseconds`), then we only
  // have to look back one year to find a transition. But if it doesn't use DST,
  // then we need to look all the way back to 1847 (the earliest rule in the
  // TZDB) to see if it had other offset transitions in the past. Looping back
  // from a far-future date to 1847 is very slow (minutes of 100% CPU!), and is
  // also unnecessary because DST rules aren't put into the TZDB more than a few
  // years in the future because the political changes in time zones happen with
  // only a few years' warning. Therefore, if a far-future date is provided,
  // then we'll run the check in two parts:
  // 1. First, we'll look back for up to one year to see if the latest TZDB
  //    rules have DST.
  // 2. If not, then we'll "fast-reverse" back to a few years later than the
  //    current system time, and then look back to 1847. This reduces the
  //    worst-case loop from 273K years to 175 years, for a ~1500x improvement
  //    in worst-case perf.
  const afterLatestRule = afterLatestPossibleTzdbRuleChange();
  const isFarFuture = JSBI.greaterThan(epochNanoseconds, afterLatestRule);
  const lowercap = isFarFuture ? JSBI.subtract(epochNanoseconds, ABOUT_ONE_YEAR_NANOS) : BEFORE_FIRST_OFFSET_TRANSITION;
  let rightNanos = JSBI.subtract(epochNanoseconds, ONE);
  const rightOffsetNs = GetIANATimeZoneOffsetNanoseconds(rightNanos, id);
  let leftNanos = rightNanos;
  let leftOffsetNs = rightOffsetNs;
  while (rightOffsetNs === leftOffsetNs && JSBI.greaterThan(rightNanos, lowercap)) {
    leftNanos = JSBI.subtract(rightNanos, TWO_WEEKS_NANOS);
    leftOffsetNs = GetIANATimeZoneOffsetNanoseconds(leftNanos, id);
    if (rightOffsetNs === leftOffsetNs) {
      rightNanos = leftNanos;
    }
  }
  if (rightOffsetNs === leftOffsetNs) {
    if (isFarFuture) {
      // There was no DST after looking back one year, which means that the most
      // recent TZDB rules don't have any recurring transitions. To check for
      // transitions in older rules, back up to a few years after the current
      // date and then look all the way back to 1847. Note that we move back one
      // day from the latest possible rule so that when the recursion runs it
      // won't consider the new time to be "far future" because the system clock
      // has advanced in the meantime.
      const newTimeToCheck = JSBI.subtract(afterLatestRule, DAY_NANOS);
      return GetIANATimeZonePreviousTransition(newTimeToCheck, id);
    }
    return null;
  }
  const result = bisect(
    (epochNs: JSBI) => GetIANATimeZoneOffsetNanoseconds(epochNs, id),
    leftNanos,
    rightNanos,
    leftOffsetNs,
    rightOffsetNs
  );
  return result;
}

// ts-prune-ignore-next TODO: remove this after tests are converted to TS
export function parseFromEnUsFormat(datetime: string) {
  const parts = datetime.split(/[^\w]+/);

  if (parts.length !== 7) {
    throw new RangeError(`expected 7 parts in "${datetime}`);
  }

  const month = +parts[0];
  const day = +parts[1];
  let year = +parts[2];
  const era = parts[3].toUpperCase();
  if (era === 'B' || era === 'BC') {
    year = -year + 1;
  } else if (era !== 'A' && era !== 'AD') {
    throw new RangeError(`Unknown era ${era} in "${datetime}`);
  }
  let hour = +parts[4];
  if (hour === 24) {
    // bugs.chromium.org/p/chromium/issues/detail?id=1045791
    hour = 0;
  }
  const minute = +parts[5];
  const second = +parts[6];

  if (
    !NumberIsFinite(year) ||
    !NumberIsFinite(month) ||
    !NumberIsFinite(day) ||
    !NumberIsFinite(hour) ||
    !NumberIsFinite(minute) ||
    !NumberIsFinite(second)
  ) {
    throw new RangeError(`Invalid number in "${datetime}`);
  }

  return { year, month, day, hour, minute, second };
}

// ts-prune-ignore-next TODO: remove this after tests are converted to TS
export function GetFormatterParts(timeZone: string, epochMilliseconds: number) {
  const formatter = getIntlDateTimeFormatEnUsForTimeZone(timeZone);
  // Using `format` instead of `formatToParts` for compatibility with older clients
  const datetime = formatter.format(new Date(epochMilliseconds));
  return parseFromEnUsFormat(datetime);
}

export function GetIANATimeZoneEpochValue(
  id: string,
  year: number,
  month: number,
  day: number,
  hour: number,
  minute: number,
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number
) {
  const ns = GetEpochFromISOParts(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond);
  if (ns === null) throw new RangeError('DateTime outside of supported range');
  let nsEarlier = JSBI.subtract(ns, DAY_NANOS);
  if (JSBI.lessThan(nsEarlier, NS_MIN)) nsEarlier = ns;
  let nsLater = JSBI.add(ns, DAY_NANOS);
  if (JSBI.greaterThan(nsLater, NS_MAX)) nsLater = ns;
  const earliest = GetIANATimeZoneOffsetNanoseconds(nsEarlier, id);
  const latest = GetIANATimeZoneOffsetNanoseconds(nsLater, id);
  const found = earliest === latest ? [earliest] : [earliest, latest];
  return found
    .map((offsetNanoseconds) => {
      const epochNanoseconds = JSBI.subtract(ns, JSBI.BigInt(offsetNanoseconds));
      const parts = GetIANATimeZoneDateTimeParts(epochNanoseconds, id);
      if (
        year !== parts.year ||
        month !== parts.month ||
        day !== parts.day ||
        hour !== parts.hour ||
        minute !== parts.minute ||
        second !== parts.second ||
        millisecond !== parts.millisecond ||
        microsecond !== parts.microsecond ||
        nanosecond !== parts.nanosecond
      ) {
        return undefined;
      }
      return epochNanoseconds;
    })
    .filter((x) => x !== undefined) as JSBI[];
}

export function LeapYear(year: number) {
  if (undefined === year) return false;
  const isDiv4 = year % 4 === 0;
  const isDiv100 = year % 100 === 0;
  const isDiv400 = year % 400 === 0;
  return isDiv4 && (!isDiv100 || isDiv400);
}

export function ISODaysInMonth(year: number, month: number) {
  const DoM = {
    standard: [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31],
    leapyear: [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
  };
  return DoM[LeapYear(year) ? 'leapyear' : 'standard'][month - 1];
}

export function DayOfWeek(year: number, month: number, day: number) {
  const m = month + (month < 3 ? 10 : -2);
  const Y = year - (month < 3 ? 1 : 0);

  const c = MathFloor(Y / 100);
  const y = Y - c * 100;
  const d = day;

  const pD = d;
  const pM = MathFloor(2.6 * m - 0.2);
  const pY = y + MathFloor(y / 4);
  const pC = MathFloor(c / 4) - 2 * c;

  const dow = (pD + pM + pY + pC) % 7;

  return dow + (dow <= 0 ? 7 : 0);
}

export function DayOfYear(year: number, month: number, day: number) {
  let days = day;
  for (let m = month - 1; m > 0; m--) {
    days += ISODaysInMonth(year, m);
  }
  return days;
}

export function WeekOfYear(year: number, month: number, day: number) {
  const doy = DayOfYear(year, month, day);
  const dow = DayOfWeek(year, month, day) || 7;
  const doj = DayOfWeek(year, 1, 1);

  const week = MathFloor((doy - dow + 10) / 7);

  if (week < 1) {
    if (doj === 5 || (doj === 6 && LeapYear(year - 1))) {
      return 53;
    } else {
      return 52;
    }
  }
  if (week === 53) {
    if ((LeapYear(year) ? 366 : 365) - doy < 4 - dow) {
      return 1;
    }
  }

  return week;
}

export function DurationSign(
  y: number,
  mon: number,
  w: number,
  d: number,
  h: number,
  min: number,
  s: number,
  ms: number,
  µs: number,
  ns: number
) {
  for (const prop of [y, mon, w, d, h, min, s, ms, µs, ns]) {
    if (prop !== 0) return prop < 0 ? -1 : 1;
  }
  return 0;
}

function BalanceISOYearMonth(yearParam: number, monthParam: number) {
  let year = yearParam;
  let month = monthParam;
  if (!NumberIsFinite(year) || !NumberIsFinite(month)) throw new RangeError('infinity is out of range');
  month -= 1;
  year += MathFloor(month / 12);
  month %= 12;
  if (month < 0) month += 12;
  month += 1;
  return { year, month };
}

function BalanceISODate(yearParam: number, monthParam: number, dayParam: number) {
  let year = yearParam;
  let month = monthParam;
  let day = dayParam;
  if (!NumberIsFinite(day)) throw new RangeError('infinity is out of range');
  ({ year, month } = BalanceISOYearMonth(year, month));
  let daysInYear = 0;
  let testYear = month > 2 ? year : year - 1;
  while (((daysInYear = LeapYear(testYear) ? 366 : 365), day < -daysInYear)) {
    year -= 1;
    testYear -= 1;
    day += daysInYear;
  }
  testYear += 1;
  while (((daysInYear = LeapYear(testYear) ? 366 : 365), day > daysInYear)) {
    year += 1;
    testYear += 1;
    day -= daysInYear;
  }

  while (day < 1) {
    ({ year, month } = BalanceISOYearMonth(year, month - 1));
    day += ISODaysInMonth(year, month);
  }
  while (day > ISODaysInMonth(year, month)) {
    day -= ISODaysInMonth(year, month);
    ({ year, month } = BalanceISOYearMonth(year, month + 1));
  }

  return { year, month, day };
}

function BalanceISODateTime(
  yearParam: number,
  monthParam: number,
  dayParam: number,
  hourParam: number,
  minuteParam: number,
  secondParam: number,
  millisecondParam: number,
  microsecondParam: number,
  nanosecondParam: number
) {
  const { deltaDays, hour, minute, second, millisecond, microsecond, nanosecond } = BalanceTime(
    hourParam,
    minuteParam,
    secondParam,
    millisecondParam,
    microsecondParam,
    nanosecondParam
  );
  const { year, month, day } = BalanceISODate(yearParam, monthParam, dayParam + deltaDays);
  return { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond };
}

function BalanceTime(
  hourParam: number,
  minuteParam: number,
  secondParam: number,
  millisecondParam: number,
  microsecondParam: number,
  nanosecondParam: number
) {
  let hour = hourParam;
  let minute = minuteParam;
  let second = secondParam;
  let millisecond = millisecondParam;
  let microsecond = microsecondParam;
  let nanosecond = nanosecondParam;
  if (
    !NumberIsFinite(hour) ||
    !NumberIsFinite(minute) ||
    !NumberIsFinite(second) ||
    !NumberIsFinite(millisecond) ||
    !NumberIsFinite(microsecond) ||
    !NumberIsFinite(nanosecond)
  ) {
    throw new RangeError('infinity is out of range');
  }

  microsecond += MathFloor(nanosecond / 1000);
  nanosecond = NonNegativeModulo(nanosecond, 1000);

  millisecond += MathFloor(microsecond / 1000);
  microsecond = NonNegativeModulo(microsecond, 1000);

  second += MathFloor(millisecond / 1000);
  millisecond = NonNegativeModulo(millisecond, 1000);

  minute += MathFloor(second / 60);
  second = NonNegativeModulo(second, 60);

  hour += MathFloor(minute / 60);
  minute = NonNegativeModulo(minute, 60);

  const deltaDays = MathFloor(hour / 24);
  hour = NonNegativeModulo(hour, 24);

  return { deltaDays, hour, minute, second, millisecond, microsecond, nanosecond };
}

export function TotalDurationNanoseconds(
  daysParam: number,
  hoursParam: number,
  minutesParam: number,
  secondsParam: number,
  millisecondsParam: number,
  microsecondsParam: number,
  nanosecondsParam: number,
  offsetShift: number
) {
  const days: JSBI = JSBI.BigInt(daysParam);
  let nanoseconds: JSBI = JSBI.BigInt(nanosecondsParam);
  if (daysParam !== 0) nanoseconds = JSBI.subtract(JSBI.BigInt(nanosecondsParam), JSBI.BigInt(offsetShift));
  const hours = JSBI.add(JSBI.BigInt(hoursParam), JSBI.multiply(days, JSBI.BigInt(24)));
  const minutes = JSBI.add(JSBI.BigInt(minutesParam), JSBI.multiply(hours, SIXTY));
  const seconds = JSBI.add(JSBI.BigInt(secondsParam), JSBI.multiply(minutes, SIXTY));
  const milliseconds = JSBI.add(JSBI.BigInt(millisecondsParam), JSBI.multiply(seconds, THOUSAND));
  const microseconds = JSBI.add(JSBI.BigInt(microsecondsParam), JSBI.multiply(milliseconds, THOUSAND));
  return JSBI.add(JSBI.BigInt(nanoseconds), JSBI.multiply(microseconds, THOUSAND));
}

function NanosecondsToDays(nanosecondsParam: JSBI, relativeTo: ReturnType<typeof ToRelativeTemporalObject>) {
  const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
  const sign = MathSign(JSBI.toNumber(nanosecondsParam));
  let nanoseconds = JSBI.BigInt(nanosecondsParam);
  let dayLengthNs = 86400e9;
  if (sign === 0) return { days: 0, nanoseconds: ZERO, dayLengthNs };
  if (!IsTemporalZonedDateTime(relativeTo)) {
    let days: JSBI;
    ({ quotient: days, remainder: nanoseconds } = divmod(nanoseconds, JSBI.BigInt(dayLengthNs)));
    return { days: JSBI.toNumber(days), nanoseconds, dayLengthNs };
  }

  const startNs = GetSlot(relativeTo, EPOCHNANOSECONDS);
  const start = GetSlot(relativeTo, INSTANT);
  const endNs = JSBI.add(startNs, nanoseconds);
  const end = new TemporalInstant(endNs);
  const timeZone = GetSlot(relativeTo, TIME_ZONE);
  const calendar = GetSlot(relativeTo, CALENDAR);

  // Find the difference in days only.
  const dtStart = BuiltinTimeZoneGetPlainDateTimeFor(timeZone, start, calendar);
  const dtEnd = BuiltinTimeZoneGetPlainDateTimeFor(timeZone, end, calendar);
  let { days } = DifferenceISODateTime(
    GetSlot(dtStart, ISO_YEAR),
    GetSlot(dtStart, ISO_MONTH),
    GetSlot(dtStart, ISO_DAY),
    GetSlot(dtStart, ISO_HOUR),
    GetSlot(dtStart, ISO_MINUTE),
    GetSlot(dtStart, ISO_SECOND),
    GetSlot(dtStart, ISO_MILLISECOND),
    GetSlot(dtStart, ISO_MICROSECOND),
    GetSlot(dtStart, ISO_NANOSECOND),
    GetSlot(dtEnd, ISO_YEAR),
    GetSlot(dtEnd, ISO_MONTH),
    GetSlot(dtEnd, ISO_DAY),
    GetSlot(dtEnd, ISO_HOUR),
    GetSlot(dtEnd, ISO_MINUTE),
    GetSlot(dtEnd, ISO_SECOND),
    GetSlot(dtEnd, ISO_MILLISECOND),
    GetSlot(dtEnd, ISO_MICROSECOND),
    GetSlot(dtEnd, ISO_NANOSECOND),
    calendar,
    'day',
    ObjectCreate(null)
  );
  let intermediateNs = AddZonedDateTime(start, timeZone, calendar, 0, 0, 0, days, 0, 0, 0, 0, 0, 0);
  // may disambiguate

  // If clock time after addition was in the middle of a skipped period, the
  // endpoint was disambiguated to a later clock time. So it's possible that
  // the resulting disambiguated result is later than endNs. If so, then back
  // up one day and try again. Repeat if necessary (some transitions are
  // > 24 hours) until either there's zero days left or the date duration is
  // back inside the period where it belongs. Note that this case only can
  // happen for positive durations because the only direction that
  // `disambiguation: 'compatible'` can change clock time is forwards.
  if (sign === 1) {
    while (days > 0 && JSBI.greaterThan(intermediateNs, endNs)) {
      --days;
      intermediateNs = AddZonedDateTime(start, timeZone, calendar, 0, 0, 0, days, 0, 0, 0, 0, 0, 0);
      // may do disambiguation
    }
  }
  nanoseconds = JSBI.subtract(endNs, intermediateNs);

  let isOverflow = false;
  let relativeInstant = new TemporalInstant(intermediateNs);
  do {
    // calculate length of the next day (day that contains the time remainder)
    const oneDayFartherNs = AddZonedDateTime(relativeInstant, timeZone, calendar, 0, 0, 0, sign, 0, 0, 0, 0, 0, 0);
    const relativeNs = GetSlot(relativeInstant, EPOCHNANOSECONDS);
    dayLengthNs = JSBI.toNumber(JSBI.subtract(oneDayFartherNs, relativeNs));
    isOverflow = JSBI.greaterThan(
      JSBI.multiply(JSBI.subtract(nanoseconds, JSBI.BigInt(dayLengthNs)), JSBI.BigInt(sign)),
      ZERO
    );
    if (isOverflow) {
      nanoseconds = JSBI.subtract(nanoseconds, JSBI.BigInt(dayLengthNs));
      relativeInstant = new TemporalInstant(oneDayFartherNs);
      days += sign;
    }
  } while (isOverflow);
  return { days, nanoseconds, dayLengthNs: MathAbs(dayLengthNs) };
}

export function BalanceDuration(
  daysParam: number,
  hoursParam: number,
  minutesParam: number,
  secondsParam: number,
  millisecondsParam: number,
  microsecondsParam: number,
  nanosecondsParam: number,
  largestUnit: Temporal.DateTimeUnit,
  relativeTo: ReturnType<typeof ToRelativeTemporalObject> = undefined
) {
  let days = daysParam;
  let nanosecondsBigInt: JSBI,
    microsecondsBigInt: JSBI,
    millisecondsBigInt: JSBI,
    secondsBigInt: JSBI,
    minutesBigInt: JSBI,
    hoursBigInt: JSBI;
  if (IsTemporalZonedDateTime(relativeTo)) {
    const endNs = AddZonedDateTime(
      GetSlot(relativeTo, INSTANT),
      GetSlot(relativeTo, TIME_ZONE),
      GetSlot(relativeTo, CALENDAR),
      0,
      0,
      0,
      days,
      hoursParam,
      minutesParam,
      secondsParam,
      millisecondsParam,
      microsecondsParam,
      nanosecondsParam
    );
    const startNs = GetSlot(relativeTo, EPOCHNANOSECONDS);
    nanosecondsBigInt = JSBI.subtract(endNs, startNs);
  } else {
    nanosecondsBigInt = TotalDurationNanoseconds(
      days,
      hoursParam,
      minutesParam,
      secondsParam,
      millisecondsParam,
      microsecondsParam,
      nanosecondsParam,
      0
    );
  }
  if (largestUnit === 'year' || largestUnit === 'month' || largestUnit === 'week' || largestUnit === 'day') {
    ({ days, nanoseconds: nanosecondsBigInt } = NanosecondsToDays(nanosecondsBigInt, relativeTo));
  } else {
    days = 0;
  }

  const sign = JSBI.lessThan(nanosecondsBigInt, ZERO) ? -1 : 1;
  nanosecondsBigInt = abs(nanosecondsBigInt);
  microsecondsBigInt = millisecondsBigInt = secondsBigInt = minutesBigInt = hoursBigInt = ZERO;

  switch (largestUnit) {
    case 'year':
    case 'month':
    case 'week':
    case 'day':
    case 'hour':
      ({ quotient: microsecondsBigInt, remainder: nanosecondsBigInt } = divmod(nanosecondsBigInt, THOUSAND));
      ({ quotient: millisecondsBigInt, remainder: microsecondsBigInt } = divmod(microsecondsBigInt, THOUSAND));
      ({ quotient: secondsBigInt, remainder: millisecondsBigInt } = divmod(millisecondsBigInt, THOUSAND));
      ({ quotient: minutesBigInt, remainder: secondsBigInt } = divmod(secondsBigInt, SIXTY));
      ({ quotient: hoursBigInt, remainder: minutesBigInt } = divmod(minutesBigInt, SIXTY));
      break;
    case 'minute':
      ({ quotient: microsecondsBigInt, remainder: nanosecondsBigInt } = divmod(nanosecondsBigInt, THOUSAND));
      ({ quotient: millisecondsBigInt, remainder: microsecondsBigInt } = divmod(microsecondsBigInt, THOUSAND));
      ({ quotient: secondsBigInt, remainder: millisecondsBigInt } = divmod(millisecondsBigInt, THOUSAND));
      ({ quotient: minutesBigInt, remainder: secondsBigInt } = divmod(secondsBigInt, SIXTY));
      break;
    case 'second':
      ({ quotient: microsecondsBigInt, remainder: nanosecondsBigInt } = divmod(nanosecondsBigInt, THOUSAND));
      ({ quotient: millisecondsBigInt, remainder: microsecondsBigInt } = divmod(microsecondsBigInt, THOUSAND));
      ({ quotient: secondsBigInt, remainder: millisecondsBigInt } = divmod(millisecondsBigInt, THOUSAND));
      break;
    case 'millisecond':
      ({ quotient: microsecondsBigInt, remainder: nanosecondsBigInt } = divmod(nanosecondsBigInt, THOUSAND));
      ({ quotient: millisecondsBigInt, remainder: microsecondsBigInt } = divmod(microsecondsBigInt, THOUSAND));
      break;
    case 'microsecond':
      ({ quotient: microsecondsBigInt, remainder: nanosecondsBigInt } = divmod(nanosecondsBigInt, THOUSAND));
      break;
    case 'nanosecond':
      break;
    default:
      throw new Error('assert not reached');
  }

  const hours = JSBI.toNumber(hoursBigInt) * sign;
  const minutes = JSBI.toNumber(minutesBigInt) * sign;
  const seconds = JSBI.toNumber(secondsBigInt) * sign;
  const milliseconds = JSBI.toNumber(millisecondsBigInt) * sign;
  const microseconds = JSBI.toNumber(microsecondsBigInt) * sign;
  const nanoseconds = JSBI.toNumber(nanosecondsBigInt) * sign;

  return { days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

export function UnbalanceDurationRelative(
  yearsParam: number,
  monthsParam: number,
  weeksParam: number,
  daysParam: number,
  largestUnit: Temporal.DateTimeUnit,
  relativeToParam: ReturnType<typeof ToRelativeTemporalObject>
) {
  let years = yearsParam;
  let months = monthsParam;
  let weeks = weeksParam;
  let days = daysParam;
  const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
  const sign = DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);

  let calendar;
  let relativeTo: Temporal.PlainDate | undefined;
  if (relativeToParam) {
    relativeTo = ToTemporalDate(relativeToParam);
    calendar = GetSlot(relativeTo, CALENDAR);
  }

  const oneYear = new TemporalDuration(sign);
  const oneMonth = new TemporalDuration(0, sign);
  const oneWeek = new TemporalDuration(0, 0, sign);

  switch (largestUnit) {
    case 'year':
      // no-op
      break;
    case 'month':
      {
        if (!calendar) throw new RangeError('a starting point is required for months balancing');
        // balance years down to months
        const dateAdd = calendar.dateAdd;
        const dateUntil = calendar.dateUntil;
        let relativeToDateOnly: Temporal.PlainDateLike = relativeTo as Temporal.PlainDateLike;
        while (MathAbs(years) > 0) {
          const newRelativeTo = CalendarDateAdd(calendar, relativeToDateOnly, oneYear, undefined, dateAdd);
          const untilOptions = ObjectCreate(null);
          untilOptions.largestUnit = 'month';
          const untilResult = CalendarDateUntil(calendar, relativeToDateOnly, newRelativeTo, untilOptions, dateUntil);
          const oneYearMonths = GetSlot(untilResult, MONTHS);
          relativeToDateOnly = newRelativeTo;
          months += oneYearMonths;
          years -= sign;
        }
      }
      break;
    case 'week':
      if (!calendar) throw new RangeError('a starting point is required for weeks balancing');
      // balance years down to days
      while (MathAbs(years) > 0) {
        let oneYearDays;
        ({ relativeTo, days: oneYearDays } = MoveRelativeDate(calendar, relativeTo as Temporal.PlainDate, oneYear));
        days += oneYearDays;
        years -= sign;
      }

      // balance months down to days
      while (MathAbs(months) > 0) {
        let oneMonthDays;
        ({ relativeTo, days: oneMonthDays } = MoveRelativeDate(calendar, relativeTo as Temporal.PlainDate, oneMonth));
        days += oneMonthDays;
        months -= sign;
      }
      break;
    default:
      // balance years down to days
      while (MathAbs(years) > 0) {
        if (!calendar) throw new RangeError('a starting point is required for balancing calendar units');
        let oneYearDays;
        ({ relativeTo, days: oneYearDays } = MoveRelativeDate(calendar, relativeTo as Temporal.PlainDate, oneYear));
        days += oneYearDays;
        years -= sign;
      }

      // balance months down to days
      while (MathAbs(months) > 0) {
        if (!calendar) throw new RangeError('a starting point is required for balancing calendar units');
        let oneMonthDays;
        ({ relativeTo, days: oneMonthDays } = MoveRelativeDate(calendar, relativeTo as Temporal.PlainDate, oneMonth));
        days += oneMonthDays;
        months -= sign;
      }

      // balance weeks down to days
      while (MathAbs(weeks) > 0) {
        if (!calendar) throw new RangeError('a starting point is required for balancing calendar units');
        let oneWeekDays;
        ({ relativeTo, days: oneWeekDays } = MoveRelativeDate(calendar, relativeTo as Temporal.PlainDate, oneWeek));
        days += oneWeekDays;
        weeks -= sign;
      }
      break;
  }

  return { years, months, weeks, days };
}

export function BalanceDurationRelative(
  yearsParam: number,
  monthsParam: number,
  weeksParam: number,
  daysParam: number,
  largestUnit: Temporal.DateTimeUnit,
  relativeToParam: ReturnType<typeof ToRelativeTemporalObject>
) {
  let years = yearsParam;
  let months = monthsParam;
  let weeks = weeksParam;
  let days = daysParam;
  const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
  const sign = DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
  if (sign === 0) return { years, months, weeks, days };

  let calendar;
  let relativeTo: Temporal.PlainDate | undefined;
  if (relativeToParam) {
    relativeTo = ToTemporalDate(relativeToParam);
    calendar = GetSlot(relativeTo, CALENDAR);
  }

  const oneYear = new TemporalDuration(sign);
  const oneMonth = new TemporalDuration(0, sign);
  const oneWeek = new TemporalDuration(0, 0, sign);

  switch (largestUnit) {
    case 'year': {
      if (!calendar) throw new RangeError('a starting point is required for years balancing');
      // balance days up to years
      let newRelativeTo, oneYearDays;
      ({ relativeTo: newRelativeTo, days: oneYearDays } = MoveRelativeDate(
        calendar,
        relativeTo as Temporal.PlainDate,
        oneYear
      ));
      while (MathAbs(days) >= MathAbs(oneYearDays)) {
        days -= oneYearDays;
        years += sign;
        relativeTo = newRelativeTo;
        ({ relativeTo: newRelativeTo, days: oneYearDays } = MoveRelativeDate(calendar, relativeTo, oneYear));
      }

      // balance days up to months
      let oneMonthDays;
      ({ relativeTo: newRelativeTo, days: oneMonthDays } = MoveRelativeDate(
        calendar,
        relativeTo as Temporal.PlainDate,
        oneMonth
      ));
      while (MathAbs(days) >= MathAbs(oneMonthDays)) {
        days -= oneMonthDays;
        months += sign;
        relativeTo = newRelativeTo;
        ({ relativeTo: newRelativeTo, days: oneMonthDays } = MoveRelativeDate(calendar, relativeTo, oneMonth));
      }

      // balance months up to years
      const dateAdd = calendar.dateAdd;
      newRelativeTo = CalendarDateAdd(calendar, relativeTo as Temporal.PlainDate, oneYear, undefined, dateAdd);
      const dateUntil = calendar.dateUntil;
      const untilOptions = ObjectCreate(null);
      untilOptions.largestUnit = 'month';
      let untilResult = CalendarDateUntil(
        calendar,
        relativeTo as Temporal.PlainDate,
        newRelativeTo,
        untilOptions,
        dateUntil
      );
      let oneYearMonths = GetSlot(untilResult, MONTHS);
      while (MathAbs(months) >= MathAbs(oneYearMonths)) {
        months -= oneYearMonths;
        years += sign;
        relativeTo = newRelativeTo;
        newRelativeTo = CalendarDateAdd(calendar, relativeTo, oneYear, undefined, dateAdd);
        const untilOptions = ObjectCreate(null);
        untilOptions.largestUnit = 'month';
        untilResult = CalendarDateUntil(calendar, relativeTo, newRelativeTo, untilOptions, dateUntil);
        oneYearMonths = GetSlot(untilResult, MONTHS);
      }
      break;
    }
    case 'month': {
      if (!calendar) throw new RangeError('a starting point is required for months balancing');
      // balance days up to months
      let newRelativeTo, oneMonthDays;
      ({ relativeTo: newRelativeTo, days: oneMonthDays } = MoveRelativeDate(
        calendar,
        relativeTo as Temporal.PlainDate,
        oneMonth
      ));
      while (MathAbs(days) >= MathAbs(oneMonthDays)) {
        days -= oneMonthDays;
        months += sign;
        relativeTo = newRelativeTo;
        ({ relativeTo: newRelativeTo, days: oneMonthDays } = MoveRelativeDate(calendar, relativeTo, oneMonth));
      }
      break;
    }
    case 'week': {
      if (!calendar) throw new RangeError('a starting point is required for weeks balancing');
      // balance days up to weeks
      let newRelativeTo, oneWeekDays;
      ({ relativeTo: newRelativeTo, days: oneWeekDays } = MoveRelativeDate(
        calendar,
        relativeTo as Temporal.PlainDate,
        oneWeek
      ));
      while (MathAbs(days) >= MathAbs(oneWeekDays)) {
        days -= oneWeekDays;
        weeks += sign;
        relativeTo = newRelativeTo;
        ({ relativeTo: newRelativeTo, days: oneWeekDays } = MoveRelativeDate(calendar, relativeTo, oneWeek));
      }
      break;
    }
    default:
      // no-op
      break;
  }

  return { years, months, weeks, days };
}

export function CalculateOffsetShift(
  relativeTo: ReturnType<typeof ToRelativeTemporalObject>,
  y: number,
  mon: number,
  w: number,
  d: number
) {
  if (IsTemporalZonedDateTime(relativeTo)) {
    const instant = GetSlot(relativeTo, INSTANT);
    const timeZone = GetSlot(relativeTo, TIME_ZONE);
    const calendar = GetSlot(relativeTo, CALENDAR);
    const offsetBefore = GetOffsetNanosecondsFor(timeZone, instant);
    const after = AddZonedDateTime(instant, timeZone, calendar, y, mon, w, d, 0, 0, 0, 0, 0, 0);
    const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
    const instantAfter = new TemporalInstant(after);
    const offsetAfter = GetOffsetNanosecondsFor(timeZone, instantAfter);
    return offsetAfter - offsetBefore;
  }
  return 0;
}

export function CreateNegatedTemporalDuration(duration: Temporal.Duration) {
  const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
  return new TemporalDuration(
    -GetSlot(duration, YEARS),
    -GetSlot(duration, MONTHS),
    -GetSlot(duration, WEEKS),
    -GetSlot(duration, DAYS),
    -GetSlot(duration, HOURS),
    -GetSlot(duration, MINUTES),
    -GetSlot(duration, SECONDS),
    -GetSlot(duration, MILLISECONDS),
    -GetSlot(duration, MICROSECONDS),
    -GetSlot(duration, NANOSECONDS)
  );
}

export function ConstrainToRange(value: number | undefined, min: number, max: number) {
  // Math.Max accepts undefined values and returns NaN. Undefined values are
  // used for optional params in the method below.
  return MathMin(max, MathMax(min, value as number));
}
function ConstrainISODate(year: number, monthParam: number, dayParam?: number) {
  const month = ConstrainToRange(monthParam, 1, 12);
  const day = ConstrainToRange(dayParam, 1, ISODaysInMonth(year, month));
  return { year, month, day };
}

function ConstrainTime(
  hourParam: number,
  minuteParam: number,
  secondParam: number,
  millisecondParam: number,
  microsecondParam: number,
  nanosecondParam: number
) {
  const hour = ConstrainToRange(hourParam, 0, 23);
  const minute = ConstrainToRange(minuteParam, 0, 59);
  const second = ConstrainToRange(secondParam, 0, 59);
  const millisecond = ConstrainToRange(millisecondParam, 0, 999);
  const microsecond = ConstrainToRange(microsecondParam, 0, 999);
  const nanosecond = ConstrainToRange(nanosecondParam, 0, 999);
  return { hour, minute, second, millisecond, microsecond, nanosecond };
}

export function RejectToRange(value: number, min: number, max: number) {
  if (value < min || value > max) throw new RangeError(`value out of range: ${min} <= ${value} <= ${max}`);
}

function RejectISODate(year: number, month: number, day: number) {
  RejectToRange(month, 1, 12);
  RejectToRange(day, 1, ISODaysInMonth(year, month));
}

function RejectDateRange(year: number, month: number, day: number) {
  // Noon avoids trouble at edges of DateTime range (excludes midnight)
  RejectDateTimeRange(year, month, day, 12, 0, 0, 0, 0, 0);
}

export function RejectTime(
  hour: number,
  minute: number,
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number
) {
  RejectToRange(hour, 0, 23);
  RejectToRange(minute, 0, 59);
  RejectToRange(second, 0, 59);
  RejectToRange(millisecond, 0, 999);
  RejectToRange(microsecond, 0, 999);
  RejectToRange(nanosecond, 0, 999);
}

function RejectDateTime(
  year: number,
  month: number,
  day: number,
  hour: number,
  minute: number,
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number
) {
  RejectISODate(year, month, day);
  RejectTime(hour, minute, second, millisecond, microsecond, nanosecond);
}

function RejectDateTimeRange(
  year: number,
  month: number,
  day: number,
  hour: number,
  minute: number,
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number
) {
  RejectToRange(year, YEAR_MIN, YEAR_MAX);
  // Reject any DateTime 24 hours or more outside the Instant range
  if (
    (year === YEAR_MIN &&
      null ==
        GetEpochFromISOParts(year, month, day + 1, hour, minute, second, millisecond, microsecond, nanosecond - 1)) ||
    (year === YEAR_MAX &&
      null ==
        GetEpochFromISOParts(year, month, day - 1, hour, minute, second, millisecond, microsecond, nanosecond + 1))
  ) {
    throw new RangeError('DateTime outside of supported range');
  }
}

export function ValidateEpochNanoseconds(epochNanoseconds: JSBI) {
  if (JSBI.lessThan(epochNanoseconds, NS_MIN) || JSBI.greaterThan(epochNanoseconds, NS_MAX)) {
    throw new RangeError('Instant outside of supported range');
  }
}

function RejectYearMonthRange(year: number, month: number) {
  RejectToRange(year, YEAR_MIN, YEAR_MAX);
  if (year === YEAR_MIN) {
    RejectToRange(month, 4, 12);
  } else if (year === YEAR_MAX) {
    RejectToRange(month, 1, 9);
  }
}

export function RejectDuration(
  y: number,
  mon: number,
  w: number,
  d: number,
  h: number,
  min: number,
  s: number,
  ms: number,
  µs: number,
  ns: number
) {
  const sign = DurationSign(y, mon, w, d, h, min, s, ms, µs, ns);
  for (const prop of [y, mon, w, d, h, min, s, ms, µs, ns]) {
    if (!NumberIsFinite(prop)) throw new RangeError('infinite values not allowed as duration fields');
    const propSign = MathSign(prop);
    if (propSign !== 0 && propSign !== sign) throw new RangeError('mixed-sign values not allowed as duration fields');
  }
}

export function DifferenceISODate<Allowed extends Temporal.DateTimeUnit>(
  y1: number,
  m1: number,
  d1: number,
  y2: number,
  m2: number,
  d2: number,
  largestUnit: Allowed
) {
  switch (largestUnit) {
    case 'year':
    case 'month': {
      const sign = -CompareISODate(y1, m1, d1, y2, m2, d2);
      if (sign === 0) return { years: 0, months: 0, weeks: 0, days: 0 };

      const start = { year: y1, month: m1, day: d1 };
      const end = { year: y2, month: m2, day: d2 };

      let years = end.year - start.year;
      let mid = AddISODate(y1, m1, d1, years, 0, 0, 0, 'constrain');
      let midSign = -CompareISODate(mid.year, mid.month, mid.day, y2, m2, d2);
      if (midSign === 0) {
        return largestUnit === 'year'
          ? { years, months: 0, weeks: 0, days: 0 }
          : { years: 0, months: years * 12, weeks: 0, days: 0 };
      }
      let months = end.month - start.month;
      if (midSign !== sign) {
        years -= sign;
        months += sign * 12;
      }
      mid = AddISODate(y1, m1, d1, years, months, 0, 0, 'constrain');
      midSign = -CompareISODate(mid.year, mid.month, mid.day, y2, m2, d2);
      if (midSign === 0) {
        return largestUnit === 'year'
          ? { years, months, weeks: 0, days: 0 }
          : { years: 0, months: months + years * 12, weeks: 0, days: 0 };
      }
      if (midSign !== sign) {
        // The end date is later in the month than mid date (or earlier for
        // negative durations). Back up one month.
        months -= sign;
        if (months === -sign) {
          years -= sign;
          months = 11 * sign;
        }
        mid = AddISODate(y1, m1, d1, years, months, 0, 0, 'constrain');
      }

      let days = 0;
      // If we get here, months and years are correct (no overflow), and `mid`
      // is within the range from `start` to `end`. To count the days between
      // `mid` and `end`, there are 3 cases:
      // 1) same month: use simple subtraction
      // 2) end is previous month from intermediate (negative duration)
      // 3) end is next month from intermediate (positive duration)
      if (mid.month === end.month) {
        // 1) same month: use simple subtraction
        days = end.day - mid.day;
      } else if (sign < 0) {
        // 2) end is previous month from intermediate (negative duration)
        // Example: intermediate: Feb 1, end: Jan 30, DaysInMonth = 31, days = -2
        days = -mid.day - (ISODaysInMonth(end.year, end.month) - end.day);
      } else {
        // 3) end is next month from intermediate (positive duration)
        // Example: intermediate: Jan 29, end: Feb 1, DaysInMonth = 31, days = 3
        days = end.day + (ISODaysInMonth(mid.year, mid.month) - mid.day);
      }

      if (largestUnit === 'month') {
        months += years * 12;
        years = 0;
      }
      return { years, months, weeks: 0, days };
    }
    case 'week':
    case 'day': {
      let larger, smaller, sign;
      if (CompareISODate(y1, m1, d1, y2, m2, d2) < 0) {
        smaller = { year: y1, month: m1, day: d1 };
        larger = { year: y2, month: m2, day: d2 };
        sign = 1;
      } else {
        smaller = { year: y2, month: m2, day: d2 };
        larger = { year: y1, month: m1, day: d1 };
        sign = -1;
      }
      let days = DayOfYear(larger.year, larger.month, larger.day) - DayOfYear(smaller.year, smaller.month, smaller.day);
      for (let year = smaller.year; year < larger.year; ++year) {
        days += LeapYear(year) ? 366 : 365;
      }
      let weeks = 0;
      if (largestUnit === 'week') {
        weeks = MathFloor(days / 7);
        days %= 7;
      }
      weeks *= sign;
      days *= sign;
      return { years: 0, months: 0, weeks, days };
    }
    default:
      throw new Error('assert not reached');
  }
}

function DifferenceTime(
  h1: number,
  min1: number,
  s1: number,
  ms1: number,
  µs1: number,
  ns1: number,
  h2: number,
  min2: number,
  s2: number,
  ms2: number,
  µs2: number,
  ns2: number
) {
  let hours = h2 - h1;
  let minutes = min2 - min1;
  let seconds = s2 - s1;
  let milliseconds = ms2 - ms1;
  let microseconds = µs2 - µs1;
  let nanoseconds = ns2 - ns1;

  const sign = DurationSign(0, 0, 0, 0, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
  hours *= sign;
  minutes *= sign;
  seconds *= sign;
  milliseconds *= sign;
  microseconds *= sign;
  nanoseconds *= sign;

  let deltaDays = 0;
  ({
    deltaDays,
    hour: hours,
    minute: minutes,
    second: seconds,
    millisecond: milliseconds,
    microsecond: microseconds,
    nanosecond: nanoseconds
  } = BalanceTime(hours, minutes, seconds, milliseconds, microseconds, nanoseconds));

  if (deltaDays != 0) throw new Error('assertion failure in DifferenceTime: _bt_.[[Days]] should be 0');
  hours *= sign;
  minutes *= sign;
  seconds *= sign;
  milliseconds *= sign;
  microseconds *= sign;
  nanoseconds *= sign;

  return { hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

function DifferenceInstant(
  ns1: JSBI,
  ns2: JSBI,
  increment: number,
  unit: keyof typeof nsPerTimeUnit,
  roundingMode: Temporal.RoundingMode
) {
  const diff = JSBI.subtract(ns2, ns1);

  const remainder = JSBI.remainder(diff, JSBI.BigInt(86400e9));
  const wholeDays = JSBI.subtract(diff, remainder);
  const roundedRemainder = RoundNumberToIncrement(remainder, nsPerTimeUnit[unit] * increment, roundingMode);
  const roundedDiff = JSBI.add(wholeDays, roundedRemainder);

  const nanoseconds = JSBI.toNumber(JSBI.remainder(roundedDiff, THOUSAND));
  const microseconds = JSBI.toNumber(JSBI.remainder(JSBI.divide(roundedDiff, THOUSAND), THOUSAND));
  const milliseconds = JSBI.toNumber(JSBI.remainder(JSBI.divide(roundedDiff, MILLION), THOUSAND));
  const seconds = JSBI.toNumber(JSBI.divide(roundedDiff, BILLION));
  return { seconds, milliseconds, microseconds, nanoseconds };
}

function DifferenceISODateTime(
  y1Param: number,
  mon1Param: number,
  d1Param: number,
  h1: number,
  min1: number,
  s1: number,
  ms1: number,
  µs1: number,
  ns1: number,
  y2: number,
  mon2: number,
  d2: number,
  h2: number,
  min2: number,
  s2: number,
  ms2: number,
  µs2: number,
  ns2: number,
  calendar: Temporal.CalendarProtocol,
  largestUnit: Temporal.DateTimeUnit,
  options: Temporal.DifferenceOptions<Temporal.DateTimeUnit>
) {
  let y1 = y1Param;
  let mon1 = mon1Param;
  let d1 = d1Param;

  let { hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = DifferenceTime(
    h1,
    min1,
    s1,
    ms1,
    µs1,
    ns1,
    h2,
    min2,
    s2,
    ms2,
    µs2,
    ns2
  );

  const timeSign = DurationSign(0, 0, 0, 0, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
  const dateSign = CompareISODate(y2, mon2, d2, y1, mon1, d1);
  if (dateSign === -timeSign) {
    ({ year: y1, month: mon1, day: d1 } = BalanceISODate(y1, mon1, d1 - timeSign));
    ({ hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
      -timeSign,
      hours,
      minutes,
      seconds,
      milliseconds,
      microseconds,
      nanoseconds,
      largestUnit
    ));
  }

  const date1 = CreateTemporalDate(y1, mon1, d1, calendar);
  const date2 = CreateTemporalDate(y2, mon2, d2, calendar);
  const dateLargestUnit = LargerOfTwoTemporalUnits('day', largestUnit);
  const untilOptions = MergeLargestUnitOption(options, dateLargestUnit);
  // TODO untilOptions doesn't want to compile as it seems that smallestUnit is not clamped?
  // Type 'SmallestUnit<DateTimeUnit> | undefined' is not assignable to type
  //      'SmallestUnit<"year" | "month" | "day" | "week"> | undefined'.
  // Type '"hour"' is not assignable to type
  //      'SmallestUnit<"year" | "month" | "day" | "week"> | undefined'.ts(2345)
  let { years, months, weeks, days } = CalendarDateUntil(calendar, date1, date2, untilOptions as any);
  // Signs of date part and time part may not agree; balance them together
  ({ days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
    days,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds,
    largestUnit
  ));
  return { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

function DifferenceZonedDateTime(
  ns1: JSBI,
  ns2: JSBI,
  timeZone: Temporal.TimeZoneProtocol,
  calendar: Temporal.CalendarProtocol,
  largestUnit: Temporal.DateTimeUnit,
  options: Temporal.DifferenceOptions<Temporal.DateTimeUnit>
) {
  const nsDiff = JSBI.subtract(ns2, ns1);
  if (JSBI.equal(nsDiff, ZERO)) {
    return {
      years: 0,
      months: 0,
      weeks: 0,
      days: 0,
      hours: 0,
      minutes: 0,
      seconds: 0,
      milliseconds: 0,
      microseconds: 0,
      nanoseconds: 0
    };
  }

  // Find the difference in dates only.
  const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
  const start = new TemporalInstant(ns1);
  const end = new TemporalInstant(ns2);
  const dtStart = BuiltinTimeZoneGetPlainDateTimeFor(timeZone, start, calendar);
  const dtEnd = BuiltinTimeZoneGetPlainDateTimeFor(timeZone, end, calendar);
  let { years, months, weeks, days } = DifferenceISODateTime(
    GetSlot(dtStart, ISO_YEAR),
    GetSlot(dtStart, ISO_MONTH),
    GetSlot(dtStart, ISO_DAY),
    GetSlot(dtStart, ISO_HOUR),
    GetSlot(dtStart, ISO_MINUTE),
    GetSlot(dtStart, ISO_SECOND),
    GetSlot(dtStart, ISO_MILLISECOND),
    GetSlot(dtStart, ISO_MICROSECOND),
    GetSlot(dtStart, ISO_NANOSECOND),
    GetSlot(dtEnd, ISO_YEAR),
    GetSlot(dtEnd, ISO_MONTH),
    GetSlot(dtEnd, ISO_DAY),
    GetSlot(dtEnd, ISO_HOUR),
    GetSlot(dtEnd, ISO_MINUTE),
    GetSlot(dtEnd, ISO_SECOND),
    GetSlot(dtEnd, ISO_MILLISECOND),
    GetSlot(dtEnd, ISO_MICROSECOND),
    GetSlot(dtEnd, ISO_NANOSECOND),
    calendar,
    largestUnit,
    options
  );
  const intermediateNs = AddZonedDateTime(start, timeZone, calendar, years, months, weeks, 0, 0, 0, 0, 0, 0, 0);
  // may disambiguate
  let timeRemainderNs = JSBI.subtract(ns2, intermediateNs);
  const intermediate = CreateTemporalZonedDateTime(intermediateNs, timeZone, calendar);
  ({ nanoseconds: timeRemainderNs, days } = NanosecondsToDays(timeRemainderNs, intermediate));

  // Finally, merge the date and time durations and return the merged result.
  const { hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
    0,
    0,
    0,
    0,
    0,
    0,
    JSBI.toNumber(timeRemainderNs),
    'hour'
  );
  return { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

type DifferenceOperation = 'since' | 'until';

// TODO: does it make sense to explicitly union the other and options types for each operation?
export function DifferenceTemporalInstant(
  operation: DifferenceOperation,
  instant: Temporal.Instant,
  otherParam: InstantParams['until'][0],
  optionsParam: InstantParams['until'][1] | undefined
): Temporal.Duration {
  const other = ToTemporalInstant(otherParam);
  let first, second;
  if (operation === 'until') {
    [first, second] = [instant, other];
  } else {
    [first, second] = [other, instant];
  }
  const options = GetOptionsObject(optionsParam);
  const smallestUnit = GetTemporalUnit(options, 'smallestUnit', 'time', 'nanosecond');
  const defaultLargestUnit = LargerOfTwoTemporalUnits('second', smallestUnit);
  let largestUnit = GetTemporalUnit(options, 'largestUnit', 'time', 'auto');
  if (largestUnit === 'auto') largestUnit = defaultLargestUnit;
  if (LargerOfTwoTemporalUnits(largestUnit, smallestUnit) !== largestUnit) {
    throw new RangeError(`largestUnit ${largestUnit} cannot be smaller than smallestUnit ${smallestUnit}`);
  }
  const roundingMode = ToTemporalRoundingMode(options, 'trunc');
  const MAX_DIFFERENCE_INCREMENTS = {
    hour: 24,
    minute: 60,
    second: 60,
    millisecond: 1000,
    microsecond: 1000,
    nanosecond: 1000
  };
  const roundingIncrement = ToTemporalRoundingIncrement(options, MAX_DIFFERENCE_INCREMENTS[smallestUnit], false);
  const onens = GetSlot(first, EPOCHNANOSECONDS);
  const twons = GetSlot(second, EPOCHNANOSECONDS);
  let { seconds, milliseconds, microseconds, nanoseconds } = DifferenceInstant(
    onens,
    twons,
    roundingIncrement,
    smallestUnit,
    roundingMode
  );
  let hours, minutes;
  ({ hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
    0,
    0,
    0,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds,
    largestUnit
  ));
  const Duration = GetIntrinsic('%Temporal.Duration%');
  return new Duration(0, 0, 0, 0, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
}

export function DifferenceTemporalPlainDate(
  operation: DifferenceOperation,
  plainDate: Temporal.PlainDate,
  otherParam: PlainDateParams['until'][0],
  optionsParam: PlainDateParams['until'][1]
): Temporal.Duration {
  const sign = operation === 'since' ? -1 : 1;
  const other = ToTemporalDate(otherParam);
  const calendar = GetSlot(plainDate, CALENDAR);
  const otherCalendar = GetSlot(other, CALENDAR);
  const calendarId = ToString(calendar);
  const otherCalendarId = ToString(otherCalendar);
  if (calendarId !== otherCalendarId) {
    throw new RangeError(`cannot compute difference between dates of ${calendarId} and ${otherCalendarId} calendars`);
  }

  const options = GetOptionsObject(optionsParam);
  const smallestUnit = GetTemporalUnit(options, 'smallestUnit', 'date', 'day');
  const defaultLargestUnit = LargerOfTwoTemporalUnits('day', smallestUnit);
  let largestUnit = GetTemporalUnit(options, 'largestUnit', 'date', 'auto');
  if (largestUnit === 'auto') largestUnit = defaultLargestUnit;
  if (LargerOfTwoTemporalUnits(largestUnit, smallestUnit) !== largestUnit) {
    throw new RangeError(`largestUnit ${largestUnit} cannot be smaller than smallestUnit ${smallestUnit}`);
  }
  let roundingMode = ToTemporalRoundingMode(options, 'trunc');
  if (operation === 'since') roundingMode = NegateTemporalRoundingMode(roundingMode);
  const roundingIncrement = ToTemporalRoundingIncrement(options, undefined, false);

  const untilOptions = MergeLargestUnitOption(options, largestUnit);
  let { years, months, weeks, days } = CalendarDateUntil(calendar, plainDate, other, untilOptions);

  if (smallestUnit !== 'day' || roundingIncrement !== 1) {
    ({ years, months, weeks, days } = RoundDuration(
      years,
      months,
      weeks,
      days,
      0,
      0,
      0,
      0,
      0,
      0,
      roundingIncrement,
      smallestUnit,
      roundingMode,
      plainDate
    ));
  }

  const Duration = GetIntrinsic('%Temporal.Duration%');
  return new Duration(sign * years, sign * months, sign * weeks, sign * days, 0, 0, 0, 0, 0, 0);
}

export function DifferenceTemporalPlainDateTime(
  operation: DifferenceOperation,
  plainDateTime: Temporal.PlainDateTime,
  otherParam: PlainDateTimeParams['until'][0],
  optionsParam: PlainDateTimeParams['until'][1]
): Temporal.Duration {
  const sign = operation === 'since' ? -1 : 1;
  const other = ToTemporalDateTime(otherParam);
  const calendar = GetSlot(plainDateTime, CALENDAR);
  const otherCalendar = GetSlot(other, CALENDAR);
  const calendarId = ToString(calendar);
  const otherCalendarId = ToString(otherCalendar);
  if (calendarId !== otherCalendarId) {
    throw new RangeError(`cannot compute difference between dates of ${calendarId} and ${otherCalendarId} calendars`);
  }
  const options = GetOptionsObject(optionsParam);
  const smallestUnit = GetTemporalUnit(options, 'smallestUnit', 'datetime', 'nanosecond');
  const defaultLargestUnit = LargerOfTwoTemporalUnits('day', smallestUnit);
  let largestUnit = GetTemporalUnit(options, 'largestUnit', 'datetime', 'auto');
  if (largestUnit === 'auto') largestUnit = defaultLargestUnit;
  if (LargerOfTwoTemporalUnits(largestUnit, smallestUnit) !== largestUnit) {
    throw new RangeError(`largestUnit ${largestUnit} cannot be smaller than smallestUnit ${smallestUnit}`);
  }
  let roundingMode = ToTemporalRoundingMode(options, 'trunc');
  if (operation === 'since') roundingMode = NegateTemporalRoundingMode(roundingMode);
  const roundingIncrement = ToTemporalDateTimeRoundingIncrement(options, smallestUnit);

  let { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
    DifferenceISODateTime(
      GetSlot(plainDateTime, ISO_YEAR),
      GetSlot(plainDateTime, ISO_MONTH),
      GetSlot(plainDateTime, ISO_DAY),
      GetSlot(plainDateTime, ISO_HOUR),
      GetSlot(plainDateTime, ISO_MINUTE),
      GetSlot(plainDateTime, ISO_SECOND),
      GetSlot(plainDateTime, ISO_MILLISECOND),
      GetSlot(plainDateTime, ISO_MICROSECOND),
      GetSlot(plainDateTime, ISO_NANOSECOND),
      GetSlot(other, ISO_YEAR),
      GetSlot(other, ISO_MONTH),
      GetSlot(other, ISO_DAY),
      GetSlot(other, ISO_HOUR),
      GetSlot(other, ISO_MINUTE),
      GetSlot(other, ISO_SECOND),
      GetSlot(other, ISO_MILLISECOND),
      GetSlot(other, ISO_MICROSECOND),
      GetSlot(other, ISO_NANOSECOND),
      calendar,
      largestUnit,
      options
    );

  const relativeTo = TemporalDateTimeToDate(plainDateTime);
  ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = RoundDuration(
    years,
    months,
    weeks,
    days,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds,
    roundingIncrement,
    smallestUnit,
    roundingMode,
    relativeTo
  ));
  ({ days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
    days,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds,
    largestUnit
  ));

  const Duration = GetIntrinsic('%Temporal.Duration%');
  return new Duration(
    sign * years,
    sign * months,
    sign * weeks,
    sign * days,
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds
  );
}

export function DifferenceTemporalPlainTime(
  operation: DifferenceOperation,
  plainTime: Temporal.PlainTime,
  otherParam: PlainTimeParams['until'][0],
  optionsParam: PlainTimeParams['until'][1]
): Temporal.Duration {
  const sign = operation === 'since' ? -1 : 1;
  const other = ToTemporalTime(otherParam);
  const options = GetOptionsObject(optionsParam);
  let largestUnit = GetTemporalUnit(options, 'largestUnit', 'time', 'auto');
  if (largestUnit === 'auto') largestUnit = 'hour';
  const smallestUnit = GetTemporalUnit(options, 'smallestUnit', 'time', 'nanosecond');
  if (LargerOfTwoTemporalUnits(largestUnit, smallestUnit) !== largestUnit) {
    throw new RangeError(`largestUnit ${largestUnit} cannot be smaller than smallestUnit ${smallestUnit}`);
  }
  let roundingMode = ToTemporalRoundingMode(options, 'trunc');
  if (operation === 'since') roundingMode = NegateTemporalRoundingMode(roundingMode);
  const MAX_INCREMENTS = {
    hour: 24,
    minute: 60,
    second: 60,
    millisecond: 1000,
    microsecond: 1000,
    nanosecond: 1000
  };
  const roundingIncrement = ToTemporalRoundingIncrement(options, MAX_INCREMENTS[smallestUnit], false);
  let { hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = DifferenceTime(
    GetSlot(plainTime, ISO_HOUR),
    GetSlot(plainTime, ISO_MINUTE),
    GetSlot(plainTime, ISO_SECOND),
    GetSlot(plainTime, ISO_MILLISECOND),
    GetSlot(plainTime, ISO_MICROSECOND),
    GetSlot(plainTime, ISO_NANOSECOND),
    GetSlot(other, ISO_HOUR),
    GetSlot(other, ISO_MINUTE),
    GetSlot(other, ISO_SECOND),
    GetSlot(other, ISO_MILLISECOND),
    GetSlot(other, ISO_MICROSECOND),
    GetSlot(other, ISO_NANOSECOND)
  );
  ({ hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = RoundDuration(
    0,
    0,
    0,
    0,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds,
    roundingIncrement,
    smallestUnit,
    roundingMode
  ));
  ({ hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
    0,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds,
    largestUnit
  ));
  const Duration = GetIntrinsic('%Temporal.Duration%');
  return new Duration(
    0,
    0,
    0,
    0,
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds
  );
}

export function DifferenceTemporalPlainYearMonth(
  operation: DifferenceOperation,
  yearMonth: Temporal.PlainYearMonth,
  otherParam: PlainYearMonthParams['until'][0],
  optionsParam: PlainYearMonthParams['until'][1]
): Temporal.Duration {
  const sign = operation === 'since' ? -1 : 1;
  const other = ToTemporalYearMonth(otherParam);
  const calendar = GetSlot(yearMonth, CALENDAR);
  const otherCalendar = GetSlot(other, CALENDAR);
  const calendarID = ToString(calendar);
  const otherCalendarID = ToString(otherCalendar);
  if (calendarID !== otherCalendarID) {
    throw new RangeError(`cannot compute difference between months of ${calendarID} and ${otherCalendarID} calendars`);
  }
  const options = GetOptionsObject(optionsParam);
  const ALLOWED_UNITS = SINGULAR_PLURAL_UNITS.reduce((allowed, [p, s, c]) => {
    if (c === 'date' && s !== 'week' && s !== 'day') allowed.push(s, p);
    return allowed;
  }, [] as Array<Temporal.DateTimeUnit | Temporal.PluralUnit<Temporal.DateTimeUnit>>);
  const smallestUnit = GetTemporalUnit(options, 'smallestUnit', 'date', 'month');
  if (smallestUnit === 'week' || smallestUnit === 'day') {
    throw new RangeError(`smallestUnit must be one of ${ALLOWED_UNITS.join(', ')}, not ${smallestUnit}`);
  }
  let largestUnit = GetTemporalUnit(options, 'largestUnit', 'date', 'auto');
  if (largestUnit === 'week' || largestUnit === 'day') {
    throw new RangeError(`largestUnit must be one of ${ALLOWED_UNITS.join(', ')}, not ${largestUnit}`);
  }
  if (largestUnit === 'auto') largestUnit = 'year';
  if (LargerOfTwoTemporalUnits(largestUnit, smallestUnit) !== largestUnit) {
    throw new RangeError(`largestUnit ${largestUnit} cannot be smaller than smallestUnit ${smallestUnit}`);
  }
  let roundingMode = ToTemporalRoundingMode(options, 'trunc');
  if (operation === 'since') roundingMode = NegateTemporalRoundingMode(roundingMode);
  const roundingIncrement = ToTemporalRoundingIncrement(options, undefined, false);

  const fieldNames = CalendarFields(calendar, ['monthCode', 'year'] as const);
  const otherFields = PrepareTemporalFields(other, fieldNames, []);
  otherFields.day = 1;
  const thisFields = PrepareTemporalFields(yearMonth, fieldNames, []);
  thisFields.day = 1;
  // The calls to PrepareTemporalFields don't mark day as a required property,
  // and TS doesn't automatically narrow the type of the object because of the
  // assignments above, so we must "cast" the inputs.
  const otherDate = CalendarDateFromFields(calendar, otherFields as typeof otherFields & { day: number });
  const thisDate = CalendarDateFromFields(calendar, thisFields as typeof thisFields & { day: number });

  const untilOptions = MergeLargestUnitOption(options, largestUnit);
  let { years, months } = CalendarDateUntil(calendar, thisDate, otherDate, untilOptions);

  if (smallestUnit !== 'month' || roundingIncrement !== 1) {
    ({ years, months } = RoundDuration(
      years,
      months,
      0,
      0,
      0,
      0,
      0,
      0,
      0,
      0,
      roundingIncrement,
      smallestUnit,
      roundingMode,
      thisDate
    ));
  }

  const Duration = GetIntrinsic('%Temporal.Duration%');
  return new Duration(sign * years, sign * months, 0, 0, 0, 0, 0, 0, 0, 0);
}

export function DifferenceTemporalZonedDateTime(
  operation: DifferenceOperation,
  zonedDateTime: Temporal.ZonedDateTime,
  otherParam: ZonedDateTimeParams['until'][0],
  optionsParam: ZonedDateTimeParams['until'][1]
): Temporal.Duration {
  const sign = operation === 'since' ? -1 : 1;
  const other = ToTemporalZonedDateTime(otherParam);
  const calendar = GetSlot(zonedDateTime, CALENDAR);
  const otherCalendar = GetSlot(other, CALENDAR);
  const calendarId = ToString(calendar);
  const otherCalendarId = ToString(otherCalendar);
  if (calendarId !== otherCalendarId) {
    throw new RangeError(`cannot compute difference between dates of ${calendarId} and ${otherCalendarId} calendars`);
  }
  const options = GetOptionsObject(optionsParam);
  const smallestUnit = GetTemporalUnit(options, 'smallestUnit', 'datetime', 'nanosecond');
  const defaultLargestUnit = LargerOfTwoTemporalUnits('hour', smallestUnit);
  let largestUnit = GetTemporalUnit(options, 'largestUnit', 'datetime', 'auto');
  if (largestUnit === 'auto') largestUnit = defaultLargestUnit;
  if (LargerOfTwoTemporalUnits(largestUnit, smallestUnit) !== largestUnit) {
    throw new RangeError(`largestUnit ${largestUnit} cannot be smaller than smallestUnit ${smallestUnit}`);
  }
  let roundingMode = ToTemporalRoundingMode(options, 'trunc');
  if (operation === 'since') roundingMode = NegateTemporalRoundingMode(roundingMode);
  const roundingIncrement = ToTemporalDateTimeRoundingIncrement(options, smallestUnit);

  const ns1 = GetSlot(zonedDateTime, EPOCHNANOSECONDS);
  const ns2 = GetSlot(other, EPOCHNANOSECONDS);
  let years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds;
  if (largestUnit !== 'year' && largestUnit !== 'month' && largestUnit !== 'week' && largestUnit !== 'day') {
    // The user is only asking for a time difference, so return difference of instants.
    years = 0;
    months = 0;
    weeks = 0;
    days = 0;
    ({ seconds, milliseconds, microseconds, nanoseconds } = DifferenceInstant(
      ns1,
      ns2,
      roundingIncrement,
      // TODO this doesn't type-check as it includes >= day-size units
      // This is probably safe as the typing for ToSmallestTemporalUnit isn't
      // very good.
      smallestUnit as any,
      roundingMode
    ));
    ({ hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
      0,
      0,
      0,
      seconds,
      milliseconds,
      microseconds,
      nanoseconds,
      largestUnit
    ));
  } else {
    const timeZone = GetSlot(zonedDateTime, TIME_ZONE);
    if (!TimeZoneEquals(timeZone, GetSlot(other, TIME_ZONE))) {
      throw new RangeError(
        "When calculating difference between time zones, largestUnit must be 'hours' " +
          'or smaller because day lengths can vary between time zones due to DST or time zone offset changes.'
      );
    }
    const untilOptions = MergeLargestUnitOption(options, largestUnit);
    ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
      DifferenceZonedDateTime(ns1, ns2, timeZone, calendar, largestUnit, untilOptions));
    ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = RoundDuration(
      years,
      months,
      weeks,
      days,
      hours,
      minutes,
      seconds,
      milliseconds,
      microseconds,
      nanoseconds,
      roundingIncrement,
      smallestUnit,
      roundingMode,
      zonedDateTime
    ));
    ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
      AdjustRoundedDurationDays(
        years,
        months,
        weeks,
        days,
        hours,
        minutes,
        seconds,
        milliseconds,
        microseconds,
        nanoseconds,
        roundingIncrement,
        smallestUnit,
        roundingMode,
        zonedDateTime
      ));
  }

  const Duration = GetIntrinsic('%Temporal.Duration%');
  return new Duration(
    sign * years,
    sign * months,
    sign * weeks,
    sign * days,
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds
  );
}

export function AddISODate(
  yearParam: number,
  monthParam: number,
  dayParam: number,
  yearsParam: number,
  monthsParam: number,
  weeksParam: number,
  daysParam: number,
  overflow: Temporal.ArithmeticOptions['overflow']
) {
  let year = yearParam;
  let month = monthParam;
  let day = dayParam;
  let years = yearsParam;
  let months = monthsParam;
  let weeks = weeksParam;
  let days = daysParam;

  year += years;
  month += months;
  ({ year, month } = BalanceISOYearMonth(year, month));
  ({ year, month, day } = RegulateISODate(year, month, day, overflow));
  days += 7 * weeks;
  day += days;
  ({ year, month, day } = BalanceISODate(year, month, day));
  return { year, month, day };
}

function AddTime(
  hourParam: number,
  minuteParam: number,
  secondParam: number,
  millisecondParam: number,
  microsecondParam: number,
  nanosecondParam: number,
  hours: number,
  minutes: number,
  seconds: number,
  milliseconds: number,
  microseconds: number,
  nanoseconds: number
) {
  let hour = hourParam;
  let minute = minuteParam;
  let second = secondParam;
  let millisecond = millisecondParam;
  let microsecond = microsecondParam;
  let nanosecond = nanosecondParam;

  hour += hours;
  minute += minutes;
  second += seconds;
  millisecond += milliseconds;
  microsecond += microseconds;
  nanosecond += nanoseconds;
  let deltaDays = 0;
  ({ deltaDays, hour, minute, second, millisecond, microsecond, nanosecond } = BalanceTime(
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond
  ));
  return { deltaDays, hour, minute, second, millisecond, microsecond, nanosecond };
}

function AddDuration(
  y1: number,
  mon1: number,
  w1: number,
  d1: number,
  h1: number,
  min1: number,
  s1: number,
  ms1: number,
  µs1: number,
  ns1: number,
  y2: number,
  mon2: number,
  w2: number,
  d2: number,
  h2: number,
  min2: number,
  s2: number,
  ms2: number,
  µs2: number,
  ns2: number,
  relativeTo: ReturnType<typeof ToRelativeTemporalObject>
) {
  const largestUnit1 = DefaultTemporalLargestUnit(y1, mon1, w1, d1, h1, min1, s1, ms1, µs1, ns1);
  const largestUnit2 = DefaultTemporalLargestUnit(y2, mon2, w2, d2, h2, min2, s2, ms2, µs2, ns2);
  const largestUnit = LargerOfTwoTemporalUnits(largestUnit1, largestUnit2);

  let years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds;
  if (!relativeTo) {
    if (largestUnit === 'year' || largestUnit === 'month' || largestUnit === 'week') {
      throw new RangeError('relativeTo is required for years, months, or weeks arithmetic');
    }
    years = months = weeks = 0;
    ({ days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
      d1 + d2,
      h1 + h2,
      min1 + min2,
      s1 + s2,
      ms1 + ms2,
      µs1 + µs2,
      ns1 + ns2,
      largestUnit
    ));
  } else if (IsTemporalDate(relativeTo)) {
    const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
    const calendar = GetSlot(relativeTo, CALENDAR);

    const dateDuration1 = new TemporalDuration(y1, mon1, w1, d1, 0, 0, 0, 0, 0, 0);
    const dateDuration2 = new TemporalDuration(y2, mon2, w2, d2, 0, 0, 0, 0, 0, 0);
    const dateAdd = calendar.dateAdd;
    const intermediate = CalendarDateAdd(calendar, relativeTo, dateDuration1, undefined, dateAdd);
    const end = CalendarDateAdd(calendar, intermediate, dateDuration2, undefined, dateAdd);

    const dateLargestUnit = LargerOfTwoTemporalUnits('day', largestUnit);
    const differenceOptions = ObjectCreate(null);
    differenceOptions.largestUnit = dateLargestUnit;
    ({ years, months, weeks, days } = CalendarDateUntil(calendar, relativeTo, end, differenceOptions));
    // Signs of date part and time part may not agree; balance them together
    ({ days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
      days,
      h1 + h2,
      min1 + min2,
      s1 + s2,
      ms1 + ms2,
      µs1 + µs2,
      ns1 + ns2,
      largestUnit
    ));
  } else {
    // relativeTo is a ZonedDateTime
    const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
    const timeZone = GetSlot(relativeTo, TIME_ZONE);
    const calendar = GetSlot(relativeTo, CALENDAR);
    const intermediateNs = AddZonedDateTime(
      GetSlot(relativeTo, INSTANT),
      timeZone,
      calendar,
      y1,
      mon1,
      w1,
      d1,
      h1,
      min1,
      s1,
      ms1,
      µs1,
      ns1
    );
    const endNs = AddZonedDateTime(
      new TemporalInstant(intermediateNs),
      timeZone,
      calendar,
      y2,
      mon2,
      w2,
      d2,
      h2,
      min2,
      s2,
      ms2,
      µs2,
      ns2
    );
    if (largestUnit !== 'year' && largestUnit !== 'month' && largestUnit !== 'week' && largestUnit !== 'day') {
      // The user is only asking for a time difference, so return difference of instants.
      years = 0;
      months = 0;
      weeks = 0;
      days = 0;
      ({ seconds, milliseconds, microseconds, nanoseconds } = DifferenceInstant(
        GetSlot(relativeTo, EPOCHNANOSECONDS),
        endNs,
        1,
        'nanosecond',
        'halfExpand'
      ));
      ({ hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
        0,
        0,
        0,
        seconds,
        milliseconds,
        microseconds,
        nanoseconds,
        largestUnit
      ));
    } else {
      ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
        DifferenceZonedDateTime(
          GetSlot(relativeTo, EPOCHNANOSECONDS),
          endNs,
          timeZone,
          calendar,
          largestUnit,
          ObjectCreate(null)
        ));
    }
  }

  RejectDuration(years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
  return { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

function AddInstant(epochNanoseconds: JSBI, h: number, min: number, s: number, ms: number, µs: number, ns: number) {
  let sum = ZERO;
  sum = JSBI.add(sum, JSBI.BigInt(ns));
  sum = JSBI.add(sum, JSBI.multiply(JSBI.BigInt(µs), THOUSAND));
  sum = JSBI.add(sum, JSBI.multiply(JSBI.BigInt(ms), MILLION));
  sum = JSBI.add(sum, JSBI.multiply(JSBI.BigInt(s), BILLION));
  sum = JSBI.add(sum, JSBI.multiply(JSBI.BigInt(min), JSBI.BigInt(60 * 1e9)));
  sum = JSBI.add(sum, JSBI.multiply(JSBI.BigInt(h), JSBI.BigInt(60 * 60 * 1e9)));

  const result = JSBI.add(epochNanoseconds, sum);
  ValidateEpochNanoseconds(result);
  return result;
}

function AddDateTime(
  year: number,
  month: number,
  day: number,
  hourParam: number,
  minuteParam: number,
  secondParam: number,
  millisecondParam: number,
  microsecondParam: number,
  nanosecondParam: number,
  calendar: Temporal.CalendarProtocol,
  years: number,
  months: number,
  weeks: number,
  daysParam: number,
  hours: number,
  minutes: number,
  seconds: number,
  milliseconds: number,
  microseconds: number,
  nanoseconds: number,
  options?: Temporal.ArithmeticOptions
) {
  let days = daysParam;
  // Add the time part
  let { deltaDays, hour, minute, second, millisecond, microsecond, nanosecond } = AddTime(
    hourParam,
    minuteParam,
    secondParam,
    millisecondParam,
    microsecondParam,
    nanosecondParam,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds
  );
  days += deltaDays;

  // Delegate the date part addition to the calendar
  const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
  const datePart = CreateTemporalDate(year, month, day, calendar);
  const dateDuration = new TemporalDuration(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
  const addedDate = CalendarDateAdd(calendar, datePart, dateDuration, options);

  return {
    year: GetSlot(addedDate, ISO_YEAR),
    month: GetSlot(addedDate, ISO_MONTH),
    day: GetSlot(addedDate, ISO_DAY),
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond
  };
}

export function AddZonedDateTime(
  instant: Temporal.Instant,
  timeZone: Temporal.TimeZoneProtocol,
  calendar: Temporal.CalendarProtocol,
  years: number,
  months: number,
  weeks: number,
  days: number,
  h: number,
  min: number,
  s: number,
  ms: number,
  µs: number,
  ns: number,
  options?: Temporal.ArithmeticOptions
) {
  // If only time is to be added, then use Instant math. It's not OK to fall
  // through to the date/time code below because compatible disambiguation in
  // the PlainDateTime=>Instant conversion will change the offset of any
  // ZonedDateTime in the repeated clock time after a backwards transition.
  // When adding/subtracting time units and not dates, this disambiguation is
  // not expected and so is avoided below via a fast path for time-only
  // arithmetic.
  // BTW, this behavior is similar in spirit to offset: 'prefer' in `with`.
  const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
  if (DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0) === 0) {
    return AddInstant(GetSlot(instant, EPOCHNANOSECONDS), h, min, s, ms, µs, ns);
  }

  // RFC 5545 requires the date portion to be added in calendar days and the
  // time portion to be added in exact time.
  const dt = BuiltinTimeZoneGetPlainDateTimeFor(timeZone, instant, calendar);
  const datePart = CreateTemporalDate(GetSlot(dt, ISO_YEAR), GetSlot(dt, ISO_MONTH), GetSlot(dt, ISO_DAY), calendar);
  const dateDuration = new TemporalDuration(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
  const addedDate = CalendarDateAdd(calendar, datePart, dateDuration, options);
  const dtIntermediate = CreateTemporalDateTime(
    GetSlot(addedDate, ISO_YEAR),
    GetSlot(addedDate, ISO_MONTH),
    GetSlot(addedDate, ISO_DAY),
    GetSlot(dt, ISO_HOUR),
    GetSlot(dt, ISO_MINUTE),
    GetSlot(dt, ISO_SECOND),
    GetSlot(dt, ISO_MILLISECOND),
    GetSlot(dt, ISO_MICROSECOND),
    GetSlot(dt, ISO_NANOSECOND),
    calendar
  );

  // Note that 'compatible' is used below because this disambiguation behavior
  // is required by RFC 5545.
  const instantIntermediate = BuiltinTimeZoneGetInstantFor(timeZone, dtIntermediate, 'compatible');
  return AddInstant(GetSlot(instantIntermediate, EPOCHNANOSECONDS), h, min, s, ms, µs, ns);
}

type AddSubtractOperation = 'add' | 'subtract';

export function AddDurationToOrSubtractDurationFromDuration(
  operation: AddSubtractOperation,
  duration: Temporal.Duration,
  other: DurationParams['add'][0],
  optionsParam: DurationParams['add'][1]
): Temporal.Duration {
  const sign = operation === 'subtract' ? -1 : 1;
  let { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
    ToTemporalDurationRecord(other);
  const options = GetOptionsObject(optionsParam);
  const relativeTo = ToRelativeTemporalObject(options);
  ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = AddDuration(
    GetSlot(duration, YEARS),
    GetSlot(duration, MONTHS),
    GetSlot(duration, WEEKS),
    GetSlot(duration, DAYS),
    GetSlot(duration, HOURS),
    GetSlot(duration, MINUTES),
    GetSlot(duration, SECONDS),
    GetSlot(duration, MILLISECONDS),
    GetSlot(duration, MICROSECONDS),
    GetSlot(duration, NANOSECONDS),
    sign * years,
    sign * months,
    sign * weeks,
    sign * days,
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds,
    relativeTo
  ));
  const Duration = GetIntrinsic('%Temporal.Duration%');
  return new Duration(years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds);
}

export function AddDurationToOrSubtractDurationFromInstant(
  operation: AddSubtractOperation,
  instant: Temporal.Instant,
  durationLike: InstantParams['add'][0]
): Temporal.Instant {
  const sign = operation === 'subtract' ? -1 : 1;
  const { hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = ToLimitedTemporalDuration(durationLike, [
    'years',
    'months',
    'weeks',
    'days'
  ]);
  const ns = AddInstant(
    GetSlot(instant, EPOCHNANOSECONDS),
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds
  );
  const Instant = GetIntrinsic('%Temporal.Instant%');
  return new Instant(ns);
}

export function AddDurationToOrSubtractDurationFromPlainDateTime(
  operation: AddSubtractOperation,
  dateTime: Temporal.PlainDateTime,
  durationLike: PlainDateTimeParams['add'][0],
  optionsParam: PlainDateTimeParams['add'][1]
): Temporal.PlainDateTime {
  const sign = operation === 'subtract' ? -1 : 1;
  const { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
    ToTemporalDurationRecord(durationLike);
  const options = GetOptionsObject(optionsParam);
  const calendar = GetSlot(dateTime, CALENDAR);
  const { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond } = AddDateTime(
    GetSlot(dateTime, ISO_YEAR),
    GetSlot(dateTime, ISO_MONTH),
    GetSlot(dateTime, ISO_DAY),
    GetSlot(dateTime, ISO_HOUR),
    GetSlot(dateTime, ISO_MINUTE),
    GetSlot(dateTime, ISO_SECOND),
    GetSlot(dateTime, ISO_MILLISECOND),
    GetSlot(dateTime, ISO_MICROSECOND),
    GetSlot(dateTime, ISO_NANOSECOND),
    calendar,
    sign * years,
    sign * months,
    sign * weeks,
    sign * days,
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds,
    options
  );
  return CreateTemporalDateTime(year, month, day, hour, minute, second, millisecond, microsecond, nanosecond, calendar);
}

export function AddDurationToOrSubtractDurationFromPlainTime(
  operation: AddSubtractOperation,
  temporalTime: Temporal.PlainTime,
  durationLike: PlainTimeParams['add'][0]
): Temporal.PlainTime {
  const sign = operation === 'subtract' ? -1 : 1;
  const { hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = ToTemporalDurationRecord(durationLike);
  let { hour, minute, second, millisecond, microsecond, nanosecond } = AddTime(
    GetSlot(temporalTime, ISO_HOUR),
    GetSlot(temporalTime, ISO_MINUTE),
    GetSlot(temporalTime, ISO_SECOND),
    GetSlot(temporalTime, ISO_MILLISECOND),
    GetSlot(temporalTime, ISO_MICROSECOND),
    GetSlot(temporalTime, ISO_NANOSECOND),
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds
  );
  ({ hour, minute, second, millisecond, microsecond, nanosecond } = RegulateTime(
    hour,
    minute,
    second,
    millisecond,
    microsecond,
    nanosecond,
    'reject'
  ));
  const PlainTime = GetIntrinsic('%Temporal.PlainTime%');
  return new PlainTime(hour, minute, second, millisecond, microsecond, nanosecond);
}

export function AddDurationToOrSubtractDurationFromPlainYearMonth(
  operation: AddSubtractOperation,
  yearMonth: Temporal.PlainYearMonth,
  durationLike: PlainYearMonthParams['add'][0],
  optionsParam: PlainYearMonthParams['add'][1]
): Temporal.PlainYearMonth {
  let duration = ToTemporalDurationRecord(durationLike);
  if (operation === 'subtract') {
    duration = {
      years: -duration.years,
      months: -duration.months,
      weeks: -duration.weeks,
      days: -duration.days,
      hours: -duration.hours,
      minutes: -duration.minutes,
      seconds: -duration.seconds,
      milliseconds: -duration.milliseconds,
      microseconds: -duration.microseconds,
      nanoseconds: -duration.nanoseconds
    };
  }
  let { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = duration;
  ({ days } = BalanceDuration(days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds, 'day'));

  const options = GetOptionsObject(optionsParam);

  const calendar = GetSlot(yearMonth, CALENDAR);
  const fieldNames = CalendarFields(calendar, ['monthCode', 'year'] as const);
  const fields = PrepareTemporalFields(yearMonth, fieldNames, []);
  const sign = DurationSign(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
  fields.day = sign < 0 ? ToPositiveInteger(CalendarDaysInMonth(calendar, yearMonth)) : 1;
  // PrepareTemporalFields returns a type where 'day' is potentially undefined,
  // and TS doesn't narrow the type as a result of the assignment above, so we
  // cast the fields input to the new type.
  const startDate = CalendarDateFromFields(calendar, fields as typeof fields & { day: number });
  const Duration = GetIntrinsic('%Temporal.Duration%');
  const durationToAdd = new Duration(years, months, weeks, days, 0, 0, 0, 0, 0, 0);
  const optionsCopy = ObjectAssign(ObjectCreate(null), options);
  const addedDate = CalendarDateAdd(calendar, startDate, durationToAdd, options);
  const addedDateFields = PrepareTemporalFields(addedDate, fieldNames, []);

  return CalendarYearMonthFromFields(calendar, addedDateFields, optionsCopy);
}

export function AddDurationToOrSubtractDurationFromZonedDateTime(
  operation: AddSubtractOperation,
  zonedDateTime: Temporal.ZonedDateTime,
  durationLike: ZonedDateTimeParams['add'][0],
  optionsParam: ZonedDateTimeParams['add'][1]
): Temporal.ZonedDateTime {
  const sign = operation === 'subtract' ? -1 : 1;
  const { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
    ToTemporalDurationRecord(durationLike);
  const options = GetOptionsObject(optionsParam);
  const timeZone = GetSlot(zonedDateTime, TIME_ZONE);
  const calendar = GetSlot(zonedDateTime, CALENDAR);
  const epochNanoseconds = AddZonedDateTime(
    GetSlot(zonedDateTime, INSTANT),
    timeZone,
    calendar,
    sign * years,
    sign * months,
    sign * weeks,
    sign * days,
    sign * hours,
    sign * minutes,
    sign * seconds,
    sign * milliseconds,
    sign * microseconds,
    sign * nanoseconds,
    options
  );
  return CreateTemporalZonedDateTime(epochNanoseconds, timeZone, calendar);
}

function RoundNumberToIncrement(quantity: JSBI, increment: number, mode: Temporal.RoundingMode) {
  if (increment === 1) return quantity;
  let { quotient, remainder } = divmod(quantity, JSBI.BigInt(increment));
  if (JSBI.equal(remainder, ZERO)) return quantity;
  const sign = JSBI.lessThan(remainder, ZERO) ? -1 : 1;
  switch (mode) {
    case 'ceil':
      if (sign > 0) quotient = JSBI.add(quotient, JSBI.BigInt(sign));
      break;
    case 'floor':
      if (sign < 0) quotient = JSBI.add(quotient, JSBI.BigInt(sign));
      break;
    case 'trunc':
      // no change needed, because divmod is a truncation
      break;
    case 'halfExpand':
      // "half up away from zero"
      if (JSBI.toNumber(abs(JSBI.multiply(remainder, JSBI.BigInt(2)))) >= increment) {
        quotient = JSBI.add(quotient, JSBI.BigInt(sign));
      }
      break;
  }
  return JSBI.multiply(quotient, JSBI.BigInt(increment));
}

export function RoundInstant(
  epochNs: JSBI,
  increment: number,
  unit: keyof typeof nsPerTimeUnit,
  roundingMode: Temporal.RoundingMode
) {
  // Note: NonNegativeModulo, but with BigInt
  let remainder = JSBI.remainder(epochNs, JSBI.BigInt(86400e9));
  if (JSBI.lessThan(remainder, ZERO)) remainder = JSBI.add(remainder, JSBI.BigInt(86400e9));
  const wholeDays = JSBI.subtract(epochNs, remainder);
  const roundedRemainder = RoundNumberToIncrement(remainder, nsPerTimeUnit[unit] * increment, roundingMode);
  return JSBI.add(wholeDays, roundedRemainder);
}

export function RoundISODateTime(
  yearParam: number,
  monthParam: number,
  dayParam: number,
  hourParam: number,
  minuteParam: number,
  secondParam: number,
  millisecondParam: number,
  microsecondParam: number,
  nanosecondParam: number,
  increment: number,
  unit: UnitSmallerThanOrEqualTo<'day'>,
  roundingMode: Temporal.RoundingMode,
  dayLengthNs = 86400e9
) {
  const { deltaDays, hour, minute, second, millisecond, microsecond, nanosecond } = RoundTime(
    hourParam,
    minuteParam,
    secondParam,
    millisecondParam,
    microsecondParam,
    nanosecondParam,
    increment,
    unit,
    roundingMode,
    dayLengthNs
  );
  const { year, month, day } = BalanceISODate(yearParam, monthParam, dayParam + deltaDays);
  return { year, month, day, hour, minute, second, millisecond, microsecond, nanosecond };
}

export function RoundTime(
  hour: number,
  minute: number,
  second: number,
  millisecond: number,
  microsecond: number,
  nanosecond: number,
  increment: number,
  unit: keyof typeof nsPerTimeUnit | 'day',
  roundingMode: Temporal.RoundingMode,
  dayLengthNs = 86400e9
) {
  let quantity = ZERO;
  switch (unit) {
    case 'day':
    case 'hour':
      quantity = JSBI.BigInt(hour);
    // fall through
    case 'minute':
      quantity = JSBI.add(JSBI.multiply(quantity, SIXTY), JSBI.BigInt(minute));
    // fall through
    case 'second':
      quantity = JSBI.add(JSBI.multiply(quantity, SIXTY), JSBI.BigInt(second));
    // fall through
    case 'millisecond':
      quantity = JSBI.add(JSBI.multiply(quantity, THOUSAND), JSBI.BigInt(millisecond));
    // fall through
    case 'microsecond':
      quantity = JSBI.add(JSBI.multiply(quantity, THOUSAND), JSBI.BigInt(microsecond));
    // fall through
    case 'nanosecond':
      quantity = JSBI.add(JSBI.multiply(quantity, THOUSAND), JSBI.BigInt(nanosecond));
  }
  const nsPerUnit = unit === 'day' ? dayLengthNs : nsPerTimeUnit[unit];
  const rounded = RoundNumberToIncrement(quantity, nsPerUnit * increment, roundingMode);
  const result = JSBI.toNumber(JSBI.divide(rounded, JSBI.BigInt(nsPerUnit)));
  switch (unit) {
    case 'day':
      return { deltaDays: result, hour: 0, minute: 0, second: 0, millisecond: 0, microsecond: 0, nanosecond: 0 };
    case 'hour':
      return BalanceTime(result, 0, 0, 0, 0, 0);
    case 'minute':
      return BalanceTime(hour, result, 0, 0, 0, 0);
    case 'second':
      return BalanceTime(hour, minute, result, 0, 0, 0);
    case 'millisecond':
      return BalanceTime(hour, minute, second, result, 0, 0);
    case 'microsecond':
      return BalanceTime(hour, minute, second, millisecond, result, 0);
    case 'nanosecond':
      return BalanceTime(hour, minute, second, millisecond, microsecond, result);
    default:
      throw new Error(`Invalid unit ${unit}`);
  }
}

function DaysUntil(
  earlier: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime,
  later: Temporal.PlainDate | Temporal.PlainDateTime | Temporal.ZonedDateTime
) {
  return DifferenceISODate(
    GetSlot(earlier, ISO_YEAR),
    GetSlot(earlier, ISO_MONTH),
    GetSlot(earlier, ISO_DAY),
    GetSlot(later, ISO_YEAR),
    GetSlot(later, ISO_MONTH),
    GetSlot(later, ISO_DAY),
    'day'
  ).days;
}

function MoveRelativeDate(
  calendar: Temporal.CalendarProtocol,
  relativeToParam: NonNullable<ReturnType<typeof ToRelativeTemporalObject>>,
  duration: Temporal.Duration
) {
  const later = CalendarDateAdd(calendar, relativeToParam, duration, undefined);
  const days = DaysUntil(relativeToParam, later);
  return { relativeTo: later, days };
}

export function MoveRelativeZonedDateTime(
  relativeTo: Temporal.ZonedDateTime,
  years: number,
  months: number,
  weeks: number,
  days: number
) {
  const timeZone = GetSlot(relativeTo, TIME_ZONE);
  const calendar = GetSlot(relativeTo, CALENDAR);
  const intermediateNs = AddZonedDateTime(
    GetSlot(relativeTo, INSTANT),
    timeZone,
    calendar,
    years,
    months,
    weeks,
    days,
    0,
    0,
    0,
    0,
    0,
    0
  );
  return CreateTemporalZonedDateTime(intermediateNs, timeZone, calendar);
}

export function AdjustRoundedDurationDays(
  yearsParam: number,
  monthsParam: number,
  weeksParam: number,
  daysParam: number,
  hoursParam: number,
  minutesParam: number,
  secondsParam: number,
  millisecondsParam: number,
  microsecondsParam: number,
  nanosecondsParam: number,
  increment: number,
  unit: Temporal.DateTimeUnit,
  roundingMode: Temporal.RoundingMode,
  relativeTo: ReturnType<typeof ToRelativeTemporalObject>
) {
  let years = yearsParam;
  let months = monthsParam;
  let weeks = weeksParam;
  let days = daysParam;
  let hours = hoursParam;
  let minutes = minutesParam;
  let seconds = secondsParam;
  let milliseconds = millisecondsParam;
  let microseconds = microsecondsParam;
  let nanoseconds = nanosecondsParam;
  if (
    !IsTemporalZonedDateTime(relativeTo) ||
    unit === 'year' ||
    unit === 'month' ||
    unit === 'week' ||
    unit === 'day' ||
    (unit === 'nanosecond' && increment === 1)
  ) {
    return { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
  }

  // There's one more round of rounding possible: if relativeTo is a
  // ZonedDateTime, the time units could have rounded up into enough hours
  // to exceed the day length. If this happens, grow the date part by a
  // single day and re-run exact time rounding on the smaller remainder. DO
  // NOT RECURSE, because once the extra hours are sucked up into the date
  // duration, there's no way for another full day to come from the next
  // round of rounding. And if it were possible (e.g. contrived calendar
  // with 30-minute-long "days") then it'd risk an infinite loop.
  let timeRemainderNs = TotalDurationNanoseconds(
    0,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds,
    0
  );
  const direction = MathSign(JSBI.toNumber(timeRemainderNs));

  const timeZone = GetSlot(relativeTo, TIME_ZONE);
  const calendar = GetSlot(relativeTo, CALENDAR);
  const dayStart = AddZonedDateTime(
    GetSlot(relativeTo, INSTANT),
    timeZone,
    calendar,
    years,
    months,
    weeks,
    days,
    0,
    0,
    0,
    0,
    0,
    0
  );
  const TemporalInstant = GetIntrinsic('%Temporal.Instant%');
  const dayEnd = AddZonedDateTime(
    new TemporalInstant(dayStart),
    timeZone,
    calendar,
    0,
    0,
    0,
    direction,
    0,
    0,
    0,
    0,
    0,
    0
  );
  const dayLengthNs = JSBI.subtract(dayEnd, dayStart);

  if (
    JSBI.greaterThanOrEqual(JSBI.multiply(JSBI.subtract(timeRemainderNs, dayLengthNs), JSBI.BigInt(direction)), ZERO)
  ) {
    ({ years, months, weeks, days } = AddDuration(
      years,
      months,
      weeks,
      days,
      0,
      0,
      0,
      0,
      0,
      0,
      0,
      0,
      0,
      direction,
      0,
      0,
      0,
      0,
      0,
      0,
      relativeTo
    ));
    timeRemainderNs = RoundInstant(JSBI.subtract(timeRemainderNs, dayLengthNs), increment, unit, roundingMode);
    ({ hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = BalanceDuration(
      0,
      0,
      0,
      0,
      0,
      0,
      JSBI.toNumber(timeRemainderNs),
      'hour'
    ));
  }
  return { years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds };
}

export function RoundDuration(
  yearsParam: number,
  monthsParam: number,
  weeksParam: number,
  daysParam: number,
  hoursParam: number,
  minutesParam: number,
  secondsParam: number,
  millisecondsParam: number,
  microsecondsParam: number,
  nanosecondsParam: number,
  increment: number,
  unit: Temporal.DateTimeUnit,
  roundingMode: Temporal.RoundingMode,
  relativeToParam: ReturnType<typeof ToRelativeTemporalObject> = undefined
) {
  let years = yearsParam;
  let months = monthsParam;
  let weeks = weeksParam;
  let days = daysParam;
  let hours = hoursParam;
  let minutes = minutesParam;
  let seconds = secondsParam;
  let milliseconds = millisecondsParam;
  let microseconds = microsecondsParam;
  let nanoseconds = JSBI.BigInt(nanosecondsParam);
  const TemporalDuration = GetIntrinsic('%Temporal.Duration%');
  let calendar, zdtRelative;
  // A cast is used below because relativeTo will be either PlainDate or
  // undefined for the rest of this long method (after any ZDT=>PlainDate
  // conversion below), and TS isn't smart enough to know that the type has
  // changed. See https://github.com/microsoft/TypeScript/issues/27706.
  let relativeTo = relativeToParam as Temporal.PlainDate | undefined;
  if (relativeTo) {
    if (IsTemporalZonedDateTime(relativeTo)) {
      zdtRelative = relativeTo;
      relativeTo = ToTemporalDate(relativeTo);
    } else if (!IsTemporalDate(relativeTo)) {
      throw new TypeError('starting point must be PlainDate or ZonedDateTime');
    }
    calendar = GetSlot(relativeTo, CALENDAR);
  }

  // First convert time units up to days, if rounding to days or higher units.
  // If rounding relative to a ZonedDateTime, then some days may not be 24h.
  // TS doesn't know that `dayLengthNs` is only used if the unit is day or
  // larger. We'll cast away `undefined` when it's used lower down below.
  let dayLengthNs: JSBI | undefined;
  if (unit === 'year' || unit === 'month' || unit === 'week' || unit === 'day') {
    nanoseconds = TotalDurationNanoseconds(0, hours, minutes, seconds, milliseconds, microseconds, nanosecondsParam, 0);
    let intermediate;
    if (zdtRelative) {
      intermediate = MoveRelativeZonedDateTime(zdtRelative, years, months, weeks, days);
    }
    let deltaDays;
    let dayLength: number;
    ({ days: deltaDays, nanoseconds, dayLengthNs: dayLength } = NanosecondsToDays(nanoseconds, intermediate));
    dayLengthNs = JSBI.BigInt(dayLength);
    days += deltaDays;
    hours = minutes = seconds = milliseconds = microseconds = 0;
  }

  let total: number;
  switch (unit) {
    case 'year': {
      if (!calendar) throw new RangeError('A starting point is required for years rounding');

      // convert months and weeks to days by calculating difference(
      // relativeTo + years, relativeTo + { years, months, weeks })
      const yearsDuration = new TemporalDuration(years);
      const dateAdd = calendar.dateAdd;
      const yearsLater = CalendarDateAdd(calendar, relativeTo as Temporal.PlainDate, yearsDuration, undefined, dateAdd);
      const yearsMonthsWeeks = new TemporalDuration(years, months, weeks);
      const yearsMonthsWeeksLater = CalendarDateAdd(
        calendar,
        relativeTo as Temporal.PlainDate,
        yearsMonthsWeeks,
        undefined,
        dateAdd
      );
      const monthsWeeksInDays = DaysUntil(yearsLater, yearsMonthsWeeksLater);
      relativeTo = yearsLater;
      days += monthsWeeksInDays;

      const daysLater = CalendarDateAdd(calendar, relativeTo, { days }, undefined, dateAdd);
      const untilOptions = ObjectCreate(null);
      untilOptions.largestUnit = 'year';
      const yearsPassed = CalendarDateUntil(calendar, relativeTo, daysLater, untilOptions).years;
      years += yearsPassed;
      const oldRelativeTo = relativeTo;
      relativeTo = CalendarDateAdd(calendar, relativeTo, { years: yearsPassed }, undefined, dateAdd);
      const daysPassed = DaysUntil(oldRelativeTo, relativeTo);
      days -= daysPassed;
      const oneYear = new TemporalDuration(days < 0 ? -1 : 1);
      let { days: oneYearDays } = MoveRelativeDate(calendar, relativeTo, oneYear);

      // Note that `nanoseconds` below (here and in similar code for months,
      // weeks, and days further below) isn't actually nanoseconds for the
      // full date range.  Instead, it's a BigInt representation of total
      // days multiplied by the number of nanoseconds in the last day of
      // the duration. This lets us do days-or-larger rounding using BigInt
      // math which reduces precision loss.
      oneYearDays = MathAbs(oneYearDays);
      // dayLengthNs is never undefined if unit is `day` or larger.
      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
      const divisor = JSBI.multiply(JSBI.BigInt(oneYearDays), dayLengthNs!);
      nanoseconds = JSBI.add(
        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
        JSBI.add(JSBI.multiply(divisor, JSBI.BigInt(years)), JSBI.multiply(JSBI.BigInt(days), dayLengthNs!)),
        nanoseconds
      );
      const rounded = RoundNumberToIncrement(
        nanoseconds,
        JSBI.toNumber(JSBI.multiply(divisor, JSBI.BigInt(increment))),
        roundingMode
      );
      total = JSBI.toNumber(nanoseconds) / JSBI.toNumber(divisor);
      years = JSBI.toNumber(JSBI.divide(rounded, divisor));
      nanoseconds = ZERO;
      months = weeks = days = 0;
      break;
    }
    case 'month': {
      if (!calendar) throw new RangeError('A starting point is required for months rounding');

      // convert weeks to days by calculating difference(relativeTo +
      //   { years, months }, relativeTo + { years, months, weeks })
      const yearsMonths = new TemporalDuration(years, months);
      const dateAdd = calendar.dateAdd;
      const yearsMonthsLater = CalendarDateAdd(
        calendar,
        relativeTo as Temporal.PlainDate,
        yearsMonths,
        undefined,
        dateAdd
      );
      const yearsMonthsWeeks = new TemporalDuration(years, months, weeks);
      const yearsMonthsWeeksLater = CalendarDateAdd(
        calendar,
        relativeTo as Temporal.PlainDate,
        yearsMonthsWeeks,
        undefined,
        dateAdd
      );
      const weeksInDays = DaysUntil(yearsMonthsLater, yearsMonthsWeeksLater);
      relativeTo = yearsMonthsLater;
      days += weeksInDays;

      // Months may be different lengths of days depending on the calendar,
      // convert days to months in a loop as described above under 'years'.
      const sign = MathSign(days);
      const oneMonth = new TemporalDuration(0, days < 0 ? -1 : 1);
      let oneMonthDays: number;
      ({ relativeTo, days: oneMonthDays } = MoveRelativeDate(calendar, relativeTo, oneMonth));
      while (MathAbs(days) >= MathAbs(oneMonthDays)) {
        months += sign;
        days -= oneMonthDays;
        ({ relativeTo, days: oneMonthDays } = MoveRelativeDate(calendar, relativeTo, oneMonth));
      }
      oneMonthDays = MathAbs(oneMonthDays);
      // dayLengthNs is never undefined if unit is `day` or larger.
      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
      const divisor = JSBI.multiply(JSBI.BigInt(oneMonthDays), dayLengthNs!);
      nanoseconds = JSBI.add(
        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
        JSBI.add(JSBI.multiply(divisor, JSBI.BigInt(months)), JSBI.multiply(JSBI.BigInt(days), dayLengthNs!)),
        nanoseconds
      );
      const rounded = RoundNumberToIncrement(
        nanoseconds,
        JSBI.toNumber(JSBI.multiply(divisor, JSBI.BigInt(increment))),
        roundingMode
      );
      total = JSBI.toNumber(nanoseconds) / JSBI.toNumber(divisor);
      months = JSBI.toNumber(JSBI.divide(rounded, divisor));
      nanoseconds = ZERO;
      weeks = days = 0;
      break;
    }
    case 'week': {
      if (!calendar) throw new RangeError('A starting point is required for weeks rounding');
      // Weeks may be different lengths of days depending on the calendar,
      // convert days to weeks in a loop as described above under 'years'.
      const sign = MathSign(days);
      const oneWeek = new TemporalDuration(0, 0, days < 0 ? -1 : 1);
      let oneWeekDays;
      ({ relativeTo, days: oneWeekDays } = MoveRelativeDate(calendar, relativeTo as Temporal.PlainDate, oneWeek));
      while (MathAbs(days) >= MathAbs(oneWeekDays)) {
        weeks += sign;
        days -= oneWeekDays;
        ({ relativeTo, days: oneWeekDays } = MoveRelativeDate(calendar, relativeTo, oneWeek));
      }
      oneWeekDays = MathAbs(oneWeekDays);
      // dayLengthNs is never undefined if unit is `day` or larger.
      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
      const divisor = JSBI.multiply(JSBI.BigInt(oneWeekDays), dayLengthNs!);
      nanoseconds = JSBI.add(
        // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
        JSBI.add(JSBI.multiply(divisor, JSBI.BigInt(weeks)), JSBI.multiply(JSBI.BigInt(days), dayLengthNs!)),
        nanoseconds
      );
      const rounded = RoundNumberToIncrement(
        nanoseconds,
        JSBI.toNumber(JSBI.multiply(divisor, JSBI.BigInt(increment))),
        roundingMode
      );
      total = JSBI.toNumber(nanoseconds) / JSBI.toNumber(divisor);
      weeks = JSBI.toNumber(JSBI.divide(rounded, divisor));
      nanoseconds = ZERO;
      days = 0;
      break;
    }
    case 'day': {
      // dayLengthNs is never undefined if unit is `day` or larger.
      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
      const divisor = dayLengthNs!;
      nanoseconds = JSBI.add(JSBI.multiply(divisor, JSBI.BigInt(days)), nanoseconds);
      const rounded = RoundNumberToIncrement(
        nanoseconds,
        JSBI.toNumber(JSBI.multiply(divisor, JSBI.BigInt(increment))),
        roundingMode
      );
      total = JSBI.toNumber(nanoseconds) / JSBI.toNumber(divisor);
      days = JSBI.toNumber(JSBI.divide(rounded, divisor));
      nanoseconds = ZERO;
      break;
    }
    case 'hour': {
      const divisor = 3600e9;
      let allNanoseconds = JSBI.multiply(JSBI.BigInt(hours), JSBI.BigInt(3600e9));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(minutes), JSBI.BigInt(60e9)));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(seconds), BILLION));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(milliseconds), MILLION));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(microseconds), THOUSAND));
      allNanoseconds = JSBI.add(allNanoseconds, nanoseconds);
      total = JSBI.toNumber(allNanoseconds) / divisor;
      const rounded = RoundNumberToIncrement(allNanoseconds, divisor * increment, roundingMode);
      hours = JSBI.toNumber(JSBI.divide(rounded, JSBI.BigInt(divisor)));
      nanoseconds = ZERO;
      minutes = seconds = milliseconds = microseconds = 0;
      break;
    }
    case 'minute': {
      const divisor = 60e9;
      let allNanoseconds = JSBI.multiply(JSBI.BigInt(minutes), JSBI.BigInt(60e9));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(seconds), BILLION));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(milliseconds), MILLION));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(microseconds), THOUSAND));
      allNanoseconds = JSBI.add(allNanoseconds, nanoseconds);
      total = JSBI.toNumber(allNanoseconds) / divisor;
      const rounded = RoundNumberToIncrement(allNanoseconds, divisor * increment, roundingMode);
      minutes = JSBI.toNumber(JSBI.divide(rounded, JSBI.BigInt(divisor)));
      nanoseconds = ZERO;
      seconds = milliseconds = microseconds = 0;
      break;
    }
    case 'second': {
      const divisor = 1e9;
      let allNanoseconds = JSBI.multiply(JSBI.BigInt(seconds), BILLION);
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(milliseconds), MILLION));
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(microseconds), THOUSAND));
      allNanoseconds = JSBI.add(allNanoseconds, nanoseconds);
      total = JSBI.toNumber(allNanoseconds) / divisor;
      const rounded = RoundNumberToIncrement(allNanoseconds, divisor * increment, roundingMode);
      seconds = JSBI.toNumber(JSBI.divide(rounded, JSBI.BigInt(divisor)));
      nanoseconds = ZERO;
      milliseconds = microseconds = 0;
      break;
    }
    case 'millisecond': {
      const divisor = 1e6;
      let allNanoseconds = JSBI.multiply(JSBI.BigInt(milliseconds), MILLION);
      allNanoseconds = JSBI.add(allNanoseconds, JSBI.multiply(JSBI.BigInt(microseconds), THOUSAND));
      allNanoseconds = JSBI.add(allNanoseconds, nanoseconds);
      total = JSBI.toNumber(allNanoseconds) / divisor;
      const rounded = RoundNumberToIncrement(allNanoseconds, divisor * increment, roundingMode);
      milliseconds = JSBI.toNumber(JSBI.divide(rounded, JSBI.BigInt(divisor)));
      nanoseconds = ZERO;
      microseconds = 0;
      break;
    }
    case 'microsecond': {
      const divisor = 1e3;
      let allNanoseconds = JSBI.multiply(JSBI.BigInt(microseconds), THOUSAND);
      allNanoseconds = JSBI.add(allNanoseconds, nanoseconds);
      total = JSBI.toNumber(allNanoseconds) / divisor;
      const rounded = RoundNumberToIncrement(allNanoseconds, divisor * increment, roundingMode);
      microseconds = JSBI.toNumber(JSBI.divide(rounded, JSBI.BigInt(divisor)));
      nanoseconds = ZERO;
      break;
    }
    case 'nanosecond': {
      total = JSBI.toNumber(nanoseconds);
      nanoseconds = RoundNumberToIncrement(nanoseconds, increment, roundingMode);
      break;
    }
  }
  return {
    years,
    months,
    weeks,
    days,
    hours,
    minutes,
    seconds,
    milliseconds,
    microseconds,
    nanoseconds: JSBI.toNumber(nanoseconds),
    total
  };
}

export function CompareISODate(y1: number, m1: number, d1: number, y2: number, m2: number, d2: number) {
  for (const [x, y] of [
    [y1, y2],
    [m1, m2],
    [d1, d2]
  ]) {
    if (x !== y) return ComparisonResult(x - y);
  }
  return 0;
}

function NonNegativeModulo(x: number, y: number) {
  let result = x % y;
  if (ObjectIs(result, -0)) return 0;
  if (result < 0) result += y;
  return result;
}

// Defaults to native bigint, or something "native bigint-like".
// For users of Temporal that are running in environments without native BigInt,
// the only guarantee we should give is that the returned object's toString will
// return a string containing an accurate base 10 value of this bigint. This
// form factor should correctly interop with other bigint compat libraries
// easily.
type ExternalBigInt = bigint;

export function ToBigIntExternal(arg: unknown): ExternalBigInt {
  const jsbiBI = ToBigInt(arg);
  if (typeof (globalThis as any).BigInt !== 'undefined') return (globalThis as any).BigInt(jsbiBI.toString(10));
  return jsbiBI as unknown as ExternalBigInt;
}

export function ToBigInt(arg: unknown): JSBI {
  if (arg instanceof JSBI) {
    return arg;
  }

  let prim = arg;
  if (typeof arg === 'object') {
    const toPrimFn = (arg as { [Symbol.toPrimitive]: unknown })[Symbol.toPrimitive];
    if (toPrimFn && typeof toPrimFn === 'function') {
      prim = ReflectApply(toPrimFn, arg, ['number']);
    }
  }
  switch (typeof prim) {
    case 'undefined':
    case 'object':
    case 'number':
    case 'symbol':
    default:
      throw new TypeError(`cannot convert ${typeof arg} to bigint`);
    case 'string':
      if (!prim.match(/^\s*(?:[+-]?\d+\s*)?$/)) {
        throw new SyntaxError('invalid BigInt syntax');
      }
    // eslint: no-fallthrough: false
    case 'bigint':
      try {
        return JSBI.BigInt(prim.toString());
      } catch (e: unknown) {
        if (e instanceof Error && e.message.startsWith('Invalid integer')) throw new SyntaxError(e.message);
        throw e;
      }
    case 'boolean':
      if (prim) {
        return ONE;
      } else {
        return ZERO;
      }
  }
}

// Note: This method returns values with bogus nanoseconds based on the previous iteration's
// milliseconds. That way there is a guarantee that the full nanoseconds are always going to be
// increasing at least and that the microsecond and nanosecond fields are likely to be non-zero.
export const SystemUTCEpochNanoSeconds: () => JSBI = (() => {
  let ns = JSBI.BigInt(Date.now() % 1e6);
  return () => {
    const ms = JSBI.BigInt(Date.now());
    const result = JSBI.add(JSBI.multiply(ms, MILLION), ns);
    ns = JSBI.remainder(ms, MILLION);
    if (JSBI.greaterThan(result, NS_MAX)) return NS_MAX;
    if (JSBI.lessThan(result, NS_MIN)) return NS_MIN;
    return result;
  };
})();

export function SystemTimeZone() {
  const fmt = new IntlDateTimeFormat('en-us');
  const TemporalTimeZone = GetIntrinsic('%Temporal.TimeZone%');
  return new TemporalTimeZone(ParseTemporalTimeZone(fmt.resolvedOptions().timeZone));
}

export function ComparisonResult(value: number) {
  return value < 0 ? -1 : value > 0 ? 1 : (value as 0);
}

export function GetOptionsObject<T>(options: T) {
  if (options === undefined) return ObjectCreate(null) as NonNullable<T>;
  if (IsObject(options) && options !== null) return options;
  throw new TypeError(`Options parameter must be an object, not ${options === null ? 'null' : `${typeof options}`}`);
}

export function CreateOnePropObject<K extends string, V>(propName: K, propValue: V): { [k in K]: V } {
  const o = ObjectCreate(null);
  o[propName] = propValue;
  return o;
}

type StringlyTypedKeys<T> = Exclude<keyof T, symbol | number>;
function GetOption<P extends StringlyTypedKeys<O>, O extends Partial<Record<P, unknown>>>(
  options: O,
  property: P,
  allowedValues: ReadonlyArray<O[P]>,
  fallback: undefined
): O[P];
function GetOption<
  P extends StringlyTypedKeys<O>,
  O extends Partial<Record<P, unknown>>,
  Fallback extends Required<O>[P] | undefined
>(
  options: O,
  property: P,
  allowedValues: ReadonlyArray<O[P]>,
  fallback: Fallback
): Fallback extends undefined ? O[P] | undefined : Required<O>[P];
function GetOption<
  P extends StringlyTypedKeys<O>,
  O extends Partial<Record<P, unknown>>,
  Fallback extends Required<O>[P] | undefined
>(
  options: O,
  property: P,
  allowedValues: ReadonlyArray<O[P]>,
  fallback: O[P]
): Fallback extends undefined ? O[P] | undefined : Required<O>[P] {
  let value = options[property];
  if (value !== undefined) {
    value = ToString(value) as O[P];
    if (!allowedValues.includes(value)) {
      throw new RangeError(`${property} must be one of ${allowedValues.join(', ')}, not ${value}`);
    }
    return value;
  }
  return fallback;
}

function GetNumberOption<P extends StringlyTypedKeys<T>, T extends Partial<Record<P, unknown>>>(
  options: T,
  property: P,
  minimum: number,
  maximum: number,
  fallback: number
): number {
  let valueRaw = options[property];
  if (valueRaw === undefined) return fallback;
  const value = ToNumber(valueRaw);
  if (NumberIsNaN(value) || value < minimum || value > maximum) {
    throw new RangeError(`${String(property)} must be between ${minimum} and ${maximum}, not ${value}`);
  }
  return MathFloor(value);
}

export function IsBuiltinCalendar(id: string): id is BuiltinCalendarId {
  return ArrayIncludes.call(BUILTIN_CALENDAR_IDS, id);
}

const OFFSET = new RegExp(`^${PARSE.offset.source}$`);

function bisect(
  getState: (epochNs: JSBI) => number,
  leftParam: JSBI,
  rightParam: JSBI,
  lstateParam: number = getState(leftParam),
  rstateParam: number = getState(rightParam)
) {
  // This doesn't make much sense - why do these get converted unnecessarily?
  let left = JSBI.BigInt(leftParam);
  let right = JSBI.BigInt(rightParam);
  let lstate = lstateParam;
  let rstate = rstateParam;
  while (JSBI.greaterThan(JSBI.subtract(right, left), ONE)) {
    const middle = JSBI.divide(JSBI.add(left, right), JSBI.BigInt(2));
    const mstate = getState(middle);
    if (mstate === lstate) {
      left = middle;
      lstate = mstate;
    } else if (mstate === rstate) {
      right = middle;
      rstate = mstate;
    } else {
      throw new Error(`invalid state in bisection ${lstate} - ${mstate} - ${rstate}`);
    }
  }
  return right;
}

const nsPerTimeUnit = {
  hour: 3600e9,
  minute: 60e9,
  second: 1e9,
  millisecond: 1e6,
  microsecond: 1e3,
  nanosecond: 1
};