react-day-picker/src/helpers/calculateFocusTarget.ts

import { DayFlag } from "../UI.js";
import type { CalendarDay } from "../classes/index.js";
import type { Modifiers } from "../types/index.js";

export function calculateFocusTarget(
  days: CalendarDay[],
  getModifiers: (day: CalendarDay) => Modifiers,
  isSelected: (date: Date) => boolean,
  lastFocused: CalendarDay | undefined
) {
  let focusTarget: CalendarDay | undefined;

  let index = 0;
  let found = false;

  while (index < days.length && !found) {
    const day = days[index];
    const modifiers = getModifiers(day);

    if (
      !modifiers[DayFlag.disabled] &&
      !modifiers[DayFlag.hidden] &&
      !modifiers[DayFlag.outside]
    ) {
      if (modifiers[DayFlag.focused]) {
        focusTarget = day;
        found = true;
      } else if (lastFocused?.isEqualTo(day)) {
        focusTarget = day;
        found = true;
      } else if (isSelected(day.date)) {
        focusTarget = day;
        found = true;
      } else if (modifiers[DayFlag.today]) {
        focusTarget = day;
        found = true;
      }
    }

    index++;
  }

  if (!focusTarget) {
    // return the first day that is focusable
    focusTarget = days.find((day) => {
      const m = getModifiers(day);
      return !m[DayFlag.disabled] && !m[DayFlag.hidden] && !m[DayFlag.outside];
    });
  }
  return focusTarget;
}