lib0/decoding.js

/**
 * Efficient schema-less binary decoding with support for variable length encoding.
 *
 * Use [lib0/decoding] with [lib0/encoding]. Every encoding function has a corresponding decoding function.
 *
 * Encodes numbers in little-endian order (least to most significant byte order)
 * and is compatible with Golang's binary encoding (https://golang.org/pkg/encoding/binary/)
 * which is also used in Protocol Buffers.
 *
 * ```js
 * // encoding step
 * const encoder = new encoding.createEncoder()
 * encoding.writeVarUint(encoder, 256)
 * encoding.writeVarString(encoder, 'Hello world!')
 * const buf = encoding.toUint8Array(encoder)
 * ```
 *
 * ```js
 * // decoding step
 * const decoder = new decoding.createDecoder(buf)
 * decoding.readVarUint(decoder) // => 256
 * decoding.readVarString(decoder) // => 'Hello world!'
 * decoding.hasContent(decoder) // => false - all data is read
 * ```
 *
 * @module decoding
 */

import * as buffer from './buffer.js'
import * as binary from './binary.js'
import * as math from './math.js'
import * as number from './number.js'
import * as string from './string.js'

/**
 * A Decoder handles the decoding of an Uint8Array.
 */
export class Decoder {
  /**
   * @param {Uint8Array} uint8Array Binary data to decode
   */
  constructor (uint8Array) {
    /**
     * Decoding target.
     *
     * @type {Uint8Array}
     */
    this.arr = uint8Array
    /**
     * Current decoding position.
     *
     * @type {number}
     */
    this.pos = 0
  }
}

/**
 * @function
 * @param {Uint8Array} uint8Array
 * @return {Decoder}
 */
export const createDecoder = uint8Array => new Decoder(uint8Array)

/**
 * @function
 * @param {Decoder} decoder
 * @return {boolean}
 */
export const hasContent = decoder => decoder.pos !== decoder.arr.length

/**
 * Clone a decoder instance.
 * Optionally set a new position parameter.
 *
 * @function
 * @param {Decoder} decoder The decoder instance
 * @param {number} [newPos] Defaults to current position
 * @return {Decoder} A clone of `decoder`
 */
export const clone = (decoder, newPos = decoder.pos) => {
  const _decoder = createDecoder(decoder.arr)
  _decoder.pos = newPos
  return _decoder
}

/**
 * Create an Uint8Array view of the next `len` bytes and advance the position by `len`.
 *
 * Important: The Uint8Array still points to the underlying ArrayBuffer. Make sure to discard the result as soon as possible to prevent any memory leaks.
 *            Use `buffer.copyUint8Array` to copy the result into a new Uint8Array.
 *
 * @function
 * @param {Decoder} decoder The decoder instance
 * @param {number} len The length of bytes to read
 * @return {Uint8Array}
 */
export const readUint8Array = (decoder, len) => {
  const view = buffer.createUint8ArrayViewFromArrayBuffer(decoder.arr.buffer, decoder.pos + decoder.arr.byteOffset, len)
  decoder.pos += len
  return view
}

/**
 * Read variable length Uint8Array.
 *
 * Important: The Uint8Array still points to the underlying ArrayBuffer. Make sure to discard the result as soon as possible to prevent any memory leaks.
 *            Use `buffer.copyUint8Array` to copy the result into a new Uint8Array.
 *
 * @function
 * @param {Decoder} decoder
 * @return {Uint8Array}
 */
export const readVarUint8Array = decoder => readUint8Array(decoder, readVarUint(decoder))

/**
 * Read the rest of the content as an ArrayBuffer
 * @function
 * @param {Decoder} decoder
 * @return {Uint8Array}
 */
export const readTailAsUint8Array = decoder => readUint8Array(decoder, decoder.arr.length - decoder.pos)

/**
 * Skip one byte, jump to the next position.
 * @function
 * @param {Decoder} decoder The decoder instance
 * @return {number} The next position
 */
export const skip8 = decoder => decoder.pos++

/**
 * Read one byte as unsigned integer.
 * @function
 * @param {Decoder} decoder The decoder instance
 * @return {number} Unsigned 8-bit integer
 */
export const readUint8 = decoder => decoder.arr[decoder.pos++]

/**
 * Read 2 bytes as unsigned integer.
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.
 */
export const readUint16 = decoder => {
  const uint =
    decoder.arr[decoder.pos] +
    (decoder.arr[decoder.pos + 1] << 8)
  decoder.pos += 2
  return uint
}

/**
 * Read 4 bytes as unsigned integer.
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.
 */
export const readUint32 = decoder => {
  const uint =
    (decoder.arr[decoder.pos] +
    (decoder.arr[decoder.pos + 1] << 8) +
    (decoder.arr[decoder.pos + 2] << 16) +
    (decoder.arr[decoder.pos + 3] << 24)) >>> 0
  decoder.pos += 4
  return uint
}

/**
 * Read 4 bytes as unsigned integer in big endian order.
 * (most significant byte first)
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.
 */
export const readUint32BigEndian = decoder => {
  const uint =
    (decoder.arr[decoder.pos + 3] +
    (decoder.arr[decoder.pos + 2] << 8) +
    (decoder.arr[decoder.pos + 1] << 16) +
    (decoder.arr[decoder.pos] << 24)) >>> 0
  decoder.pos += 4
  return uint
}

/**
 * Look ahead without incrementing the position
 * to the next byte and read it as unsigned integer.
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.
 */
export const peekUint8 = decoder => decoder.arr[decoder.pos]

/**
 * Look ahead without incrementing the position
 * to the next byte and read it as unsigned integer.
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.
 */
export const peekUint16 = decoder =>
  decoder.arr[decoder.pos] +
  (decoder.arr[decoder.pos + 1] << 8)

/**
 * Look ahead without incrementing the position
 * to the next byte and read it as unsigned integer.
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.
 */
export const peekUint32 = decoder => (
  decoder.arr[decoder.pos] +
  (decoder.arr[decoder.pos + 1] << 8) +
  (decoder.arr[decoder.pos + 2] << 16) +
  (decoder.arr[decoder.pos + 3] << 24)
) >>> 0

/**
 * Read unsigned integer (32bit) with variable length.
 * 1/8th of the storage is used as encoding overhead.
 *  * numbers < 2^7 is stored in one bytlength
 *  * numbers < 2^14 is stored in two bylength
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.length
 */
export const readVarUint = decoder => {
  let num = 0
  let mult = 1
  while (true) {
    const r = decoder.arr[decoder.pos++]
    // num = num | ((r & binary.BITS7) << len)
    num = num + (r & binary.BITS7) * mult // shift $r << (7*#iterations) and add it to num
    mult *= 128 // next iteration, shift 7 "more" to the left
    if (r < binary.BIT8) {
      return num
    }
    /* istanbul ignore if */
    if (num > number.MAX_SAFE_INTEGER) {
      throw new Error('Integer out of range!')
    }
  }
}

/**
 * Read signed integer (32bit) with variable length.
 * 1/8th of the storage is used as encoding overhead.
 *  * numbers < 2^7 is stored in one bytlength
 *  * numbers < 2^14 is stored in two bylength
 * @todo This should probably create the inverse ~num if number is negative - but this would be a breaking change.
 *
 * @function
 * @param {Decoder} decoder
 * @return {number} An unsigned integer.length
 */
export const readVarInt = decoder => {
  let r = decoder.arr[decoder.pos++]
  let num = r & binary.BITS6
  let mult = 64
  const sign = (r & binary.BIT7) > 0 ? -1 : 1
  if ((r & binary.BIT8) === 0) {
    // don't continue reading
    return sign * num
  }
  while (true) {
    r = decoder.arr[decoder.pos++]
    // num = num | ((r & binary.BITS7) << len)
    num = num + (r & binary.BITS7) * mult
    mult *= 128
    if (r < binary.BIT8) {
      return sign * num
    }
    /* istanbul ignore if */
    if (num > number.MAX_SAFE_INTEGER) {
      throw new Error('Integer out of range!')
    }
  }
}

/**
 * Look ahead and read varUint without incrementing position
 *
 * @function
 * @param {Decoder} decoder
 * @return {number}
 */
export const peekVarUint = decoder => {
  const pos = decoder.pos
  const s = readVarUint(decoder)
  decoder.pos = pos
  return s
}

/**
 * Look ahead and read varUint without incrementing position
 *
 * @function
 * @param {Decoder} decoder
 * @return {number}
 */
export const peekVarInt = decoder => {
  const pos = decoder.pos
  const s = readVarInt(decoder)
  decoder.pos = pos
  return s
}

/**
 * We don't test this function anymore as we use native decoding/encoding by default now.
 * Better not modify this anymore..
 *
 * Transforming utf8 to a string is pretty expensive. The code performs 10x better
 * when String.fromCodePoint is fed with all characters as arguments.
 * But most environments have a maximum number of arguments per functions.
 * For effiency reasons we apply a maximum of 10000 characters at once.
 *
 * @function
 * @param {Decoder} decoder
 * @return {String} The read String.
 */
/* istanbul ignore next */
export const _readVarStringPolyfill = decoder => {
  let remainingLen = readVarUint(decoder)
  if (remainingLen === 0) {
    return ''
  } else {
    let encodedString = String.fromCodePoint(readUint8(decoder)) // remember to decrease remainingLen
    if (--remainingLen < 100) { // do not create a Uint8Array for small strings
      while (remainingLen--) {
        encodedString += String.fromCodePoint(readUint8(decoder))
      }
    } else {
      while (remainingLen > 0) {
        const nextLen = remainingLen < 10000 ? remainingLen : 10000
        // this is dangerous, we create a fresh array view from the existing buffer
        const bytes = decoder.arr.subarray(decoder.pos, decoder.pos + nextLen)
        decoder.pos += nextLen
        // Starting with ES5.1 we can supply a generic array-like object as arguments
        encodedString += String.fromCodePoint.apply(null, /** @type {any} */ (bytes))
        remainingLen -= nextLen
      }
    }
    return decodeURIComponent(escape(encodedString))
  }
}

/**
 * @function
 * @param {Decoder} decoder
 * @return {String} The read String
 */
export const _readVarStringNative = decoder =>
  /** @type any */ (string.utf8TextDecoder).decode(readVarUint8Array(decoder))

/**
 * Read string of variable length
 * * varUint is used to store the length of the string
 *
 * @function
 * @param {Decoder} decoder
 * @return {String} The read String
 *
 */
/* istanbul ignore next */
export const readVarString = string.utf8TextDecoder ? _readVarStringNative : _readVarStringPolyfill

/**
 * Look ahead and read varString without incrementing position
 *
 * @function
 * @param {Decoder} decoder
 * @return {string}
 */
export const peekVarString = decoder => {
  const pos = decoder.pos
  const s = readVarString(decoder)
  decoder.pos = pos
  return s
}

/**
 * @param {Decoder} decoder
 * @param {number} len
 * @return {DataView}
 */
export const readFromDataView = (decoder, len) => {
  const dv = new DataView(decoder.arr.buffer, decoder.arr.byteOffset + decoder.pos, len)
  decoder.pos += len
  return dv
}

/**
 * @param {Decoder} decoder
 */
export const readFloat32 = decoder => readFromDataView(decoder, 4).getFloat32(0, false)

/**
 * @param {Decoder} decoder
 */
export const readFloat64 = decoder => readFromDataView(decoder, 8).getFloat64(0, false)

/**
 * @param {Decoder} decoder
 */
export const readBigInt64 = decoder => /** @type {any} */ (readFromDataView(decoder, 8)).getBigInt64(0, false)

/**
 * @param {Decoder} decoder
 */
export const readBigUint64 = decoder => /** @type {any} */ (readFromDataView(decoder, 8)).getBigUint64(0, false)

/**
 * @type {Array<function(Decoder):any>}
 */
const readAnyLookupTable = [
  decoder => undefined, // CASE 127: undefined
  decoder => null, // CASE 126: null
  readVarInt, // CASE 125: integer
  readFloat32, // CASE 124: float32
  readFloat64, // CASE 123: float64
  readBigInt64, // CASE 122: bigint
  decoder => false, // CASE 121: boolean (false)
  decoder => true, // CASE 120: boolean (true)
  readVarString, // CASE 119: string
  decoder => { // CASE 118: object<string,any>
    const len = readVarUint(decoder)
    /**
     * @type {Object<string,any>}
     */
    const obj = {}
    for (let i = 0; i < len; i++) {
      const key = readVarString(decoder)
      obj[key] = readAny(decoder)
    }
    return obj
  },
  decoder => { // CASE 117: array<any>
    const len = readVarUint(decoder)
    const arr = []
    for (let i = 0; i < len; i++) {
      arr.push(readAny(decoder))
    }
    return arr
  },
  readVarUint8Array // CASE 116: Uint8Array
]

/**
 * @param {Decoder} decoder
 */
export const readAny = decoder => readAnyLookupTable[127 - readUint8(decoder)](decoder)

/**
 * T must not be null.
 *
 * @template T
 */
export class RleDecoder extends Decoder {
  /**
   * @param {Uint8Array} uint8Array
   * @param {function(Decoder):T} reader
   */
  constructor (uint8Array, reader) {
    super(uint8Array)
    /**
     * The reader
     */
    this.reader = reader
    /**
     * Current state
     * @type {T|null}
     */
    this.s = null
    this.count = 0
  }

  read () {
    if (this.count === 0) {
      this.s = this.reader(this)
      if (hasContent(this)) {
        this.count = readVarUint(this) + 1 // see encoder implementation for the reason why this is incremented
      } else {
        this.count = -1 // read the current value forever
      }
    }
    this.count--
    return /** @type {T} */ (this.s)
  }
}

export class IntDiffDecoder extends Decoder {
  /**
   * @param {Uint8Array} uint8Array
   * @param {number} start
   */
  constructor (uint8Array, start) {
    super(uint8Array)
    /**
     * Current state
     * @type {number}
     */
    this.s = start
  }

  /**
   * @return {number}
   */
  read () {
    this.s += readVarInt(this)
    return this.s
  }
}

export class RleIntDiffDecoder extends Decoder {
  /**
   * @param {Uint8Array} uint8Array
   * @param {number} start
   */
  constructor (uint8Array, start) {
    super(uint8Array)
    /**
     * Current state
     * @type {number}
     */
    this.s = start
    this.count = 0
  }

  /**
   * @return {number}
   */
  read () {
    if (this.count === 0) {
      this.s += readVarInt(this)
      if (hasContent(this)) {
        this.count = readVarUint(this) + 1 // see encoder implementation for the reason why this is incremented
      } else {
        this.count = -1 // read the current value forever
      }
    }
    this.count--
    return /** @type {number} */ (this.s)
  }
}

export class UintOptRleDecoder extends Decoder {
  /**
   * @param {Uint8Array} uint8Array
   */
  constructor (uint8Array) {
    super(uint8Array)
    /**
     * @type {number}
     */
    this.s = 0
    this.count = 0
  }

  read () {
    if (this.count === 0) {
      this.s = readVarInt(this)
      // if the sign is negative, we read the count too, otherwise count is 1
      const isNegative = math.isNegativeZero(this.s)
      this.count = 1
      if (isNegative) {
        this.s = -this.s
        this.count = readVarUint(this) + 2
      }
    }
    this.count--
    return /** @type {number} */ (this.s)
  }
}

export class IncUintOptRleDecoder extends Decoder {
  /**
   * @param {Uint8Array} uint8Array
   */
  constructor (uint8Array) {
    super(uint8Array)
    /**
     * @type {number}
     */
    this.s = 0
    this.count = 0
  }

  read () {
    if (this.count === 0) {
      this.s = readVarInt(this)
      // if the sign is negative, we read the count too, otherwise count is 1
      const isNegative = math.isNegativeZero(this.s)
      this.count = 1
      if (isNegative) {
        this.s = -this.s
        this.count = readVarUint(this) + 2
      }
    }
    this.count--
    return /** @type {number} */ (this.s++)
  }
}

export class IntDiffOptRleDecoder extends Decoder {
  /**
   * @param {Uint8Array} uint8Array
   */
  constructor (uint8Array) {
    super(uint8Array)
    /**
     * @type {number}
     */
    this.s = 0
    this.count = 0
    this.diff = 0
  }

  /**
   * @return {number}
   */
  read () {
    if (this.count === 0) {
      const diff = readVarInt(this)
      // if the first bit is set, we read more data
      const hasCount = diff & 1
      this.diff = math.floor(diff / 2) // shift >> 1
      this.count = 1
      if (hasCount) {
        this.count = readVarUint(this) + 2
      }
    }
    this.s += this.diff
    this.count--
    return this.s
  }
}

export class StringDecoder {
  /**
   * @param {Uint8Array} uint8Array
   */
  constructor (uint8Array) {
    this.decoder = new UintOptRleDecoder(uint8Array)
    this.str = readVarString(this.decoder)
    /**
     * @type {number}
     */
    this.spos = 0
  }

  /**
   * @return {string}
   */
  read () {
    const end = this.spos + this.decoder.read()
    const res = this.str.slice(this.spos, end)
    this.spos = end
    return res
  }
}