archy/apps/web5-dwn/node_modules/hamt-sharding/src/consumable-buffer.ts

const START_MASKS = [
  0b11111111,
  0b11111110,
  0b11111100,
  0b11111000,
  0b11110000,
  0b11100000,
  0b11000000,
  0b10000000
]

const STOP_MASKS = [
  0b00000001,
  0b00000011,
  0b00000111,
  0b00001111,
  0b00011111,
  0b00111111,
  0b01111111,
  0b11111111
]

export class ConsumableBuffer {
  _value: Uint8Array
  _currentBytePos: number
  _currentBitPos: number

  constructor (value: Uint8Array) {
    this._value = value
    this._currentBytePos = value.length - 1
    this._currentBitPos = 7
  }

  availableBits (): number {
    return this._currentBitPos + 1 + this._currentBytePos * 8
  }

  totalBits (): number {
    return this._value.length * 8
  }

  take (bits: number): number {
    let pendingBits = bits
    let result = 0
    while (pendingBits > 0 && this._haveBits()) {
      const byte = this._value[this._currentBytePos]
      const availableBits = this._currentBitPos + 1
      const taking = Math.min(availableBits, pendingBits)
      const value = byteBitsToInt(byte, availableBits - taking, taking)
      result = (result << taking) + value

      pendingBits -= taking

      this._currentBitPos -= taking
      if (this._currentBitPos < 0) {
        this._currentBitPos = 7
        this._currentBytePos--
      }
    }

    return result
  }

  untake (bits: number): void {
    this._currentBitPos += bits
    while (this._currentBitPos > 7) {
      this._currentBitPos -= 8
      this._currentBytePos += 1
    }
  }

  _haveBits (): boolean {
    return this._currentBytePos >= 0
  }
}

function byteBitsToInt (byte: number, start: number, length: number): number {
  const mask = maskFor(start, length)
  return (byte & mask) >>> start
}

function maskFor (start: number, length: number): number {
  return START_MASKS[start] & STOP_MASKS[Math.min(length + start - 1, 7)]
}