uint8-varint/src/index.ts

Read/write unsigned varints from Uint8Arrays and Uint8ArrayLists

/* eslint-disable no-fallthrough */
import { allocUnsafe } from 'uint8arrays/alloc'
import type { Uint8ArrayList } from 'uint8arraylist'

const N1 = Math.pow(2, 7)
const N2 = Math.pow(2, 14)
const N3 = Math.pow(2, 21)
const N4 = Math.pow(2, 28)
const N5 = Math.pow(2, 35)
const N6 = Math.pow(2, 42)
const N7 = Math.pow(2, 49)

/** Most significant bit of a byte */
const MSB = 0x80
/** Rest of the bits in a byte */
const REST = 0x7f

export function encodingLength (value: number): number {
  if (value < N1) {
    return 1
  }

  if (value < N2) {
    return 2
  }

  if (value < N3) {
    return 3
  }

  if (value < N4) {
    return 4
  }

  if (value < N5) {
    return 5
  }

  if (value < N6) {
    return 6
  }

  if (value < N7) {
    return 7
  }

  if (Number.MAX_SAFE_INTEGER != null && value > Number.MAX_SAFE_INTEGER) {
    throw new RangeError('Could not encode varint')
  }

  return 8
}

export function encodeUint8Array (value: number, buf: Uint8Array, offset: number = 0): Uint8Array {
  switch (encodingLength(value)) {
    case 8: {
      buf[offset++] = (value & 0xFF) | MSB
      value /= 128
    }
    case 7: {
      buf[offset++] = (value & 0xFF) | MSB
      value /= 128
    }
    case 6: {
      buf[offset++] = (value & 0xFF) | MSB
      value /= 128
    }
    case 5: {
      buf[offset++] = (value & 0xFF) | MSB
      value /= 128
    }
    case 4: {
      buf[offset++] = (value & 0xFF) | MSB
      value >>>= 7
    }
    case 3: {
      buf[offset++] = (value & 0xFF) | MSB
      value >>>= 7
    }
    case 2: {
      buf[offset++] = (value & 0xFF) | MSB
      value >>>= 7
    }
    case 1: {
      buf[offset++] = (value & 0xFF)
      value >>>= 7
      break
    }
    default: throw new Error('unreachable')
  }
  return buf
}

export function encodeUint8ArrayList (value: number, buf: Uint8ArrayList, offset: number = 0): Uint8ArrayList {
  switch (encodingLength(value)) {
    case 8: {
      buf.set(offset++, (value & 0xFF) | MSB)
      value /= 128
    }
    case 7: {
      buf.set(offset++, (value & 0xFF) | MSB)
      value /= 128
    }
    case 6: {
      buf.set(offset++, (value & 0xFF) | MSB)
      value /= 128
    }
    case 5: {
      buf.set(offset++, (value & 0xFF) | MSB)
      value /= 128
    }
    case 4: {
      buf.set(offset++, (value & 0xFF) | MSB)
      value >>>= 7
    }
    case 3: {
      buf.set(offset++, (value & 0xFF) | MSB)
      value >>>= 7
    }
    case 2: {
      buf.set(offset++, (value & 0xFF) | MSB)
      value >>>= 7
    }
    case 1: {
      buf.set(offset++, (value & 0xFF))
      value >>>= 7
      break
    }
    default: throw new Error('unreachable')
  }
  return buf
}

export function decodeUint8Array (buf: Uint8Array, offset: number): number {
  let b = buf[offset]
  let res = 0

  res += b & REST
  if (b < MSB) {
    return res
  }

  b = buf[offset + 1]
  res += (b & REST) << 7
  if (b < MSB) {
    return res
  }

  b = buf[offset + 2]
  res += (b & REST) << 14
  if (b < MSB) {
    return res
  }

  b = buf[offset + 3]
  res += (b & REST) << 21
  if (b < MSB) {
    return res
  }

  b = buf[offset + 4]
  res += (b & REST) * N4
  if (b < MSB) {
    return res
  }

  b = buf[offset + 5]
  res += (b & REST) * N5
  if (b < MSB) {
    return res
  }

  b = buf[offset + 6]
  res += (b & REST) * N6
  if (b < MSB) {
    return res
  }

  b = buf[offset + 7]
  res += (b & REST) * N7
  if (b < MSB) {
    return res
  }

  throw new RangeError('Could not decode varint')
}

export function decodeUint8ArrayList (buf: Uint8ArrayList, offset: number): number {
  let b = buf.get(offset)
  let res = 0

  res += b & REST
  if (b < MSB) {
    return res
  }

  b = buf.get(offset + 1)
  res += (b & REST) << 7
  if (b < MSB) {
    return res
  }

  b = buf.get(offset + 2)
  res += (b & REST) << 14
  if (b < MSB) {
    return res
  }

  b = buf.get(offset + 3)
  res += (b & REST) << 21
  if (b < MSB) {
    return res
  }

  b = buf.get(offset + 4)
  res += (b & REST) * N4
  if (b < MSB) {
    return res
  }

  b = buf.get(offset + 5)
  res += (b & REST) * N5
  if (b < MSB) {
    return res
  }

  b = buf.get(offset + 6)
  res += (b & REST) * N6
  if (b < MSB) {
    return res
  }

  b = buf.get(offset + 7)
  res += (b & REST) * N7
  if (b < MSB) {
    return res
  }

  throw new RangeError('Could not decode varint')
}

export function encode (value: number): Uint8Array
export function encode (value: number, buf: Uint8Array, offset?: number): Uint8Array
export function encode (value: number, buf: Uint8ArrayList, offset?: number): Uint8ArrayList
export function encode <T extends Uint8Array | Uint8ArrayList = Uint8Array> (value: number, buf?: T, offset: number = 0): T {
  if (buf == null) {
    buf = allocUnsafe(encodingLength(value)) as T
  }
  if (buf instanceof Uint8Array) {
    return encodeUint8Array(value, buf, offset) as T
  } else {
    return encodeUint8ArrayList(value, buf, offset) as T
  }
}

export function decode (buf: Uint8ArrayList | Uint8Array, offset: number = 0): number {
  if (buf instanceof Uint8Array) {
    return decodeUint8Array(buf, offset)
  } else {
    return decodeUint8ArrayList(buf, offset)
  }
}