✏ 编辑: large-numbers.js

'use strict'
// Tar can encode large and negative numbers using a leading byte of
// 0xff for negative, and 0x80 for positive.

const encode = (num, buf) => {
  if (!Number.isSafeInteger(num))
    // The number is so large that javascript cannot represent it with integer
    // precision.
    throw Error('cannot encode number outside of javascript safe integer range')
  else if (num < 0)
    encodeNegative(num, buf)
  else
    encodePositive(num, buf)
  return buf
}

const encodePositive = (num, buf) => {
  buf[0] = 0x80

for (var i = buf.length; i > 1; i--) {
    buf[i - 1] = num & 0xff
    num = Math.floor(num / 0x100)
  }
}

const encodeNegative = (num, buf) => {
  buf[0] = 0xff
  var flipped = false
  num = num * -1
  for (var i = buf.length; i > 1; i--) {
    var byte = num & 0xff
    num = Math.floor(num / 0x100)
    if (flipped)
      buf[i - 1] = onesComp(byte)
    else if (byte === 0)
      buf[i - 1] = 0
    else {
      flipped = true
      buf[i - 1] = twosComp(byte)
    }
  }
}

const parse = (buf) => {
  const pre = buf[0]
  const value = pre === 0x80 ? pos(buf.slice(1, buf.length))
    : pre === 0xff ? twos(buf)
    : null
  if (value === null)
    throw Error('invalid base256 encoding')

if (!Number.isSafeInteger(value))
    // The number is so large that javascript cannot represent it with integer
    // precision.
    throw Error('parsed number outside of javascript safe integer range')

return value
}

const twos = (buf) => {
  var len = buf.length
  var sum = 0
  var flipped = false
  for (var i = len - 1; i > -1; i--) {
    var byte = buf[i]
    var f
    if (flipped)
      f = onesComp(byte)
    else if (byte === 0)
      f = byte
    else {
      flipped = true
      f = twosComp(byte)
    }
    if (f !== 0)
      sum -= f * Math.pow(256, len - i - 1)
  }
  return sum
}

const pos = (buf) => {
  var len = buf.length
  var sum = 0
  for (var i = len - 1; i > -1; i--) {
    var byte = buf[i]
    if (byte !== 0)
      sum += byte * Math.pow(256, len - i - 1)
  }
  return sum
}

const onesComp = byte => (0xff ^ byte) & 0xff

const twosComp = byte => ((0xff ^ byte) + 1) & 0xff

module.exports = {
  encode,
  parse,
}

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