100 lines
2.1 KiB
JavaScript
100 lines
2.1 KiB
JavaScript
![]() |
'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,
|
||
|
}
|