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'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,
}