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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. The trailing byte in the
// section will always be 0x20, or in some implementations 0x00.
// this module encodes and decodes these things.
const encode = exports.encode = (num, buf) => {
buf[buf.length - 1] = 0x20
if (num < 0)
encodeNegative(num, buf)
else
encodePositive(num, buf)
return buf
}
const encodePositive = (num, buf) => {
buf[0] = 0x80
for (var i = buf.length - 2; i > 0; i--) {
if (num === 0)
buf[i] = 0
else {
buf[i] = num % 0x100
num = Math.floor(num / 0x100)
}
}
}
const encodeNegative = (num, buf) => {
buf[0] = 0xff
var flipped = false
num = num * -1
for (var i = buf.length - 2; i > 0; i--) {
var byte
if (num === 0)
byte = 0
else {
byte = num % 0x100
num = Math.floor(num / 0x100)
}
if (flipped)
buf[i] = onesComp(byte)
else if (byte === 0)
buf[i] = 0
else {
flipped = true
buf[i] = twosComp(byte)
}
}
}
const parse = exports.parse = (buf) => {
var post = buf[buf.length - 1]
var pre = buf[0]
return pre === 0x80 ? pos(buf.slice(1, buf.length - 1))
: twos(buf.slice(1, buf.length - 1))
}
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 * -1
}
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
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