100 lines
2.1 KiB
JavaScript
100 lines
2.1 KiB
JavaScript
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'use strict'
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// Tar can encode large and negative numbers using a leading byte of
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// 0xff for negative, and 0x80 for positive.
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const encode = (num, buf) => {
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if (!Number.isSafeInteger(num))
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// The number is so large that javascript cannot represent it with integer
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// precision.
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throw Error('cannot encode number outside of javascript safe integer range')
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else if (num < 0)
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encodeNegative(num, buf)
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else
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encodePositive(num, buf)
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return buf
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}
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const encodePositive = (num, buf) => {
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buf[0] = 0x80
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for (var i = buf.length; i > 1; i--) {
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buf[i - 1] = num & 0xff
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num = Math.floor(num / 0x100)
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}
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}
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const encodeNegative = (num, buf) => {
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buf[0] = 0xff
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var flipped = false
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num = num * -1
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for (var i = buf.length; i > 1; i--) {
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var byte = num & 0xff
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num = Math.floor(num / 0x100)
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if (flipped)
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buf[i - 1] = onesComp(byte)
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else if (byte === 0)
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buf[i - 1] = 0
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else {
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flipped = true
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buf[i - 1] = twosComp(byte)
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}
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}
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}
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const parse = (buf) => {
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const pre = buf[0]
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const value = pre === 0x80 ? pos(buf.slice(1, buf.length))
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: pre === 0xff ? twos(buf)
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: null
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if (value === null)
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throw Error('invalid base256 encoding')
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if (!Number.isSafeInteger(value))
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// The number is so large that javascript cannot represent it with integer
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// precision.
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throw Error('parsed number outside of javascript safe integer range')
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return value
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}
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const twos = (buf) => {
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var len = buf.length
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var sum = 0
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var flipped = false
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for (var i = len - 1; i > -1; i--) {
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var byte = buf[i]
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var f
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if (flipped)
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f = onesComp(byte)
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else if (byte === 0)
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f = byte
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else {
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flipped = true
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f = twosComp(byte)
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}
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if (f !== 0)
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sum -= f * Math.pow(256, len - i - 1)
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}
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return sum
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}
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const pos = (buf) => {
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var len = buf.length
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var sum = 0
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for (var i = len - 1; i > -1; i--) {
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var byte = buf[i]
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if (byte !== 0)
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sum += byte * Math.pow(256, len - i - 1)
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}
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return sum
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}
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const onesComp = byte => (0xff ^ byte) & 0xff
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const twosComp = byte => ((0xff ^ byte) + 1) & 0xff
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module.exports = {
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encode,
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parse,
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}
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