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