108 lines
3.6 KiB
JavaScript
108 lines
3.6 KiB
JavaScript
"use strict";
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/**
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* Implementation of atob() according to the HTML and Infra specs, except that
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* instead of throwing INVALID_CHARACTER_ERR we return null.
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*/
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function atob(data) {
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// Web IDL requires DOMStrings to just be converted using ECMAScript
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// ToString, which in our case amounts to using a template literal.
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data = `${data}`;
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// "Remove all ASCII whitespace from data."
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data = data.replace(/[ \t\n\f\r]/g, "");
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// "If data's length divides by 4 leaving no remainder, then: if data ends
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// with one or two U+003D (=) code points, then remove them from data."
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if (data.length % 4 === 0) {
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data = data.replace(/==?$/, "");
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}
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// "If data's length divides by 4 leaving a remainder of 1, then return
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// failure."
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//
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// "If data contains a code point that is not one of
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//
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// U+002B (+)
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// U+002F (/)
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// ASCII alphanumeric
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//
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// then return failure."
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if (data.length % 4 === 1 || /[^+/0-9A-Za-z]/.test(data)) {
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return null;
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}
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// "Let output be an empty byte sequence."
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let output = "";
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// "Let buffer be an empty buffer that can have bits appended to it."
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//
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// We append bits via left-shift and or. accumulatedBits is used to track
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// when we've gotten to 24 bits.
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let buffer = 0;
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let accumulatedBits = 0;
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// "Let position be a position variable for data, initially pointing at the
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// start of data."
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//
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// "While position does not point past the end of data:"
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for (let i = 0; i < data.length; i++) {
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// "Find the code point pointed to by position in the second column of
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// Table 1: The Base 64 Alphabet of RFC 4648. Let n be the number given in
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// the first cell of the same row.
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//
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// "Append to buffer the six bits corresponding to n, most significant bit
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// first."
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//
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// atobLookup() implements the table from RFC 4648.
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buffer <<= 6;
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buffer |= atobLookup(data[i]);
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accumulatedBits += 6;
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// "If buffer has accumulated 24 bits, interpret them as three 8-bit
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// big-endian numbers. Append three bytes with values equal to those
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// numbers to output, in the same order, and then empty buffer."
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if (accumulatedBits === 24) {
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output += String.fromCharCode((buffer & 0xff0000) >> 16);
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output += String.fromCharCode((buffer & 0xff00) >> 8);
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output += String.fromCharCode(buffer & 0xff);
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buffer = accumulatedBits = 0;
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}
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// "Advance position by 1."
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}
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// "If buffer is not empty, it contains either 12 or 18 bits. If it contains
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// 12 bits, then discard the last four and interpret the remaining eight as
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// an 8-bit big-endian number. If it contains 18 bits, then discard the last
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// two and interpret the remaining 16 as two 8-bit big-endian numbers. Append
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// the one or two bytes with values equal to those one or two numbers to
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// output, in the same order."
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if (accumulatedBits === 12) {
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buffer >>= 4;
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output += String.fromCharCode(buffer);
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} else if (accumulatedBits === 18) {
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buffer >>= 2;
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output += String.fromCharCode((buffer & 0xff00) >> 8);
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output += String.fromCharCode(buffer & 0xff);
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}
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// "Return output."
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return output;
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}
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/**
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* A lookup table for atob(), which converts an ASCII character to the
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* corresponding six-bit number.
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*/
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function atobLookup(chr) {
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if (/[A-Z]/.test(chr)) {
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return chr.charCodeAt(0) - "A".charCodeAt(0);
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}
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if (/[a-z]/.test(chr)) {
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return chr.charCodeAt(0) - "a".charCodeAt(0) + 26;
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}
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if (/[0-9]/.test(chr)) {
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return chr.charCodeAt(0) - "0".charCodeAt(0) + 52;
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}
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if (chr === "+") {
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return 62;
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}
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if (chr === "/") {
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return 63;
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}
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// Throw exception; should not be hit in tests
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return undefined;
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}
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module.exports = atob;
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