110 lines
3.9 KiB
JavaScript
110 lines
3.9 KiB
JavaScript
'use strict';
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/**
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* Implementation of atob() according to the HTML spec, except that instead of
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* throwing INVALID_CHARACTER_ERR we return null.
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*/
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function atob(input) {
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// WebIDL requires DOMStrings to just be converted using ECMAScript
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// ToString, which in our case amounts to calling String().
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input = String(input);
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// "Remove all space characters from input."
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input = input.replace(/[ \t\n\f\r]/g, '');
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// "If the length of input divides by 4 leaving no remainder, then: if
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// input ends with one or two U+003D EQUALS SIGN (=) characters, remove
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// them from input."
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if (input.length % 4 == 0 && /==?$/.test(input)) {
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input = input.replace(/==?$/, '');
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}
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// "If the length of input divides by 4 leaving a remainder of 1, throw an
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// INVALID_CHARACTER_ERR exception and abort these steps."
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//
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// "If input contains a character that is not in the following list of
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// characters and character ranges, throw an INVALID_CHARACTER_ERR
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// exception and abort these steps:
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//
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// U+002B PLUS SIGN (+)
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// U+002F SOLIDUS (/)
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// U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9)
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// U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z
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// U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z"
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if (input.length % 4 == 1 || !/^[+/0-9A-Za-z]*$/.test(input)) {
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return null;
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}
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// "Let output be a string, initially empty."
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var output = '';
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// "Let buffer be a buffer that can have bits appended to it, initially
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// empty."
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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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var buffer = 0;
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var accumulatedBits = 0;
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// "While position does not point past the end of input, run these
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// substeps:"
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for (var i = 0; i < input.length; i++) {
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// "Find the character pointed to by position in the first column of
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// the following table. Let n be the number given in the second cell of
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// the same row."
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//
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// "Append to buffer the six bits corresponding to number, most
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// significant bit first."
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//
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// atobLookup() implements the table from the spec.
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buffer <<= 6;
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buffer |= atobLookup(input[i]);
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// "If buffer has accumulated 24 bits, interpret them as three 8-bit
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// big-endian numbers. Append the three characters with code points
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// equal to those numbers to output, in the same order, and then empty
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// buffer."
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accumulatedBits += 6;
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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 one character."
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}
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// "If buffer is not empty, it contains either 12 or 18 bits. If it
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// contains 12 bits, discard the last four and interpret the remaining
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// eight as an 8-bit big-endian number. If it contains 18 bits, discard the
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// last two and interpret the remaining 16 as two 8-bit big-endian numbers.
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// Append the one or two characters with code points equal to those one or
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// two numbers to 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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}
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module.exports = atob;
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