'use strict';

/**

* Implementation of atob() according to the HTML spec, except that instead of
* throwing INVALID_CHARACTER_ERR we return null.
*/

function atob(input) {

// WebIDL requires DOMStrings to just be converted using ECMAScript
// ToString, which in our case amounts to calling String().
input = String(input);
// "Remove all space characters from input."
input = input.replace(/[ \t\n\f\r]/g, '');
// "If the length of input divides by 4 leaving no remainder, then: if
// input ends with one or two U+003D EQUALS SIGN (=) characters, remove
// them from input."
if (input.length % 4 == 0 && /==?$/.test(input)) {
  input = input.replace(/==?$/, '');
}
// "If the length of input divides by 4 leaving a remainder of 1, throw an
// INVALID_CHARACTER_ERR exception and abort these steps."
//
// "If input contains a character that is not in the following list of
// characters and character ranges, throw an INVALID_CHARACTER_ERR
// exception and abort these steps:
//
// U+002B PLUS SIGN (+)
// U+002F SOLIDUS (/)
// U+0030 DIGIT ZERO (0) to U+0039 DIGIT NINE (9)
// U+0041 LATIN CAPITAL LETTER A to U+005A LATIN CAPITAL LETTER Z
// U+0061 LATIN SMALL LETTER A to U+007A LATIN SMALL LETTER Z"
if (input.length % 4 == 1 || !/^[+/0-9A-Za-z]*$/.test(input)) {
  return null;
}
// "Let output be a string, initially empty."
var output = '';
// "Let buffer be a buffer that can have bits appended to it, initially
// empty."
//
// We append bits via left-shift and or.  accumulatedBits is used to track
// when we've gotten to 24 bits.
var buffer = 0;
var accumulatedBits = 0;
// "While position does not point past the end of input, run these
// substeps:"
for (var i = 0; i < input.length; i++) {
  // "Find the character pointed to by position in the first column of
  // the following table. Let n be the number given in the second cell of
  // the same row."
  //
  // "Append to buffer the six bits corresponding to number, most
  // significant bit first."
  //
  // atobLookup() implements the table from the spec.
  buffer <<= 6;
  buffer |= atobLookup(input[i]);
  // "If buffer has accumulated 24 bits, interpret them as three 8-bit
  // big-endian numbers. Append the three characters with code points
  // equal to those numbers to output, in the same order, and then empty
  // buffer."
  accumulatedBits += 6;
  if (accumulatedBits == 24) {
    output += String.fromCharCode((buffer & 0xff0000) >> 16);
    output += String.fromCharCode((buffer & 0xff00) >> 8);
    output += String.fromCharCode(buffer & 0xff);
    buffer = accumulatedBits = 0;
  }
  // "Advance position by one character."
}
// "If buffer is not empty, it contains either 12 or 18 bits. If it
// contains 12 bits, discard the last four and interpret the remaining
// eight as an 8-bit big-endian number. If it contains 18 bits, discard the
// last two and interpret the remaining 16 as two 8-bit big-endian numbers.
// Append the one or two characters with code points equal to those one or
// two numbers to output, in the same order."
if (accumulatedBits == 12) {
  buffer >>= 4;
  output += String.fromCharCode(buffer);
} else if (accumulatedBits == 18) {
  buffer >>= 2;
  output += String.fromCharCode((buffer & 0xff00) >> 8);
  output += String.fromCharCode(buffer & 0xff);
}
// "Return output."
return output;

} /**

* A lookup table for atob(), which converts an ASCII character to the
* corresponding six-bit number.
*/

function atobLookup(chr) {

if (/[A-Z]/.test(chr)) {
  return chr.charCodeAt(0) - 'A'.charCodeAt(0);
}
if (/[a-z]/.test(chr)) {
  return chr.charCodeAt(0) - 'a'.charCodeAt(0) + 26;
}
if (/[0-9]/.test(chr)) {
  return chr.charCodeAt(0) - '0'.charCodeAt(0) + 52;
}
if (chr == '+') {
  return 62;
}
if (chr == '/') {
  return 63;
}
// Throw exception; should not be hit in tests

}

module.exports = atob;