ascii85.js

String.prototype.bytes = function () {
  var bytes = [], i = 0;
  while ( true ) {
    var byte = this.charCodeAt(i);
    if ( isNaN(byte) ) break;
    bytes.push(byte);
    i += 1;
  }
  return bytes;
}

String.prototype.toAscii85 = function () {
  var encode_base85_block = function (block) {
    var padding_length = 0, bytes = [];

    for ( var i = 0; i < 4; ++i ) {
      if ( block[i] === undefined ) {
        block[i] = 0;
        padding_length += 1;
      }
    }

    if ( padding_length === 0 && block.every(function (v) { return v === 0; }) ) {
      return [122];
    }

    var n = (block[0] << 24) + (block[1] << 16) + (block[2] << 8) + block[3];
    while ( n < 0 ) {
      n += 4294967296;
    }

    while ( true ) {
      var new_n = Math.floor(n / 85);
      bytes.push(n - new_n * 85 + 33);
      n = new_n;
      if ( bytes.length === 5 ) break;
    }

    return bytes.reverse().slice(0, 5 - padding_length);
  }, bytes = this.bytes(), result_bytes = [60, 126];

  for ( var i = 0, n = bytes.length ; i < n; i = i + 4 ) {
    result_bytes = result_bytes.concat(encode_base85_block([bytes[i], bytes[i + 1], bytes[i + 2], bytes[i + 3]]));
  }
  result_bytes.push(126);
  result_bytes.push(62);

  return result_bytes.map(function (v) { return String.fromCharCode(v); }).join('');
};

String.prototype.fromAscii85 = function () {
  var decode_base85_block = function (block) {
    var padding_length = 0;
    for ( var i = 0; i < 5; ++i ) {
      if ( block[i] === undefined ) {
        block[i] = 117;
        padding_length += 1;
      }
    }

    var n = block.reduce(function (m, v, index) {
      return m + (v - 33) * Math.pow(85, 4 - index);
    }, 0);

    var bytes = [];
    bytes.push(n >> 24)
    bytes.push((n & 0xffffff) >> 16)
    bytes.push((n & 0xffff) >> 8)
    bytes.push(n & 0xff)

    bytes = bytes.map(function (v) {
      while ( v < 0 ) {
        v += 256;
      }
      return v;
    });

    return bytes.slice(0, 4 - padding_length);
  }

  var subject = this.replace(/\s/g, '').replace(/^<~/, '').replace(/~>$/, '').replace(/z/g, '!!!!!');
  var result_bytes = [];
  var bytes = subject.bytes();
  for ( var i = 0, n = bytes.length; i < n; i += 5 ) {
    var block = [bytes[i], bytes[i + 1], bytes[i + 2], bytes[i + 3], bytes[i + 4]];
    result_bytes = result_bytes.concat(decode_base85_block(block));
  }

  return result_bytes.map(function (v) { return String.fromCharCode(v); }).join('');
};