decode = function (charCode) {
  var bigA = 65;     // 'A'
  var bigZ = 90;     // 'Z'

  var littleA = 97;  // 'a'
  var littleZ = 122; // 'z'

  var zero = 48;     // '0'
  var nine = 57;     // '9'

  var plus = 43;     // '+'
  var slash = 47;    // '/'

  var littleOffset = 26;
  var numberOffset = 52;

  // 0 - 25: ABCDEFGHIJKLMNOPQRSTUVWXYZ
  if (bigA <= charCode && charCode <= bigZ) {
    return (charCode - bigA);
  }

  // 26 - 51: abcdefghijklmnopqrstuvwxyz
  if (littleA <= charCode && charCode <= littleZ) {
    return (charCode - littleA + littleOffset);
  }

  // 52 - 61: 0123456789
  if (zero <= charCode && charCode <= nine) {
    return (charCode - zero + numberOffset);
  }

  // 62: +
  if (charCode == plus) {
    return 62;
  }

  // 63: /
  if (charCode == slash) {
    return 63;
  }

  // Invalid base64 digit.
  return -1;
}
Decode a single base 64 character code digit to an integer. Returns -1 on
failure.