var b64re = /^(?:[A-Za-z\d+\/]{4})*?(?:[A-Za-z\d+\/]{2}(?:==)?|[A-Za-z\d+\/]{3}=?)?$/;
var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

module.exports = function(string) {
  // atob can work with strings with whitespaces, even inside the encoded part,
  // but only \t, \n, \f, \r and ' ', which can be stripped.
  string = String(string).replace(/[\t\n\f\r ]+/g, "");
  if (!b64re.test(string))
      throw new TypeError("Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.");

  // Adding the padding if missing, for semplicity
  string += "==".slice(2 - (string.length & 3));
  var bitmap, result = "",
      r1, r2, i = 0;
  for (; i < string.length;) {
      bitmap = b64.indexOf(string.charAt(i++)) << 18 | b64.indexOf(string.charAt(i++)) << 12 |
          (r1 = b64.indexOf(string.charAt(i++))) << 6 | (r2 = b64.indexOf(string.charAt(i++)));

      result += r1 === 64 ? String.fromCharCode(bitmap >> 16 & 255) :
          r2 === 64 ? String.fromCharCode(bitmap >> 16 & 255, bitmap >> 8 & 255) :
          String.fromCharCode(bitmap >> 16 & 255, bitmap >> 8 & 255, bitmap & 255);
  }
  return result;
};