实现base64解码

function base64_decode(data) {
  var b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
  var o1,
    o2,
    o3,
    h1,
    h2,
    h3,
    h4,
    bits,
    i = 0,
    ac = 0,
    dec = "",
    tmp_arr = [];
  if (!data) {
    return data;
  }
  data += "";
  do {
    h1 = b64.indexOf(data.charAt(i++));
    h2 = b64.indexOf(data.charAt(i++));
    h3 = b64.indexOf(data.charAt(i++));
    h4 = b64.indexOf(data.charAt(i++));
    bits = (h1 << 18) | (h2 << 12) | (h3 << 6) | h4;
    o1 = (bits >> 16) & 0xff;
    o2 = (bits >> 8) & 0xff;
    o3 = bits & 0xff;
    if (h3 == 64) {
      tmp_arr[ac++] = String.fromCharCode(o1);
    } else if (h4 == 64) {
      tmp_arr[ac++] = String.fromCharCode(o1, o2);
    } else {
      tmp_arr[ac++] = String.fromCharCode(o1, o2, o3);
    }
  } while (i < data.length);
  dec = tmp_arr.join("");
  dec = utf8_decode(dec);
  return dec;
}