JavaScript实现的encode64加密算法实例分析

//encode64编解码
(function() {
 var codeChar = "PaAwO65goUf7IK2vi9-xq8cFTEXLCDY1Hd3tV0ryzjbpN_BlnSs4mGRkQWMZJeuh";
 window.encode64 = function(str) {
  var s = "";
  var a = strToBytes(str);
  //取得字串的字节数组, 数组长度是字串长度的2倍.
  var res = a.length % 3;
  //3个字节一组进行处理, 余下特殊处理
  var i = 2, v;
  for (; i < a.length; i += 3) {
  //每3个字节用4个字符表示, 
  //相当于3个字符(实际上是6个字节)用8个字符编码(实际为16个字节)
  //看起来容量膨胀了很多, 但是在启用压缩的情况下, 这些又被抵消掉了
   v = a[i - 2] + (a[i - 1] << 8) + (a[i] << 16);
   s += codeChar.charAt(v & 0x3f);
   s += codeChar.charAt((v  6) & 0x3f);
   s += codeChar.charAt((v  12) & 0x3f);
   s += codeChar.charAt((v  18));
  }
  if (res == 1) {//字节余一位时候, 补2个字符, 64*64>256
   v = a[i - 2];
   s += codeChar.charAt(v & 0x3f);
   s += codeChar.charAt((v  6) & 0x3f);
  } else if (res == 2) {
  //字节余2位的时候, 补3个字节, 64*64*64>256*256, 所以是可行的
   v = a[i - 2] + (a[i - 1] << 8);
   s += codeChar.charAt(v & 0x3f);
   s += codeChar.charAt((v  6) & 0x3f);
   s += codeChar.charAt((v  12) & 0x3f);
  }
  return s;
 };
 window.decode64 = function(codeStr) {
  var dic = [];
  for (var i = 0; i < codeChar.length; i++) {
   dic[codeChar.charAt(i)] = i;
  }
  var code = [];
  var res = codeStr.length % 4;
  var i = 3, v;
  for (; i < codeStr.length; i += 4) {
   v = dic[codeStr.charAt(i - 3)];
   v += dic[codeStr.charAt(i - 2)] << 6;
   v += dic[codeStr.charAt(i - 1)] << 12;
   v += dic[codeStr.charAt(i)] << 18;
   code.push(v & 0xff, (v  8) & 0xff, (v  16) & 0xff);
  }
  if (res == 2) {
  //正确的字节数肯定是余2或3, 没有1的情况, 如果出现, 舍弃.
   v = dic[codeStr.charAt(i - 3)];
   v += dic[codeStr.charAt(i - 2)] << 6;
   code.push(v & 0xff);
  } else if (res == 3) {
   v = dic[codeStr.charAt(i - 3)];
   v += dic[codeStr.charAt(i - 2)] << 6;
   v += dic[codeStr.charAt(i - 1)] << 12;
   code.push(v & 0xff, (v  8) & 0xff);
  }
  return strFromBytes(code);
 };
})();