//encode64編解碼
(function() {
 var codeChar = "PaAwO65goUf7IK2vi9-xq8cFTEXLCDY1Hd3tV0ryzjbpN_BlnSs4mGRkQWMZJeuh";
 window.encode64 = function(str) {
  var s = "";
  var a = strToBytes(str);
  //取得字串的字節(jié)數(shù)組, 數(shù)組長度是字串長度的2倍.
  var res = a.length % 3;
  //3個(gè)字節(jié)一組進(jìn)行處理, 余下特殊處理
  var i = 2, v;
  for (; i < a.length; i += 3) {
  //每3個(gè)字節(jié)用4個(gè)字符表示, 
  //相當(dāng)于3個(gè)字符(實(shí)際上是6個(gè)字節(jié))用8個(gè)字符編碼(實(shí)際為16個(gè)字節(jié))
  //看起來容量膨脹了很多, 但是在啟用壓縮的情況下, 這些又被抵消掉了
   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) {//字節(jié)余一位時(shí)候, 補(bǔ)2個(gè)字符, 64*64>256
   v = a[i - 2];
   s += codeChar.charAt(v & 0x3f);
   s += codeChar.charAt((v >> 6) & 0x3f);
  } else if (res == 2) {
  //字節(jié)余2位的時(shí)候, 補(bǔ)3個(gè)字節(jié), 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) {
  //正確的字節(jié)數(shù)肯定是余2或3, 沒有1的情況, 如果出現(xiàn), 舍棄.
   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);
 };
})();