方法说明：

该方法使可用来重写 querystring.parse

语法：

querystring.unescape

接收参数：

无

源码：

// a safe fast alternative to decodeURIComponent

QueryString.unescapeBuffer = function(s, decodeSpaces) {

  var out = new Buffer(s.length);

  var state = 'CHAR'; // states: CHAR, HEX0, HEX1

  var n, m, hexchar;

  for (var inIndex = 0, outIndex = 0; inIndex <= s.length; inIndex++) {

    var c = s.charCodeAt(inIndex);

    switch (state) {

      case 'CHAR':

        switch (c) {

          case charCode('%'):

            n = 0;

            m = 0;

            state = 'HEX0';

            break;

          case charCode('+'):

            if (decodeSpaces) c = charCode(' ');

            // pass thru

          default:

            out[outIndex++] = c;

            break;

        }

        break;

      case 'HEX0':

        state = 'HEX1';

        hexchar = c;

        if (charCode('0') <= c && c <= charCode('9')) {

          n = c - charCode('0');

        } else if (charCode('a') <= c && c <= charCode('f')) {

          n = c - charCode('a') + 10;

        } else if (charCode('A') <= c && c <= charCode('F')) {

          n = c - charCode('A') + 10;

        } else {

          out[outIndex++] = charCode('%');

          out[outIndex++] = c;

          state = 'CHAR';

          break;

        }

        break;

      case 'HEX1':

        state = 'CHAR';

        if (charCode('0') <= c && c <= charCode('9')) {

          m = c - charCode('0');

        } else if (charCode('a') <= c && c <= charCode('f')) {

          m = c - charCode('a') + 10;

        } else if (charCode('A') <= c && c <= charCode('F')) {

          m = c - charCode('A') + 10;

        } else {

          out[outIndex++] = charCode('%');

          out[outIndex++] = hexchar;

          out[outIndex++] = c;

          break;

        }

        out[outIndex++] = 16 * n + m;

        break;

    }

  }

  // TODO support returning arbitrary buffers.

  return out.slice(0, outIndex - 1);

};