短URL构建方法

 1 /// <summary>

 2 /// 算法的理论（网上找到的资料）：

 3 /// 1.将长网址md5生成32位签名串，分为4段，每段8个字节；

 4 /// 2.对这四段循环处理，取8个字节，将他看成16进制串与0x3fffffff(30位1)与操作，即超过30位的忽略处理；

 5 /// 3.这30位分成6段，每5位的数字作为字母表的索引取得特定字符，依次进行获得6位字符串；

 6 /// 4.总的md5串可以获得4个6位串；取里面的任意一个就可作为这个长url的短url地址；

 7 /// </summary>

 8 /// <param name="url"></param>

 9 /// <returns></returns>

 public static string[] ShortUrl(string url)

 {

     //可以自定义生成MD5加密字符传前的混合KEY   

     string key = "Leejor";

     //要使用生成URL的字符   

     string[] chars = new string[]{  

     "a" , "b" , "c" , "d" , "e" , "f" , "g" , "h" ,  

     "i" , "j" , "k" , "l" , "m" , "n" , "o" , "p" ,  

     "q" , "r" , "s" , "t" , "u" , "v" , "w" , "x" ,  

     "y" , "z" , "0" , "1" , "2" , "3" , "4" , "5" ,  

     "6" , "7" , "8" , "9" , "A" , "B" , "C" , "D" ,  

     "E" , "F" , "G" , "H" , "I" , "J" , "K" , "L" ,  

     "M" , "N" , "O" , "P" , "Q" , "R" , "S" , "T" ,  

     "U" , "V" , "W" , "X" , "Y" , "Z"    };

 

     //对传入网址进行MD5加密   

     string hex = System.Web.Security.FormsAuthentication.HashPasswordForStoringInConfigFile(key + url, "md5");

 

     string[] resUrl = new string[4];

 

     for (int i = 0; i < 4; i++)

     {

         //把加密字符按照8位一组16进制与0x3FFFFFFF进行位与运算   

         int hexint = 0x3FFFFFFF & Convert.ToInt32("0x" + hex.Substring(i * 8, 8), 16);

         string outChars = string.Empty;

         for (int j = 0; j < 6; j++)

         {

             //把得到的值与0x0000003D进行位与运算，取得字符数组chars索引   

             int index = 0x0000003D & hexint;

             //把取得的字符相加   

             outChars += chars[index];

             //每次循环按位右移5位   

             hexint = hexint >> 5;

         }

         //把字符串存入对应索引的输出数组   

         resUrl[i] = outChars;

     }

     return resUrl;

 } 

这个不错：

http://blog.csdn.net/5653325/article/details/6639829