微博短地址

1. 将长网址md5生成32位签名串，分为4段，每段8个字节；
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；
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；
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；

这是算法实现：

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;
}

转换后的URL和转换前的URL，需要存到数据库中去，属于哈希Key/Value，有人推荐TTServer，日本人写的