微博短网址原理(ShortUrl)的算法

短网址一直都在微博上应用。例如腾讯微博的短网址url.cn，新浪的sinaurl.cn等。 他们是如何实现呢，本文将介绍一下该技术算法！

比如我们在腾讯微博上发布网址的时候，微博会自动判别网址，并将其转换，例如转换为：http://url.cn/3fVZf1

为什么要这样做的，原因有以下几点：

1、腾讯微博限制字数为140字一条，那么如果我们需要发一些连接上去，但是这个连接非常的长，以至于将近要占用我们内容的一半篇幅，这肯定是不能被允许的，所以短网址应运而生了。

2、短网址可以在我们项目里可以很好的对开放级URL进行管理。有一部分网址可以会涵盖seqing，暴力，广告等信息，这样我们可以通过用户的举报，完全管理这个连接将不出现在我们的应用中，应为同样的URL通过加密算法之后，得到的地址是一样的。

3、我们可以对一系列的网址进行流量，点击等统计，挖掘出大多数用户的关注点，这样有利于我们对项目的后续工作更好的作出决策。

 

其实以上三点纯属个人观点，因为在我接下来的部分项目中会应用到，所以就了解了一下，下面先来看看短网址映射算法的理论（网上找到的资料）

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;

 

很简单的理论，我们并不一定说得到的URL是唯一的，但是我们能够取出4组URL，这样几乎不会出现太大的重复。

下面来看看程序部分：

Java代码 
1.public static string[] ShortUrl(string url)   
2.        {   
3.            //可以自定义生成MD5加密字符传前的混合KEY   
4.            string key = "Leejor";   
5.            //要使用生成URL的字符   
6.            string[] chars = new string[]{   
7.                "a","b","c","d","e","f","g","h",   
8.                "i","j","k","l","m","n","o","p",   
9.                "q","r","s","t","u","v","w","x",   
10.                "y","z","0","1","2","3","4","5",   
11.                "6","7","8","9","A","B","C","D",   
12.                "E","F","G","H","I","J","K","L",   
13.                "M","N","O","P","Q","R","S","T",   
14.                "U","V","W","X","Y","Z"  
15.  
16.              };   
17.            //对传入网址进行MD5加密   
18.            string hex = System.Web.Security.FormsAuthentication.HashPasswordForStoringInConfigFile(key + url, "md5");   
19.  
20.            string[] resUrl = new string[4];   
21.  
22.            for (int i = 0; i < 4; i++)   
23.            {   
24.                //把加密字符按照8位一组16进制与0x3FFFFFFF进行位与运算   
25.                int hexint = 0x3FFFFFFF & Convert.ToInt32("0x" + hex.Substring(i * 8, 8), 16);   
26.                string outChars = string.Empty;   
27.                for (int j = 0; j < 6; j++)   
28.                {   
29.                    //把得到的值与0x0000003D进行位与运算，取得字符数组chars索引   
30.                    int index = 0x0000003D & hexint;   
31.                    //把取得的字符相加   
32.                    outChars += chars[index];   
33.                    //每次循环按位右移5位   
34.                    hexint = hexint >> 5;   
35.                }   
36.                //把字符串存入对应索引的输出数组   
37.                resUrl[i] = outChars;   
38.            }   
39.  
40.            return resUrl;   
41.        }  
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;
        }

现在可以直接使用该方法，可以等到下面四组值

ShortUrl(http://www.me3.cn")[0];  //得到值fAVfui

ShortUrl("http://www.me3.cn")[1];  //得到值3ayQry

ShortUrl("http://www.me3.cn")[2];  //得到值UZzyUr

ShortUrl("http://www.me3.cn")[3];  //得到值36rQZn