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。
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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;
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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