短網址一直都在微博上應用。例如騰訊微博的短網址url.cn，新浪的sinaurl.cn等。 他們是如何實現呢，本文將介紹一下該技術算法！
比如我們在騰訊微博上發布網址的時候，微博會自動判別網址，并將其轉換，例如轉換為：http://url.cn/3fVZf1
為什么要這樣做的，原因有以下幾點：
1、騰訊微博限制字數為140字一條，那么如果我們需要發一些連接上去，但是這個連接非常的長，以至于將近要占用我們內容的一半篇幅，這肯定是不能被允許的，所以短網址應運而生了。
2、短網址可以在我們項目里可以很好的對開放級URL進行管理。有一部分網址可以會涵蓋seqing，暴力，廣告等信息，這樣我們可以通過用戶的舉報，完全管理這個連接將不出現在我們的應用中，應為同樣的URL通過加密算法之后，得到的地址是一樣的。
3、我們可以對一系列的網址進行流量，點擊等統計，挖掘出大多數用戶的關注點，這樣有利于我們對項目的后續工作更好的作出決策。
其實以上三點純屬個人觀點，因為在我接下來的部分項目中會應用到，所以就了解了一下，下面先來看看短網址映射算法的理論（網上找到的資料）
1)將長網址md5生成32位簽名串,分為4段, 每段8個字節;
2)對這四段循環處理, 取8個字節, 將他看成16進制串與0x3fffffff(30位1)與操作, 即超過30位的忽略處理;
3)這30位分成6段, 每5位的數字作為字母表的索引取得特定字符, 依次進行獲得6位字符串;
4)總的md5串可以獲得4個6位串; 取里面的任意一個就可作為這個長url的短url地址;
很簡單的理論，我們并不一定說得到的URL是唯一的，但是我們能夠取出4組URL，這樣幾乎不會出現太大的重復。
下面來看看程序部分：
Java代碼
public static string[] ShortUrl(string url)
{
//可以自定義生成MD5加密字符傳前的混合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"
};
//對傳入網址進行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位一組16進制與0x3FFFFFFF進行位與運算
int hexint = 0x3FFFFFFF & Convert.ToInt32("0x" + hex.Substring(i * 8, 8), 16);
string outChars = string.Empty;
for (int j = 0; j < 6; j++)
{
//把得到的值與0x0000003D進行位與運算，取得字符數組chars索引
int index = 0x0000003D & hexint;
//把取得的字符相加
outChars += chars[index];
//每次循環按位右移5位
hexint = hexint >> 5;
}
//把字符串存入對應索引的輸出數組
resUrl[i] = outChars;
}
return resUrl;
}
public static string[] ShortUrl(string url)
{
//可以自定義生成MD5加密字符傳前的混合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"
};
//對傳入網址進行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位一組16進制與0x3FFFFFFF進行位與運算
int hexint = 0x3FFFFFFF & Convert.ToInt32("0x" + hex.Substring(i * 8, 8), 16);
string outChars = string.Empty;
for (int j = 0; j < 6; j++)
{
//把得到的值與0x0000003D進行位與運算，取得字符數組chars索引
int index = 0x0000003D & hexint;
//把取得的字符相加
outChars += chars[index];
//每次循環按位右移5位
hexint = hexint >> 5;
}
//把字符串存入對應索引的輸出數組
resUrl[i] = outChars;
}
return resUrl;
}
現在可以直接使用該方法，可以等到下面四組值
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
原文地址：http://haohaoker-163-com.iteye.com/blog/1094692