Java版短网址(ShortUrl)的算法

先来看看短网址映射算法的理论（网上找到的资料）：

① 将长网址用md5算法生成32位签名串，分为4段,，每段8个字符；

② 对这4段循环处理，取每段的8个字符, 将他看成16进制字符串与0x3fffffff(30位1)的位与操作，超过30位的忽略处理；

③ 将每段得到的这30位又分成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 这样一个md5字符串可以获得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。

首先，请大家了解在Java中如何用MD5来加密字符串得到32位的加密后的字符串，下面这个链接就是我已经封装好的Java MD5算法：

http://www.cnblogs.com/solove/archive/2011/10/18/2216715.html

下面来看看程序部分：

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package basic;

import util.Encript;

public class ShortUrl {
    public static void main(String[] args) {
        String url = "http://www.sunchis.com";
        for (String string : ShortText(url)) {
            print(string);
        }
    }

    public static String[] ShortText(String string){
        String key = "XuLiang";                 //自定义生成MD5加密字符串前的混合KEY
        String[] chars = new String[]{          //要使用生成URL的字符
            "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"
        };

        String hex = Encript.md5(key + string);
        int hexLen = hex.length();
        int subHexLen = hexLen / 8;
        String[] ShortStr = new String[4];

        for (int i = 0; i < subHexLen; i++) {
            String outChars = "";
            int j = i + 1;
            String subHex = hex.substring(i * 8, j * 8);
            long idx = Long.valueOf("3FFFFFFF", 16) & Long.valueOf(subHex, 16);

            for (int k = 0; k < 6; k++) {
                int index = (int) (Long.valueOf("0000003D", 16) & idx);
                outChars += chars[index];
                idx = idx >> 5;
            }
            ShortStr[i] = outChars;
        }

        return ShortStr;
    }

    private static void print(Object messagr){
        System.out.println(messagr);
    }
}

现在可以直接使用ShortText(url)方法，可以等到下面四组值：

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ShortText("http://www.sunchis.com")[0];  //得到值:Jzyqma
ShortText("http://www.sunchis.com")[1];  //得到值:QBrMzm
ShortText("http://www.sunchis.com")[2];  //得到值:bQreM3
ShortText("http://www.sunchis.com")[3];  //得到值:VNBRna