简单实现短链接算法

package com.mfyk.util;

import java.security.MessageDigest;

public class Code {

private Code(){}

/** 短链接可选字符 */

private final static char[] chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789".toCharArray();

/** md5的char值 */

private final static char[] md5chars = "1234567890EADCBF".toCharArray();

/** 30位的long类型数值,右移运算可以达30 */

static long max = Long.parseLong("3FFFFFFF", 16);

/** 61的最大值以便于从chars中选择字符 */

static long min = Long.parseLong("0000003D", 16);

public static String shortText(String md5, int start){

/** 短链接返回数据 */

StringBuilder returnValue = new StringBuilder();

String subHex = md5.substring(start*8, (start+1)*8);

/** 获取字串运算索引 */

long idx = max & Long.parseLong(subHex, 16);

/** 获取6位数的短码，6*5=30 */

for(int k = 0; k < 6; k++){

int index = (int) (min & idx);

returnValue.append(chars[index]);

idx = idx >> 5;

}

return returnValue.toString();

}

/**

* md5

* @param s

* @return

*/

public final static String md5(String s) {

try {

byte[] btInput = s.getBytes();

/** 获得MD5摘要算法的 MessageDigest 对象 */

MessageDigest mdInst = MessageDigest.getInstance("MD5");

/** 使用指定的字节更新摘要 */

mdInst.update(btInput);

/** 获得密文 */

byte[] md = mdInst.digest();

int j = md.length;

char str[] = new char[j * 2];

int k = 0;

for (int i = 0; i < j; i++) {

byte byte0 = md[i];

str[k++] = md5chars[byte0 >>> 4 & 0xf];

str[k++] = md5chars[byte0 & 0xf];

}

return new String(str);

} catch (Exception e) {

return null;

}

}

}