public class MD5 {
    /*
     * 四个链接变量
     */
    private final int A = 0x67452301;
    private final int B = 0xefcdab89;
    private final int C = 0x98badcfe;
    private final int D = 0x10325476;
    /*
     * ABCD的临时变量
     */
    private int Atemp, Btemp, Ctemp, Dtemp;

    /*
     * 常量ti公式:floor(abs(sin(i+1))×(2pow32)
     */
    private final int K[] = { 0xd76aa478, 0xe8c7b756, 0x242070db, 0xc1bdceee,
            0xf57c0faf, 0x4787c62a, 0xa8304613, 0xfd469501, 0x698098d8,
            0x8b44f7af, 0xffff5bb1, 0x895cd7be, 0x6b901122, 0xfd987193,
            0xa679438e, 0x49b40821, 0xf61e2562, 0xc040b340, 0x265e5a51,
            0xe9b6c7aa, 0xd62f105d, 0x02441453, 0xd8a1e681, 0xe7d3fbc8,
            0x21e1cde6, 0xc33707d6, 0xf4d50d87, 0x455a14ed, 0xa9e3e905,
            0xfcefa3f8, 0x676f02d9, 0x8d2a4c8a, 0xfffa3942, 0x8771f681,
            0x6d9d6122, 0xfde5380c, 0xa4beea44, 0x4bdecfa9, 0xf6bb4b60,
            0xbebfbc70, 0x289b7ec6, 0xeaa127fa, 0xd4ef3085, 0x04881d05,
            0xd9d4d039, 0xe6db99e5, 0x1fa27cf8, 0xc4ac5665, 0xf4292244,
            0x432aff97, 0xab9423a7, 0xfc93a039, 0x655b59c3, 0x8f0ccc92,
            0xffeff47d, 0x85845dd1, 0x6fa87e4f, 0xfe2ce6e0, 0xa3014314,
            0x4e0811a1, 0xf7537e82, 0xbd3af235, 0x2ad7d2bb, 0xeb86d391 };
    /*
     * 向左位移数,计算方法未知
     */
    private final int s[] = { 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 7,
            12, 17, 22, 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20,
            4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 6, 10,
            15, 21, 6, 10, 15, 21, 6, 10, 15, 21, 6, 10, 15, 21 };

    /*
     * 初始化函数
     */
    private void init() {
        Atemp = A;
        Btemp = B;
        Ctemp = C;
        Dtemp = D;
    }

    /*
     * 移动一定位数
     */
    private int shift(int a, int s) {
        return (a << s) | (a >>> (32 - s));// 右移的时候，高位一定要补零，而不是补充符号位
    }

    /*
     * 主循环
     */
    private void MainLoop(int M[]) {
        int F, g;
        int a = Atemp;
        int b = Btemp;
        int c = Ctemp;
        int d = Dtemp;
        for (int i = 0; i < 64; i++) {
            if (i < 16) {
                F = (b & c) | ((~b) & d);
                g = i;
            } else if (i < 32) {
                F = (d & b) | ((~d) & c);
                g = (5 * i + 1) % 16;
            } else if (i < 48) {
                F = b ^ c ^ d;
                g = (3 * i + 5) % 16;
            } else {
                F = c ^ (b | (~d));
                g = (7 * i) % 16;
            }
            int tmp = d;
            d = c;
            c = b;
            b = b + shift(a + F + K[i] + M[g], s[i]);
            a = tmp;
        }
        Atemp = a + Atemp;
        Btemp = b + Btemp;
        Ctemp = c + Ctemp;
        Dtemp = d + Dtemp;

    }

    /*
     * 填充函数处理后应满足bits≡448(mod512),字节就是bytes≡56（mode64)填充方式为先加一个1,其它位补零
     * 最后加上64位的原来长度
     */
    private int[] add(String str) {
        int num = ((str.length() + 8) / 64) + 1;// 分组，以512位，64个字节为一组
        int strByte[] = new int[num * 16];// 64/4=16，所以有16个整数，ABCD
        for (int i = 0; i < num * 16; i++) {// 全部初始化0
            strByte[i] = 0;
        }
        int i;
        for (i = 0; i < str.length(); i++) {
            strByte[i >> 2] |= str.charAt(i) << ((i % 4) * 8);// 一个整数存储四个字节，小端序
        }
        strByte[i >> 2] |= 0x80 << ((i % 4) * 8);// 尾部添加1
        /*
         * 添加原长度，长度指位的长度，所以要乘8，然后是小端序，所以放在倒数第二个,这里长度只用了32位
         */
        strByte[num * 16 - 2] = str.length() * 8;
        return strByte;
    }

    /*
     * 调用函数
     */
    public String getMD5(String source) {
        init();
        int strByte[] = add(source);
        for (int i = 0; i < strByte.length / 16; i++) {
            int num[] = new int[16];
            for (int j = 0; j < 16; j++) {
                num[j] = strByte[i * 16 + j];
            }
            MainLoop(num);
        }
        return changeHex(Atemp) + changeHex(Btemp) + changeHex(Ctemp)
                + changeHex(Dtemp);

    }

    /*
     * 整数变成16进制字符串
     */
    private String changeHex(int a) {
        String str = "";
        for (int i = 0; i < 4; i++) {
            str += String.format("%2s",
                    Integer.toHexString(((a >> i * 8) % (1 << 8)) & 0xff)).replace(' ', '0');
        }
        return str;
    }

    /*
     * 单例
     */
    private static MD5 instance;
    public static MD5 getInstance() {
        if (instance == null) {
            instance = new MD5();
        }
        return instance;
    }

    private MD5() {
    };
}