把string映射到64位的hashcode值方法

可以参考 xmemcached 的实现，源码如下：

 

public enum HashAlgorithm {

        /**
         * Native hash (String.hashCode()).
         */
        NATIVE_HASH,
        /**
         * CRC32_HASH as used by the perl API. This will be more consistent both
         * across multiple API users as well as java versions, but is mostly likely
         * significantly slower.
         */
        CRC32_HASH,
        /**
         * FNV hashes are designed to be fast while maintaining a low collision
         * rate. The FNV speed allows one to quickly hash lots of data while
         * maintaining a reasonable collision rate.
         * 
         * @see http://www.isthe.com/chongo/tech/comp/fnv/
         * @see http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash
         */
        FNV1_64_HASH,
        /**
         * Variation of FNV.
         */
        FNV1A_64_HASH,
        /**
         * 32-bit FNV1.
         */
        FNV1_32_HASH,
        /**
         * 32-bit FNV1a.
         */
        FNV1A_32_HASH,
        /**
         * MD5-based hash algorithm used by ketama.
         */
        KETAMA_HASH,

        /**
         * From mysql source
         */
        MYSQL_HASH,

        ELF_HASH,

        RS_HASH,

        /**
         * From lua source,it is used for long key
         */
        LUA_HASH,

        ELECTION_HASH,
        /**
         * The Jenkins One-at-a-time hash ,please see
         * http://www.burtleburtle.net/bob/hash/doobs.html
         */
        ONE_AT_A_TIME;

        private static final long FNV_64_INIT = 0xcbf29ce484222325L;
        private static final long FNV_64_PRIME = 0x100000001b3L;

        private static final long FNV_32_INIT = 2166136261L;
        private static final long FNV_32_PRIME = 16777619;

        /**
         * Compute the hash for the given key.
         * 
         * @return a positive integer hash
         */
        public long hash(final String k) {
                long rv = 0;
                switch (this) {
                case NATIVE_HASH:
                        rv = k.hashCode();
                        break;
                case CRC32_HASH:
                        // return (crc32(shift) >> 16) & 0x7fff;
                        CRC32 crc32 = new CRC32();
                        crc32.update(ByteUtils.getBytes(k));
                        rv = crc32.getValue() >> 16 & 0x7fff;
                        break;
                case FNV1_64_HASH: {
                        // Thanks to [e-mail] for the pointer
                        rv = FNV_64_INIT;
                        int len = k.length();
                        for (int i = 0; i < len; i++) {
                                rv *= FNV_64_PRIME;
                                rv ^= k.charAt(i);
                        }
                }
                        break;
                case FNV1A_64_HASH: {
                        rv = FNV_64_INIT;
                        int len = k.length();
                        for (int i = 0; i < len; i++) {
                                rv ^= k.charAt(i);
                                rv *= FNV_64_PRIME;
                        }
                }
                        break;
                case FNV1_32_HASH: {
                        rv = FNV_32_INIT;
                        int len = k.length();
                        for (int i = 0; i < len; i++) {
                                rv *= FNV_32_PRIME;
                                rv ^= k.charAt(i);
                        }
                }
                        break;
                case FNV1A_32_HASH: {
                        rv = FNV_32_INIT;
                        int len = k.length();
                        for (int i = 0; i < len; i++) {
                                rv ^= k.charAt(i);
                                rv *= FNV_32_PRIME;
                        }
                }
                        break;
                case ELECTION_HASH:
                case KETAMA_HASH:
                        byte[] bKey = computeMd5(k);
                        rv = (long) (bKey[3] & 0xFF) << 24 | (long) (bKey[2] & 0xFF) << 16
                                        | (long) (bKey[1] & 0xFF) << 8 | bKey[0] & 0xFF;
                        break;

                case MYSQL_HASH:
                        int nr2 = 4;
                        for (int i = 0; i < k.length(); i++) {
                                rv ^= ((rv & 63) + nr2) * k.charAt(i) + (rv << 8);
                                nr2 += 3;
                        }
                        break;
                case ELF_HASH:
                        long x = 0;
                        for (int i = 0; i < k.length(); i++) {
                                rv = (rv << 4) + k.charAt(i);
                                if ((x = rv & 0xF0000000L) != 0) {
                                        rv ^= x >> 24;
                                        rv &= ~x;
                                }
                        }
                        rv = rv & 0x7FFFFFFF;
                        break;
                case RS_HASH:
                        long b = 378551;
                        long a = 63689;
                        for (int i = 0; i < k.length(); i++) {
                                rv = rv * a + k.charAt(i);
                                a *= b;
                        }
                        rv = rv & 0x7FFFFFFF;
                        break;
                case LUA_HASH:
                        int step = (k.length() >> 5) + 1;
                        rv = k.length();
                        for (int len = k.length(); len >= step; len -= step) {
                                rv = rv ^ (rv << 5) + (rv >> 2) + k.charAt(len - 1);
                        }
                case ONE_AT_A_TIME:
                        try {
                                int hash = 0;
                                for (byte bt : k.getBytes("utf-8")) {
                                        hash += (bt & 0xFF);
                                        hash += (hash << 10);
                                        hash ^= (hash >>> 6);
                                }
                                hash += (hash << 3);
                                hash ^= (hash >>> 11);
                                hash += (hash << 15);
                                return hash;
                        } catch (UnsupportedEncodingException e) {
                                throw new IllegalStateException("Hash function error", e);
                        }
                default:
                        assert false;
                }

                return rv & 0xffffffffL; /* Truncate to 32-bits */
        }

        /**
         * Get the md5 of the given key.
         */
        public static byte[] computeMd5(String k) {
                MessageDigest md5;
                try {
                        md5 = MessageDigest.getInstance("MD5");
                } catch (NoSuchAlgorithmException e) {
                        throw new RuntimeException("MD5 not supported", e);
                }
                md5.reset();
                md5.update(ByteUtils.getBytes(k));
                return md5.digest();
        }

        // public static void main(String[] args) {
        // HashAlgorithm alg=HashAlgorithm.CRC32_HASH;
        // long h=0;
        // long start=System.currentTimeMillis();
        // for(int i=0;i<100000;i++)
        // h=alg.hash("MYSQL_HASH");
        // System.out.println(System.currentTimeMillis()-start);
        // }
}