JAVA 中几个有用的 Hash 算法

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package net.rubyeye.xmemcached;

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.zip.CRC32;

import net.rubyeye.xmemcached.exception.MemcachedClientException;
import net.rubyeye.xmemcached.utils.ByteUtils;

/**
 * Known hashing algorithms for locating a server for a key. Note that all hash
 * algorithms return 64-bits of hash, but only the lower 32-bits are
 * significant. This allows a positive 32-bit number to be returned for all
 * cases.
 */
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 [email protected] 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 */
	}

	private static ThreadLocal<MessageDigest> md5Local = new ThreadLocal<MessageDigest>();

	/**
	 * Get the md5 of the given key.
	 */
	public static byte[] computeMd5(String k) {
		MessageDigest md5 = md5Local.get();
		if (md5 == null) {
			try {
				md5 = MessageDigest.getInstance("MD5");
				md5Local.set(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);
	// }
}