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hash算法实现

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最近在看一个分库分表的方法,主要是通过hash的方法路由到各个表中。hash方法用到了memcached的HashAlgorithm类。

package net.spy.memcached;
 
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.zip.CRC32;
 
/**
 * 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 <a href="http://www.isthe.com/chongo/tech/comp/fnv/">fnv comparisons</a>
	 * @see <a href="http://en.wikipedia.org/wiki/Fowler_Noll_Vo_hash">fnv at wikipedia</a>
	 */
	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;
 
	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;
 
	private static MessageDigest MD5_DIGEST = null;
 
	static {
		try {
			MD5_DIGEST = MessageDigest.getInstance("MD5");
		} catch (NoSuchAlgorithmException e) {
			throw new RuntimeException("MD5 not supported", e);
		}
	}
 
	/**
	 * 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(KeyUtil.getKeyBytes(k));
				rv = (crc32.getValue() >> 16) & 0x7fff;
				break;
			case FNV1_64_HASH: {
					// Thanks to pierre@demartines.com 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 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;
			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)MD5_DIGEST.clone();
		} catch (CloneNotSupportedException e) {
			throw new RuntimeException("clone of MD5 not supported", e);
		}
		md5.update(KeyUtil.getKeyBytes(k));
		return md5.digest();
	}
}

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