memcache的分布式hash算法

	/** 
	 * Internal private hashing method.
	 *
	 * This is the original hashing algorithm from other clients.
	 * Found to be slow and have poor distribution.
	 * 
	 * @param key String to hash
	 * @return hashCode for this string using our own hashing algorithm
	 */
	private static long origCompatHashingAlg( String key ) {
		long hash   = 0;
		char[] cArr = key.toCharArray();

		for ( int i = 0; i < cArr.length; ++i ) {
			hash = (hash * 33) + cArr[i];
		}

		return hash;
	}

	/** 
	 * Internal private hashing method.
	 *
	 * This is the new hashing algorithm from other clients.
	 * Found to be fast and have very good distribution. 
	 *
	 * UPDATE: This is dog slow under java
	 * 
	 * @param key 
	 * @return 
	 */
	private static long newCompatHashingAlg( String key ) {
		CRC32 checksum = new CRC32();
		checksum.update( key.getBytes() );
		long crc = checksum.getValue();
		return (crc >> 16) & 0x7fff;
	}

	/** 
	 * Internal private hashing method.
	 *
	 * MD5 based hash algorithm for use in the consistent
	 * hashing approach.
	 * 
	 * @param key 
	 * @return 
	 */
	private static long md5HashingAlg( String key ) {
		MessageDigest md5 = MD5.get();
		md5.reset();
		md5.update( key.getBytes() );
		byte[] bKey = md5.digest();
		long res = ((long)(bKey[3]&0xFF) << 24) | ((long)(bKey[2]&0xFF) << 16) | ((long)(bKey[1]&0xFF) << 8) | (long)(bKey[0]&0xFF);
		return res;
	}

	/** 
	 * Returns a bucket to check for a given key. 
	 * 
	 * @param key String key cache is stored under
	 * @return int bucket
	 */
	private long getHash( String key, Integer hashCode ) {

		if ( hashCode != null ) {
			if ( hashingAlg == CONSISTENT_HASH )
				return hashCode.longValue() & 0xffffffffL;
			else
				return hashCode.longValue();
		}
		else {
			switch ( hashingAlg ) {
				case NATIVE_HASH:
					return (long)key.hashCode();
				case OLD_COMPAT_HASH:
					return origCompatHashingAlg( key );
				case NEW_COMPAT_HASH:
					return newCompatHashingAlg( key );
				case CONSISTENT_HASH:
					return md5HashingAlg( key );
				default:
					// use the native hash as a default
					hashingAlg = NATIVE_HASH;
					return (long)key.hashCode();
			}
		}
	}

 

默认是NATIVE_HASH这种。