/**
* Copyright 2003-2008 Luck Consulting Pty Ltd
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package net.sf.ehcache.constructs.concurrent;
import net.sf.ehcache.CacheException;
/**
* Various bits of black magic garnered from experts on the concurrency-interest@cs.oswego.edu mailing list.
*
* @author Greg Luck
* @version $Id: ConcurrencyUtil.java 744 2008-08-16 20:10:49Z gregluck $
*/
public final class ConcurrencyUtil {
private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_1 = 20;
private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_2 = 12;
private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_3 = 7;
private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_4 = 4;
/**
* Utility class therefore precent construction
*/
private ConcurrencyUtil() {
//noop;
}
/**
* Returns a hash code for non-null Object x.
* <p/>
* This function ensures that hashCodes that differ only by
* constant multiples at each bit position have a bounded
* number of collisions. (Doug Lea)
*
* @param object the object serving as a key
* @return the hash code
*/
public static int hash(Object object) {
int h = object.hashCode();
h ^= (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_1) ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_2);
return h ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_3) ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_4);
}
/**
* Selects a lock for a key. The same lock is always used for a given key.
*
* @param key
* @return the selected lock index
*/
public static int selectLock(final Object key, int numberOfLocks) throws CacheException {
int number = numberOfLocks & (numberOfLocks - 1);
if (number != 0) {
throw new CacheException("Lock number must be a power of two: " + numberOfLocks);
}
if (key == null) {
return 0;
} else {
int hash = hash(key) & (numberOfLocks - 1);
return hash;
}
}
}