/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.qpid.jms.util;

import java.util.LinkedHashMap;
import java.util.Map;

/**
 * A Simple LRU Cache based on a LinkedHashMap.
 *
 * @param <K> The type of the map key.
 * @param <V> The type of the map value.
 */
public class LRUCache<K, V> extends LinkedHashMap<K, V> {

    private static final long serialVersionUID = -342098639681884413L;
    protected int maxCacheSize = 10000;

    /**
     * Default constructor for an LRU Cache The default capacity is 10000
     */
    public LRUCache() {
        this(0,10000, 0.75f, true);
    }

    /**
     * Constructs a LRUCache with a maximum capacity
     *
     * @param maximumCacheSize
     *      The maximum number of elements to keep in the Cache before eviction starts.
     */
    public LRUCache(int maximumCacheSize) {
        this(0, maximumCacheSize, 0.75f, true);
    }

    /**
     * Constructs an empty <tt>LRUCache</tt> instance with the specified
     * initial capacity, maximumCacheSize,load factor and ordering mode.
     *
     * @param initialCapacity
     *      The initial capacity.
     * @param maximumCacheSize
     *      The maximum number of elements to keep in the Cache before eviction starts.
     * @param loadFactor
     *      The load factor to configure on the underlying map.
     * @param accessOrder the ordering mode - <tt>true</tt> for access-order,
     *                <tt>false</tt> for insertion-order.
     *
     * @throws IllegalArgumentException if the initial capacity is negative or
     *                 the load factor is non-positive.
     */
    public LRUCache(int initialCapacity, int maximumCacheSize, float loadFactor, boolean accessOrder) {
        super(initialCapacity, loadFactor, accessOrder);
        this.maxCacheSize = maximumCacheSize;
    }

    /**
     * @return Returns the maxCacheSize.
     */
    public int getMaxCacheSize() {
        return maxCacheSize;
    }

    /**
     * @param maxCacheSize The maxCacheSize to set.
     */
    public void setMaxCacheSize(int maxCacheSize) {
        this.maxCacheSize = maxCacheSize;
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
        if( size() > maxCacheSize ) {
            onCacheEviction(eldest);
            return true;
        }
        return false;
    }

    /**
     * Event point used by subclasses to perform some cleanup action when an
     * element is evicted from the cache.
     *
     * @param eldest
     *        the item being evicted from the LRUCache.
     */
    protected void onCacheEviction(Map.Entry<K,V> eldest) {
    }
}