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/*
 * Copyright 2006-2009 Odysseus Software GmbH
 *
 * 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 de.odysseus.el.tree.impl;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicInteger;

import de.odysseus.el.tree.Tree;
import de.odysseus.el.tree.TreeCache;

/**
 * Concurrent (thread-safe) FIFO tree cache (using classes from
 * java.util.concurrent). After the cache size reached a certain
 * limit, some least recently used entry are removed, when adding a new entry.
 * 
 * @author Christoph Beck
 */
public final class Cache implements TreeCache {
	private final ConcurrentMap map;
	private final ConcurrentLinkedQueue queue;
	private final AtomicInteger size;
	private final int capacity;

	/**
	 * Creates a new cache with the specified capacity
	 * and default concurrency level (16).
	 * 
	 * @param capacity
	 *            Cache size. The actual size may exceed it temporarily.
	 */
	public Cache(int capacity) {
		this(capacity, 16);
	}

	/**
	 * Creates a new cache with the specified capacity and concurrency level.
	 * 
	 * @param capacity
	 *            Cache size. The actual map size may exceed it temporarily.
	 * @param concurrencyLevel
	 *            The estimated number of concurrently updating threads. The
	 *            implementation performs internal sizing to try to accommodate
	 *            this many threads.
	 */
	public Cache(int capacity, int concurrencyLevel) {
		this.map = new ConcurrentHashMap(16, 0.75f, concurrencyLevel);
		this.queue = new ConcurrentLinkedQueue();
		this.size = new AtomicInteger();
		this.capacity = capacity;
	}
	
	public int size() {
		return size.get();
	}

	public Tree get(String expression) {
		return map.get(expression);
	}

	public void put(String expression, Tree tree) {
		if (map.putIfAbsent(expression, tree) == null) {
			queue.offer(expression);
			if (size.incrementAndGet() > capacity) {
				size.decrementAndGet();
				map.remove(queue.poll());
			}
		}
	}
}