/*
 * 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.openejb.util;

import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;

public class Memoizer<K, V> implements Computable<K, V> {
    private final ConcurrentMap<K, Future<V>> cache = new ConcurrentHashMap<K, Future<V>>();

    private final Computable<K, V> c;

    /**
     * Constructs a new <code>Memoizer</code> with the specified cache source.
     *
     * @param c is the cache value source algorithm
     * @throws NullPointerException if c is null
     */
    public Memoizer(final Computable<K, V> c) {
        if (c == null) {
            throw new NullPointerException("Computable cache value source algorithm may not be null");
        }
        this.c = c;
    }

    public V compute(final K key) throws InterruptedException {
        while (true) {
            Future<V> future = cache.get(key);
            if (future == null) {

                final Callable<V> eval = new Callable<V>() {
                    public V call() throws Exception {
                        return c.compute(key);
                    }
                };
                final FutureTask<V> futureTask = new FutureTask<V>(eval);
                future = cache.putIfAbsent(key, futureTask);
                if (future == null) {
                    future = futureTask;
                    futureTask.run();
                }
            }
            try {
                return future.get();
            } catch (final ExecutionException e) {
                e.printStackTrace();
            }
        }
    }
}
