/*

 * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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 */

package jdk.nashorn.internal;

import java.lang.ref.ReferenceQueue;

import java.lang.ref.WeakReference;

import java.util.HashMap;

import java.util.function.Function;

/**

 * This class provides a map based cache with weakly referenced values. Cleared references are

 * purged from the underlying map when values are retrieved or created.

 * It uses a {@link java.util.HashMap} to store values and needs to be externally synchronized.

 *

 * @param <K> the key type

 * @param <V> the value type

 */

public final class WeakValueCache<K, V> {

    private final HashMap<K, KeyValueReference<K, V>> map = new HashMap<>();

    private final ReferenceQueue<V> refQueue = new ReferenceQueue<>();

    /**

     * Returns the value associated with {@code key}, or {@code null} if no such value exists.

     *

     * @param key the key

     * @return the value or null if none exists

     */

    public V get(final K key) {

        // Remove cleared entries

        for (;;) {

            final KeyValueReference<?, ?> ref = (KeyValueReference) refQueue.poll();

            if (ref == null) {

                break;

            }

            map.remove(ref.key, ref);

        }

        final KeyValueReference<K, V> ref = map.get(key);

        if (ref != null) {

            return ref.get();

        }

        return null;

    }

    /**

     * Returns the value associated with {@code key}, or creates and returns a new value if

     * no value exists using the {@code creator} function.

     *

     * @param key the key

     * @param creator function to create a new value

     * @return the existing value, or a new one if none existed

     */

    public V getOrCreate(final K key, final Function<? super K, ? extends V> creator) {

        V value = get(key);

        if (value == null) {

            // Define a new value if it does not exist

            value = creator.apply(key);

            map.put(key, new KeyValueReference<>(key, value));

        }

        return value;

    }

    private static class KeyValueReference<K, V> extends WeakReference<V> {

        final K key;

        KeyValueReference(final K key, final V value) {

            super(value);

            this.key = key;

        }

    }

}