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 * except in compliance with the License. You may obtain a copy of
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 *
 * Software distributed under the License is distributed on an "AS
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 * implied. See the License for the specific language governing
 * rights and limitations under the License.
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 * Copyright (C) 2002-2004 Jan Arne Petersen <jpetersen@uni-bonn.de>
 * Copyright (C) 2002-2004 Anders Bengtsson <ndrsbngtssn@yahoo.se>
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 * Copyright (C) 2004 Stefan Matthias Aust <sma@3plus4.de>
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package org.jruby.util.collections;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

/**
 * Map-like that holds its values weakly (backed by a concurrent hash map).
 */
public class WeakValuedMap<Key, Value> {

    private final Map<Key, KeyedReference<Key, Value>> map = newMap();
    @SuppressWarnings("unchecked")
    private final ReferenceQueue<Value> deadRefs = new ReferenceQueue();

    public final void put(Key key, Value value) {
        cleanReferences();
        map.put(key, new KeyedReference<Key, Value>(value, key, deadRefs));
    }

    public final Value get(Key key) {
        cleanReferences();
        KeyedReference<Key, Value> reference = map.get(key);
        if (reference == null) return null;
        return reference.get();
    }

    public void clear() {
        cleanReferences();
        map.clear();
    }

    public int size() {
        cleanReferences();
        return map.size();
    }

    /**
     * Construct the backing store map for this WeakValuedMap. It should be capable of safe concurrent read and write.
     *
     * @return the backing store map
     */
    protected Map<Key, KeyedReference<Key, Value>> newMap() {
        return new ConcurrentHashMap<Key, KeyedReference<Key, Value>>();
    }

    protected static class KeyedReference<Key, Value> extends WeakReference<Value> {

        protected final Key key;

        public KeyedReference(Value object, Key key, ReferenceQueue<? super Value> queue) {
            super(object, queue);
            this.key = key;
        }

    }

    @SuppressWarnings("unchecked")
    private void cleanReferences() {
        KeyedReference<Key, Value> ref;
        while ( ( ref = (KeyedReference) deadRefs.poll() ) != null ) {
            map.remove( ref.key );
        }
    }
}
