/*
 *                    BioJava development code
 *
 * This code may be freely distributed and modified under the
 * terms of the GNU Lesser General Public Licence.  This should
 * be distributed with the code.  If you do not have a copy,
 * see:
 *
 *      http://www.gnu.org/copyleft/lesser.html
 *
 * Copyright for this code is held jointly by the individual
 * authors.  These should be listed in @author doc comments.
 *
 * For more information on the BioJava project and its aims,
 * or to join the biojava-l mailing list, visit the home page
 * at:
 *
 *      http://www.biojava.org/
 *
 */

package org.biojava.utils.cache;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * Map implementation which keeps weak references to values.
 * Entries are removed from the map when their value is
 * no longer reachable using normal (hard) references.  This is
 * useful for maintaining canonical copies of objects without forcing
 * these objects to remain in memory forever.
 *
 * <p>
 * Note that this is distinct from the standard library class,
 * <code>WeakHashMap</code> which has weak <em>keys</em>.
 * </p>
 *
 * @author Thomas Down
 * @since 1.3
 */

public class WeakValueHashMap extends AbstractMap {
    private final Map keyToRefMap;
    private final ReferenceQueue queue;
    private final Set iteratorRefs;
    private final ReferenceQueue iteratorRefQueue;

    public WeakValueHashMap() {
        keyToRefMap = new HashMap();
        queue = new ReferenceQueue();
        iteratorRefs = new HashSet();
        iteratorRefQueue = new ReferenceQueue();
    }

    private void diddleReferenceQueue() {
        // Avoid making behind-the-scenes modifications while iterators exist.

        if (iteratorRefs.size() > 0) {
            Reference ref;
            while ((ref = iteratorRefQueue.poll()) != null) {
                iteratorRefs.remove(ref);
            }
            if (iteratorRefs.size() > 0) {
                return;
            }
        }

        KeyedWeakReference ref;
        while ((ref = (KeyedWeakReference) queue.poll()) != null) {
            keyToRefMap.remove(ref.getKey());
        }
    }

    public Object put(Object key, Object value) {
        diddleReferenceQueue();
        Reference oldRef = (Reference) keyToRefMap.put(key, new KeyedWeakReference(key, value, queue));
        if (oldRef != null) {
            Object oldRefVal = oldRef.get();
            oldRef.clear();
            return oldRefVal;
        } else {
            return null;
        }
    }

    public Object get(Object key) {
        Reference ref = (Reference) keyToRefMap.get(key);
        if (ref != null) {
            return ref.get();
        } else {
            return null;
        }
    }

    public boolean containsKey(Object o) {
        diddleReferenceQueue();
        return keyToRefMap.containsKey(o);
    }

    public Set entrySet() {
        diddleReferenceQueue();
        return new WVEntrySet();
    }

    private class WVEntrySet extends AbstractSet {
        private Set keyRefEntrySet;

        public WVEntrySet() {
            super();
            keyRefEntrySet = keyToRefMap.entrySet();
        }

        public int size() {
            return keyRefEntrySet.size();
        }

        public Iterator iterator() {
            Iterator i = new WVEntryIterator(keyRefEntrySet.iterator());
            iteratorRefs.add(new PhantomReference(i, iteratorRefQueue));
            return i;
        }
    }

    private class WVEntryIterator implements Iterator {
        private Object cache;
        private Iterator keyRefIterator;

        public WVEntryIterator(Iterator keyRefIterator) {
            this.keyRefIterator = keyRefIterator;
        }

        public boolean hasNext() {
            if (cache == null) {
                primeCache();
            }
            return cache != null;
        }
        
        public Object next() {
            if (cache == null) {
                primeCache();
            }
            if (cache == null) {
                throw new NoSuchElementException();
            } else {
                Object o = cache;
                cache = null;
                return o;
            }
        }

        public void remove() {
            if (cache != null) {
                throw new IllegalStateException("next() not called");
            } else {
                keyRefIterator.remove();
            }
        }

        private void primeCache() {
            while (keyRefIterator.hasNext()) {
                Map.Entry krme = (Map.Entry) keyRefIterator.next();
                Object ref = ((Reference) krme.getValue()).get();
                if (ref != null) {
                    cache = new WVMapEntry(krme.getKey(), ref);
                    return;
                }
            }
        }
    }

    private static class WVMapEntry implements Map.Entry {
        private Object key;
        private Object value;
    
        private WVMapEntry(Object key, Object value) {
            this.key = key;
            this.value = value;
        }
    
        public Object getKey() {
            return key;
        }
    
        public Object getValue() {
            return value;
        }
    
        public Object setValue(Object v) {
            throw new UnsupportedOperationException();
        }
    
        public boolean equals(Object o) {
            if (! (o instanceof Map.Entry)) {
                return false;
            }
      
            Map.Entry mo = (Map.Entry) o;
            return ((key == null ? mo.getKey() == null : key.equals(mo.getKey())) &&
                    (value == null ? mo.getValue() == null : value.equals(mo.getValue())));
        }
    
        public int hashCode() {
            return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
        }
    }
}