/*******************************************************************************
 * Copyright (c) 2007, 2010 Association for Decentralized Information Management
 * in Industry THTH ry.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     VTT Technical Research Centre of Finland - initial API and implementation
 *******************************************************************************/
package org.simantics.utils.datastructures.cache;

import java.lang.ref.SoftReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Timer;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

import org.simantics.utils.threads.ThreadUtils;

/**
 * A timed (Key, Value) cache which disposes the cached entry after the
 * specified amount of time if it is not removed from the cache. The hold time
 * is given to the {@link #put(Object, Object, long)} method, separately for
 * each value.
 * 
 * <p>
 * The cached values are held as soft references, which makes them collectible
 * under memory pressure, even before their hold time has ran out.
 * 
 * @author Tuukka Lehtonen
 * 
 * @param <K> key type, held by strong references
 * @param <V> value type, held by soft references to allow collection of the
 *        cached elements when under memory pressure
 */
public class SoftTimedCache<K, V> implements ITimedCache<K, V> {

    protected class Entry {
        final K key;
        final SoftReference<V> ref;
        long holdTime;
        TimeUnit unit;

        ScheduledFuture<?> future;

        Entry(K k, V v, long holdTime, TimeUnit unit) {
            assert k != null;
            assert v != null;

            this.key = k;
            this.ref = new SoftReference<V>(v);
            this.holdTime = holdTime;
            this.unit = unit;
        }
    }

    private final Map<K, Entry> cache = Collections.synchronizedMap(new HashMap<K, Entry>());

    @SuppressWarnings("unused")
    private String name;

    private Timer timer;

    public SoftTimedCache() {
        this("Cache Timer");
    }

    public SoftTimedCache(String name) {
        this.name = name;
    }

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

    @Override
    protected void finalize() throws Throwable {
        if (timer != null) {
            timer.cancel();
        }
        clear();
        super.finalize();
    }

    public synchronized void clear() {
        Object[] entries;
        synchronized (this) {
            entries = cache.values().toArray();
            cache.clear();
        }
        for (Object o : entries) {
            @SuppressWarnings("unchecked")
            Entry e = (Entry) o;
            V v = e.ref.get();
            e.ref.clear();
            cleanup(e);
            disposeValue(v);
        }
    }

    @Override
    public void put(final K k, V v, long holdTime, TimeUnit unit) {
        Entry e = new Entry(k, v, holdTime, unit);
        synchronized (this) {
            // First dispose of a previous entry.
            dispose(k);
            // Then cache the new one.
            cache.put(k, e);

            if (unit != null && holdTime > 0) {
                schedule(e);
            }
        }
    }

    @Override
    public V release(K k) {
        Entry e;
        synchronized (this) {
            e = cache.remove(k);
        }
        if (e == null)
            return null;
        return cleanup(e);
    }

    private V cleanup(Entry e) {
        if (e.future != null) {
            if (!e.future.isCancelled()) {
                boolean ret = e.future.cancel(false);
                if (ret == false)
                    // Already disposing of this cached entry, let it be.
                    return null;
            }
        }
        return e.ref.get();
    }

    private void dispose(K k) {
        Entry e;
        synchronized (this) {
            e = cache.remove(k);
        }
        if (e == null)
            // Has been released.
            return;

        V v = e.ref.get();
        if (v != null) {
            if (e.future != null)
                e.future.cancel(false);
            e.ref.clear();
            disposeValue(v);
        }
    }

    void schedule(final Entry e) {
        e.future = ThreadUtils.getNonBlockingWorkExecutor().schedule(new Runnable() {
            @Override
            public void run() {
                // Disposal may block, must transfer to blocking work executor.
                ThreadUtils.getBlockingWorkExecutor().execute(new Runnable() {
                    @Override
                    public void run() {
                        dispose(e.key);
                    }
                });
            }
        }, e.holdTime, e.unit);
    }


    /**
     * Override to customize disposal of values when their timer elapses.
     * 
     * @param v the value to dispose of
     */
    protected void disposeValue(V v) {
        // Do nothing by default.
    }

    public class CacheEntry {
        private final Entry e;
        private final V value;
        public CacheEntry(Entry e) {
            this.e = e;
            this.value = e.ref.get();
        }
        public K getKey() {
            return e.key;
        }
        public V getValue() {
            return value;
        }
        public boolean disposeScheduled() {
            return e.future != null;
        }
        public void schedule(long holdTime, TimeUnit unit) {
            if (e.future == null) {
                e.holdTime = holdTime;
                e.unit = unit;
                SoftTimedCache.this.schedule(e);
            }
        }
    }

    public Collection<CacheEntry> getEntries() {
        ArrayList<CacheEntry> result = new ArrayList<CacheEntry>();
        synchronized (this) {
            for (Entry e : cache.values()) {
                result.add(new CacheEntry(e));
            }
        }
        return result;
    }

}