/*
 *                    PDB web 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.
 *
 *
 * Created on Mar 26, 2009
 * Created by ap3
 *
 */

package org.biojava.nbio.core.util;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;
import java.util.Set;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


/**
 * An in memory cache using soft references. (can be garbage collected)
 * This code is based on: http://java-interview-faqs.blogspot.com/2008/09/building-faster-and-efficient-cache.html
 * <p>
 * Note that entrySet() is not implemented and therefore many methods such as keySet(),
 * containsKey(), values() etc do not work.
 * <p>
 * This class is therefore best used as a cache simply to put and get items by a known key
 * @param <K> the key type
 * @param <V> the value type
 */
public class SoftHashMap<K, V> extends AbstractMap<K, V> {

        private final static Logger logger = LoggerFactory.getLogger(SoftHashMap.class);

        public static final int DEFAULT_LIMIT = 1;

        /** The internal HashMap that stores SoftReference to actual data. */
        private final Map<K, SoftReference<V>> map = new HashMap<>();

        /** Maximum Number of references you dont want GC to collect. */
        private final int max_limit;

        /** The FIFO list of hard references, order of last access. */
        private final LinkedList<V> hardCache = new LinkedList<>();

        /** Reference queue for cleared SoftReference objects. */
        private final ReferenceQueue<V> queue = new ReferenceQueue<>();

        public SoftHashMap() {
                this(1000);
        }

        /**
         * @param hardSize A maximum number of items to maintain hard references to
         * that will not be eligible for garbage collection
         */
        public SoftHashMap(int hardSize) {
                max_limit = hardSize;
        }

        @Override
        public V get(Object key) {

                V result = null;

                // We get the SoftReference represented by that key
                SoftReference<V> soft_ref = map.get(key);

                if (soft_ref != null) {
                        try {
                                // From the SoftReference we get the value, which can be
                                // null if it was not in the map, or it was removed in
                                // the clearGCCollected() method defined below

                                result = soft_ref.get();
                                if (result == null) {
                                        // If the value has been garbage collected, remove the
                                        // entry from the HashMap.
                                        map.remove(key);
                                } else {
                                        // We now add this object to the beginning of the hard
                                        // reference queue. One reference can occur more than
                                        // once, because lookups of the FIFO queue are slow, so
                                        // we don't want to search through it each time to remove
                                        // duplicates.

                                        synchronized (hardCache){
                                                hardCache.addFirst(result);
                                                if (hardCache.size() > max_limit) {
                                                        // Remove the last entry if list greater than MAX_LIMIT
                                                        hardCache.removeLast();
                                                }
                                        }
                                }
                        } catch (Exception e){
                                logger.error("Exception: ", e);
                        }
                }
                return result;
        }

        /**
         * We define our own subclass of SoftReference which contains not only the
         * value but also the key to make it easier to find the entry in the HashMap
         * after it's been garbage collected.
         */
        private static class SoftValue<K, V> extends SoftReference<V> {

                private final Object key; // always make data member final
                
                /**
                 * Did you know that an outer class can access private data members and
                 * methods of an inner class? I didn't know that! I thought it was only
                 * the inner class who could access the outer class's private
                 * information. An outer class can also access private members of an
                 * inner class inside its inner class.
                 */
                private SoftValue(V k, K key, ReferenceQueue<? super V> q) {
                        super(k, q);
                        this.key = key;
                }
        }

        /**
         * Here we go through the ReferenceQueue and remove garbage collected
         * SoftValue objects from the HashMap by looking them up using the
         * SoftValue.key data member.
         */
        @SuppressWarnings("unchecked") // every Reference in queue is stored as a SoftValue
        private void clearGCCollected() {
                SoftValue<K, V> sv;
                while ((sv = (SoftValue<K, V>) queue.poll()) != null) {
                        map.remove(sv.key); // we can access private data!
                }
        }

        /**
         * Here we put the key, value pair into the HashMap using a SoftValue
         * object.
         */
        @Override
        public synchronized V put(K key, V value) {
                clearGCCollected();
                logger.debug("Putting {} on cache. size: {}", key, size());
                map.put(key, new SoftValue<K, V>(value, key, queue));
                return value;
        }

        @Override
        public V remove(Object key) {
                clearGCCollected();
                logger.debug("Removing {} from cache. size: {}", key, size());
                return map.remove(key).get();
        }

        @Override
        public void clear() {
                synchronized (hardCache){
                        hardCache.clear();
                }

                clearGCCollected();
                logger.debug("clearing cache");
                map.clear();
        }

        @Override
        public int size() {
                clearGCCollected();
                return map.size();
        }

        @Override
        public Set<Map.Entry<K, V>> entrySet() {
                throw new UnsupportedOperationException();
        }
}