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 *   notice, this list of conditions and the following disclaimer.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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package com.jme3.asset.cache;

import com.jme3.asset.AssetKey;
import com.jme3.asset.CloneableSmartAsset;
import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.concurrent.ConcurrentHashMap;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 * WeakRefCloneAssetCache caches cloneable assets in a weak-key
 * cache, allowing them to be collected when memory is low.
 * The cache stores weak references to the asset keys, so that
 * when all clones of the original asset are collected, will cause the
 * asset to be automatically removed from the cache.
 *
 * @author Kirill Vainer
 */
public class WeakRefCloneAssetCache implements AssetCache {

    private static final Logger logger = Logger.getLogger(WeakRefAssetCache.class.getName());

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

    /**
     * Maps cloned key to AssetRef which has a weak ref to the original
     * key and a strong ref to the original asset.
     */
    private final ConcurrentHashMap smartCache
            = new ConcurrentHashMap<>();

    /**
     * Stored in the ReferenceQueue to find out when originalKey is collected
     * by GC. Once collected, the clonedKey is used to remove the asset
     * from the cache.
     */
    private static final class KeyRef extends PhantomReference {

        AssetKey clonedKey;

        public KeyRef(AssetKey originalKey, ReferenceQueue refQueue) {
            super(originalKey, refQueue);
            clonedKey = originalKey.clone();
        }
    }

    /**
     * Stores the original key and original asset.
     * The asset info contains a cloneable asset (e.g. the original, from
     * which all clones are made) and also a weak reference to the
     * original key, which is used when the clones are produced.
     */
    private static final class AssetRef extends WeakReference {

        CloneableSmartAsset asset;

        public AssetRef(CloneableSmartAsset originalAsset, AssetKey originalKey) {
            super(originalKey);
            this.asset = originalAsset;
        }
    }

    private final ThreadLocal> assetLoadStack
            = new ThreadLocal>() {
        @Override
        protected ArrayList initialValue() {
            return new ArrayList<>();
        }
    };

    private void removeCollectedAssets() {
        int removedAssets = 0;
        for (KeyRef ref; (ref = (KeyRef) refQueue.poll()) != null;) {
            // (Cannot use ref.get() since it was just collected by GC!)
            AssetKey key = ref.clonedKey;

            // Asset was collected, note that at this point the asset cache
            // might not even have this asset anymore, it is OK.
            if (smartCache.remove(key) != null) {
                removedAssets++;
            }
        }
        if (removedAssets >= 1) {
            logger.log(Level.FINE,
                    "WeakRefCloneAssetCache: {0} assets were purged from the cache.", removedAssets);
        }
    }

    @Override
    public  void addToCache(AssetKey originalKey, T obj) {
        // Make room for new asset
        removeCollectedAssets();

        CloneableSmartAsset asset = (CloneableSmartAsset) obj;

        // No circular references, since the original asset is
        // strongly referenced, we don't want the key strongly referenced.
        asset.setKey(null);

        // Start tracking the collection of originalKey
        // (this adds the KeyRef to the ReferenceQueue)
        KeyRef ref = new KeyRef(originalKey, refQueue);

        // Place the asset in the cache, but use a clone of
        // the original key.
        smartCache.put(ref.clonedKey, new AssetRef(asset, originalKey));

        // Push the original key used to load the asset
        // so that it can be set on the clone later
        ArrayList loadStack = assetLoadStack.get();
        loadStack.add(originalKey);
    }

    @Override
    public  void registerAssetClone(AssetKey key, T clone) {
        ArrayList loadStack = assetLoadStack.get();
        ((CloneableSmartAsset) clone).setKey(loadStack.remove(loadStack.size() - 1));
    }

    @Override
    public void notifyNoAssetClone() {
        ArrayList loadStack = assetLoadStack.get();
        loadStack.remove(loadStack.size() - 1);
    }

    @Override
    @SuppressWarnings("unchecked")
    public  T getFromCache(AssetKey key) {
        AssetRef smartInfo = smartCache.get(key);
        if (smartInfo == null) {
            return null;
        } else {
            // NOTE: Optimization so that registerAssetClone()
            // can check this and determine that the asset clone
            // belongs to the asset retrieved here.
            AssetKey keyForTheClone = smartInfo.get();
            if (keyForTheClone == null) {
                // The asset was JUST collected by GC
                // (between here and smartCache.get)
                return null;
            }

            // Prevent original key from getting collected
            // while an asset is loaded for it.
            ArrayList loadStack = assetLoadStack.get();
            loadStack.add(keyForTheClone);

            return (T) smartInfo.asset;
        }
    }

    @Override
    public boolean deleteFromCache(AssetKey key) {
        ArrayList loadStack = assetLoadStack.get();

        if (!loadStack.isEmpty()) {
            throw new UnsupportedOperationException("Cache cannot be modified"
                    + "while assets are being loaded");
        }

        return smartCache.remove(key) != null;
    }

    @Override
    public void clearCache() {
        ArrayList loadStack = assetLoadStack.get();

        if (!loadStack.isEmpty()) {
            throw new UnsupportedOperationException("Cache cannot be modified"
                    + "while assets are being loaded");
        }

        smartCache.clear();
    }
}