/*
 * Copyright (C) 2003-2011 eXo Platform SAS.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
package org.etk.storage.plugins.cache;

import java.lang.reflect.UndeclaredThrowableException;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

import org.etk.common.logging.Logger;
import org.etk.storage.plugins.cache.loader.LoaderVisitor;

/**
 * <p> A future cache that prevents the loading of the same resource twice.
 * This should be used when the resource to load is very exprensive or cannot 
 * be concurrently retrieved(like a classloading). </p>
 * 
 * <p> The future cache should be used with the {@link #get(Object, Object)}  method, that retrieves an object
 * from the cache. When the object is not found, then the {@link LoaderVisitor#accept(Object, Object)} method is 
 * used to retrieve the data and then this data is inserted in the cache.</p>
 * 
 * <p>The class is abstract and does not implement a cache technology by itself, the cache implement is delegated 
 * to the contractual methods {@link #get(Object)} and {@link #put(Object, Object)}. Those are intended to be used
 *  by the future cache only.</p>
 *  
 *  <p> The {@link LoaderVisitor} interface provides a source to retrieve objects to put in cache implementation is delegated 
 *  this interface is to decouble the cache from the object source.
 *  
 * Created by The eXo Platform SAS
 * Author : eXoPlatform
 *          exo@exoplatform.com
 * Jul 22, 2011  
 * @param <K> the key type parameter.
 * @param <V> the value type parameter.
 * @param <C> the context type parameter.
 */
public abstract class FutureCache<K, V, C> {

  //Defines the loader which provides a source to retrieve objects.
  private final LoaderVisitor<K,V, C> loader;
  
  private final Logger log = Logger.getLogger(FutureCache.class);
  
  private final ConcurrentMap<K, FutureTask<V>> futureEntries;
  
  public FutureCache(LoaderVisitor<K, V, C> loader) {
    this.loader = loader;
    this.futureEntries = new ConcurrentHashMap<K, FutureTask<V>>();
  }
  
  /**
   * Retrieves the cached value corresponding to the specified key from 
   * cache, it must returns null when the key does not exist. This method
   *  is intended for internal use by the future cache only.
   *  
   * @param key the key.
   * @return the cache value.
   */
  protected abstract V get(K key);
  
  /**
   * Updates the cache with a new key/value pair. This method is intended
   *  for internal use by the future cache only.
   *  
   * @param key the key
   * @param value the cache value.
   */
  protected abstract void put(K key, V value);
  
  public final V get(final C context, final K key) {
    // First we try a simple cache get
    V value = get(key);

    // If it does not succeed then we go through a process that will avoid to
    // load
    // the same resource concurrently
    if (value == null) {
      // Create our future
      FutureTask<V> future = new FutureTask<V>(new Callable<V>() {
        public V call() throws Exception {
          // Retrieve the value from the loader
          V value = loader.accept(context, key);

          //
          if (value != null) {
            // Cache it, it is made available to other threads (unless someone
            // removes it)
            put(key, value);

            // Return value
            return value;
          } else {
            return null;
          }
        }
      });

      // This boolean means we inserted in the local
      boolean inserted = true;

      //
      try {
        FutureTask<V> phantom = futureEntries.putIfAbsent(key, future);

        // Use the value that could have been inserted by another thread
        if (phantom != null) {
          future = phantom;
          inserted = false;
        } else {
          future.run();
        }

        // Returns the value
        value = future.get();
      } catch (ExecutionException e) {
        if (e.getCause() != null) {
          throw new UndeclaredThrowableException(e.getCause());
        } else {
          log.error("Computing of resource " + key + " threw an exception", e.getCause());
        }
      } catch (Exception e) {
        log.error("Retrieval of resource " + key + " threw an exception", e);
      } finally {
        // Clean up the per key map but only if our insertion succeeded and with
        // our future
        if (inserted) {
          futureEntries.remove(key, future);
        }
      }
    }

    //
    return value;
  }
  
}