/*
 * Copyright (C) 2009 eXo Platform SAS.
 *
 * This is free software; you can redistribute it and/or modify it
 * under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation; either version 2.1 of
 * the License, or (at your option) any later version.
 *
 * This software 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this software; if not, write to the Free
 * Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA, or see the FSF site: http://www.fsf.org.
 */

package org.exoplatform.commons.cache.future;

import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;

import org.exoplatform.services.log.ExoLogger;
import org.exoplatform.services.log.Log;

import lombok.Getter;

/**
 * <p>
 * A future cache that prevents the loading of the same resource twice. This
 * should be used when the resource to load is very expensive 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 Loader#retrieve(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 implementation is delegated to the contractual methods
 * {@link #get(Object)} and {@link #put(Object, Object)}. Those methods are
 * intended to be used by the future cache only.
 * </p>
 * <p>
 * The {@link Loader} interface provides a source to retrieve objects to put in
 * the cache. The goal to maintain this interface is to decouple the cache from
 * the object source.
 * </p>
 *
 * @author      <a href="mailto:julien.viet@exoplatform.com">Julien Viet</a>
 * @version     $Revision$
 * @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> {

  /** . */
  @Getter
  final Loader<K, V, C>                              loader;

  /** . */
  private final ConcurrentMap<K, Retrieval<K, V, C>> futureEntries;

  /** . */
  private final Log                                  log = ExoLogger.getLogger(FutureCache.class);

  protected FutureCache(Loader<K, V, C> loader) {
    this.loader = loader;
    this.futureEntries = new ConcurrentHashMap<>();
  }

  /**
   * Retrieves the cached value corresponding to the specified key from the
   * 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);

  /**
   * Updates the cache only local mode 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 putOnly(K key, V value);

  /**
   * Perform a cache lookup for the specified key within the specified context.
   * When the value cannot be loaded (because it does not exist or it failed or
   * anything else that does not come to my mind), the value null is returned.
   *
   * @param  context the context in which the resource is accessed
   * @param  key     the key identifying the resource
   * @return         the value
   */
  public final V get(final C context, final K key) { // NOSONAR
    if (key == null) {
      return null;
    }
    // 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
      Retrieval<K, V, C> retrieval = new Retrieval<>(context, key, this);

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

      //
      try {
        Retrieval<K, V, C> phantom = futureEntries.putIfAbsent(key, retrieval);

        // Use the value that could have been inserted by another thread
        if (phantom != null) {
          retrieval = phantom;
          inserted = false;
        } else {
          try {
            retrieval.current = Thread.currentThread();
            retrieval.future.run();
          } catch (Exception e) {
            log.error("Retrieval of resource " + key + " threw an exception", e);
          } finally {
            retrieval.current = null;
          }
        }

        // Returns the value
        if (retrieval.current == Thread.currentThread()) {
          throw new IllegalStateException("Reentrancy detected when obtaining key " + key + " with context "
              + context + " detected");
        } else {
          try {
            value = retrieval.future.get();
          } catch (ExecutionException e) {
            throw new IllegalStateException("Error retrieving value with key " + key, e);
          } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
          }
        }
      } finally {
        // Clean up the per key map but only if our insertion succeeded and with
        // our future
        if (inserted) {
          futureEntries.remove(key, retrieval);
        }
      }
    }

    //
    return value;
  }
}
