/*
    * ====================================================================
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
    * with the License.  You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   * ====================================================================
   *
   * This software consists of voluntary contributions made by many
   * individuals on behalf of the Apache Software Foundation.  For more
   * information on the Apache Software Foundation, please see
   * <http://www.apache.org/>.
   *
   */
  package org.apache.http.impl.client.cache.ehcache;
  
  import java.io.ByteArrayInputStream;
  import java.io.ByteArrayOutputStream;
  import java.io.IOException;
  
  import net.sf.ehcache.Ehcache;
  import net.sf.ehcache.Element;
  
  import org.apache.http.client.cache.HttpCacheEntry;
  import org.apache.http.client.cache.HttpCacheEntrySerializer;
  import org.apache.http.client.cache.HttpCacheStorage;
  import org.apache.http.client.cache.HttpCacheUpdateCallback;
  import org.apache.http.client.cache.HttpCacheUpdateException;
  import org.apache.http.impl.client.cache.CacheConfig;
  import org.apache.http.impl.client.cache.DefaultHttpCacheEntrySerializer;
  
  /**
   * <p>This class is a storage backend for cache entries that uses the
   * popular <a href="http://ehcache.org">Ehcache</a> cache implementation.
   * In particular, this backend allows for spillover to disk, where the
   * cache can be effectively larger than memory, and cached responses are
   * paged into and out of memory from disk as needed.</p>
   *
   * <p><b>N.B.</b> Since the Ehcache is configured ahead of time with a
   * maximum number of cache entries, this effectively ignores the
   * {@link CacheConfig#setMaxCacheEntries(int) maximum cache entries}
   * specified by a provided {@link CacheConfig}.</p>
   *
   * <p>Please refer to the <a href="http://ehcache.org/documentation/index.html">
   * Ehcache documentation</a> for details on how to configure the Ehcache
   * itself.</p>
   * @since 4.1
   */
  public class EhcacheHttpCacheStorage implements HttpCacheStorage {
  
      private final Ehcache cache;
      private final HttpCacheEntrySerializer serializer;
      private final int maxUpdateRetries;
  
      /**
       * Constructs a storage backend using the provided Ehcache
       * with default configuration options.
       * @param cache where to store cached origin responses
       */
      public EhcacheHttpCacheStorage(final Ehcache cache) {
          this(cache, CacheConfig.DEFAULT, new DefaultHttpCacheEntrySerializer());
      }
  
      /**
       * Constructs a storage backend using the provided Ehcache
       * with the given configuration options.
       * @param cache where to store cached origin responses
       * @param config cache storage configuration options - note that
       *   the setting for max object size <b>will be ignored</b> and
       *   should be configured in the Ehcache instead.
       */
      public EhcacheHttpCacheStorage(final Ehcache cache, final CacheConfig config){
          this(cache, config, new DefaultHttpCacheEntrySerializer());
      }
  
      /**
       * Constructs a storage backend using the provided Ehcache
       * with the given configuration options, but using an alternative
       * cache entry serialization strategy.
       * @param cache where to store cached origin responses
       * @param config cache storage configuration options - note that
       *   the setting for max object size <b>will be ignored</b> and
       *   should be configured in the Ehcache instead.
       * @param serializer alternative serialization mechanism
       */
      public EhcacheHttpCacheStorage(final Ehcache cache, final CacheConfig config, final HttpCacheEntrySerializer serializer){
          this.cache = cache;
         this.maxUpdateRetries = config.getMaxUpdateRetries();
         this.serializer = serializer;
     }
 
     @Override
     public synchronized void putEntry(final String key, final HttpCacheEntry entry) throws IOException {
         final ByteArrayOutputStream bos = new ByteArrayOutputStream();
         serializer.writeTo(entry, bos);
         cache.put(new Element(key, bos.toByteArray()));
     }
 
     @Override
     public synchronized HttpCacheEntry getEntry(final String key) throws IOException {
         final Element e = cache.get(key);
         if(e == null){
             return null;
         }
 
         final byte[] data = (byte[])e.getObjectValue();
         return serializer.readFrom(new ByteArrayInputStream(data));
     }
 
     @Override
     public synchronized void removeEntry(final String key) {
         cache.remove(key);
     }
 
     @Override
     public synchronized void updateEntry(final String key, final HttpCacheUpdateCallback callback)
             throws IOException, HttpCacheUpdateException {
         int numRetries = 0;
         do{
             final Element oldElement = cache.get(key);
 
             HttpCacheEntry existingEntry = null;
             if(oldElement != null){
                 final byte[] data = (byte[])oldElement.getObjectValue();
                 existingEntry = serializer.readFrom(new ByteArrayInputStream(data));
             }
 
             final HttpCacheEntry updatedEntry = callback.update(existingEntry);
 
             if (existingEntry == null) {
                 putEntry(key, updatedEntry);
                 return;
             } else {
                 // Attempt to do a CAS replace, if we fail then retry
                 // While this operation should work fine within this instance, multiple instances
                 //  could trample each others' data
                 final ByteArrayOutputStream bos = new ByteArrayOutputStream();
                 serializer.writeTo(updatedEntry, bos);
                 final Element newElement = new Element(key, bos.toByteArray());
                 if (cache.replace(oldElement, newElement)) {
                     return;
                 }else{
                     numRetries++;
                 }
             }
         }while(numRetries <= maxUpdateRetries);
         throw new HttpCacheUpdateException("Failed to update");
     }
 }