/*******************************************************************************

"FreePastry" Peer-to-Peer Application Development Substrate

Copyright 2002-2007, Rice University. Copyright 2006-2007, Max Planck Institute 
for Software Systems.  All rights reserved.

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modification, are permitted provided that the following conditions are
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- Redistributions of source code must retain the above copyright
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- Redistributions in binary form must reproduce the above copyright
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documentation and/or other materials provided with the distribution.

- Neither the name of Rice  University (RICE), Max Planck Institute for Software 
Systems (MPI-SWS) nor the names of its contributors may be used to endorse or 
promote products derived from this software without specific prior written 
permission.

This software is provided by RICE, MPI-SWS and the contributors on an "as is" 
basis, without any representations or warranties of any kind, express or implied 
including, but not limited to, representations or warranties of 
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*******************************************************************************/ 

package rice.persistence;

/*
 * @(#) LRUCache.java
 *
 * @author Ansley Post
 * @author Alan Mislove
 *
 * @version $Id: LRUCache.java 3986 2007-11-29 18:49:28Z jstewart $
 */
import java.io.*;
import java.util.*;
import java.util.zip.*;

import rice.*;
import rice.Continuation.*;
import rice.environment.Environment;
import rice.environment.logging.Logger;
import rice.p2p.commonapi.*;
import rice.p2p.util.*;

/**
 * This class is an encapsulation of a least-recently-used (LRU)
 * cache.  It uses the provided storage service in order to
 * store the cached data.  If the Storage provides non-corruption
 * services, these services will also be provided by this cache.
 */
public class LRUCache implements Cache {

  // the maximum size of the cache
  private int maximumSize;

  // the back-end storage used by this cache
  private Storage storage;

  // the list of keys, in MRU -> LRU order
  private LinkedList order;

  protected Environment environment;
  
  /**
   * Builds a LRU cache given a storage object to store the cached
   * data in and a maximum cache size.
   *
   * @param storage The storage service to use as a back-end storage
   * @param maximumSize The maximum size, in bytes, of storage to use
   */
  public LRUCache(Storage storage, int maximumSize, Environment env) {
    this.environment = env;
    this.storage = storage;
    this.maximumSize = maximumSize;

    this.order = new LinkedList();
    Iterator i = storage.scan().getIterator();
    
    while (i.hasNext())
      order.addLast(i.next());
  }
  
  /**
   * Returns the backing storage object
   *
   * @return The storage backing this cache
   */
  public Storage getStorage() {
    return storage;
  }
  
  /**
   * Renames the given object to the new id.  This method is potentially faster
   * than store/cache and unstore/uncache.
   *
   * @param oldId The id of the object in question.
   * @param newId The new id of the object in question.
   * @param c The command to run once the operation is complete
   */
  public void rename(final Id oldId, final Id newId, Continuation c) {
    storage.rename(oldId, newId, new StandardContinuation(c) {
      public void receiveResult(Object o) {
        synchronized (order) {
          order.remove(oldId);
          order.addFirst(newId);
        }
      }
    });
  }

  /**
   * Caches an object in this storage. This method is non-blocking.
   * If the object has already been stored at the location id, this
   * method has the effect of calling <code>uncachr(id)</code> followed
   * by <code>cache(id, obj)</code>. This method finishes by calling
   * receiveResult() on the provided continuation with whether or not
   * the object was cached.  Note that the object may not actually be
   * cached due to the cache replacement policy.
   *
   * Returns <code>True</code> if the cache actaully stores the object, else
   * <code>False</code> (through receiveResult on c).
   *
   * @param id The object's id.
   * @param metadata The object's metadata
   * @param obj The object to cache.
   * @param c The command to run once the operation is complete
   */
  public void cache(final Id id, final Serializable metadata, final Serializable obj, final Continuation c) {
    final int size = getSize(obj);

    if (exists(id)) {
      synchronized (order) {
        order.remove(id);
        order.addFirst(id);
      }
      
      c.receiveResult(new Boolean(true));
      return;
    }
      
    if (size > maximumSize) {
      c.receiveResult(new Boolean(false));
      return;
    }
    
    //System.out.println("\nCaching object of size " + size + " with ID " + id + " num " + order.size());
    
    final Continuation store = new Continuation() {
      public void receiveResult(Object o) {
        synchronized (order) {
          order.addFirst(id);
        }
        
        storage.store(id, metadata, obj, c);
      }

      public void receiveException(Exception e) {
        c.receiveException(e);
      }
    };

    long totalSize = storage.getTotalSize();

    if (maximumSize - size < totalSize)
      resize(maximumSize - size, store);
    else 
      store.receiveResult(Boolean.TRUE);
  }

  /**
   * Removes the object from the list of cached objects. This method is
   * non-blocking. If the object was not in the cached list in the first place,
   * nothing happens and <code>False</code> is returned.
   *
   * Returns <code>True</code> if the action succeeds, else
   * <code>False</code>  (through receiveResult on c).
   *
   * @param pid The object's id
   * @param c The command to run once the operation is complete
   */
  public void uncache(Id id, Continuation c) {
    synchronized (order) {
      order.remove(id);
    }
    
    storage.unstore(id, c);
  }

  /**
   * Returns whether or not an object is cached in the location <code>id</code>.
   *
   * @param id The id of the object in question.
   * @return Whether or not an object is present at id.
   */
  public boolean exists(Id id) {
    return storage.exists(id);
  }

  /**
   * Returns the object identified by the given id.
   *
   * @param id The id of the object in question.
   * @param c The command to run once the operation is complete
   * @return The object, or <code>null</code> if there is no cooresponding
   * object (through receiveResult on c).
   */
  public void getObject(Id id, Continuation c) {
    if (! exists(id)) {
      c.receiveResult(null);
      return;
    }

    synchronized (this) {
      order.remove(id);
      order.addFirst(id);
    }

    storage.getObject(id, c);
  }
  
  /**
   * Returns the metadata associated with the provided object, or null if
   * no metadata exists.  The metadata must be stored in memory, so this 
   * operation is guaranteed to be fast and non-blocking.
   *
   * @param id The id for which the metadata is needed
   * @return The metadata, or null of non exists
   */
  public Serializable getMetadata(Id id) {
    return storage.getMetadata(id);
  }
  
  /**
   * Updates the metadata stored under the given key to be the provided
   * value.  As this may require a disk access, the requestor must
   * also provide a continuation to return the result to.  
   *
   * @param id The id for the metadata 
   * @param metadata The metadata to store
   * @param c The command to run once the operation is complete
   */
  public void setMetadata(Id id, Serializable metadata, Continuation command) {
    storage.setMetadata(id, metadata, command);
  }

  /**
   * Return the objects identified by the given range of ids. The IdSet 
   * returned contains the Ids of the stored objects. The range is
   * partially inclusive, the lower range is inclusive, and the upper
   * exclusive.
   *
   *
   * NOTE: This method blocks so if the behavior of this method changes and
   * uses the disk, this method may be deprecated.
   *
   * @param range The range to query  
   * @return The idset containg the keys 
   */
   public synchronized IdSet scan(IdRange range){
     return storage.scan(range);
   }
  
  /**
   * Return all objects currently stored by this catalog
   *
   * NOTE: This method blocks so if the behavior of this method changes and
   * no longer stored in memory, this method may be deprecated.
   *
   * @return The idset containg the keys 
   */
  public synchronized IdSet scan() {
    return storage.scan();
  }
  
  /**
   * Returns a map which contains keys mapping ids to the associated 
   * metadata.  
   *
   * @param range The range to query  
   * @return The map containg the keys 
   */
  public SortedMap scanMetadata(IdRange range) {
    return storage.scanMetadata(range);
  }
  
  /**
   * Returns a map which contains keys mapping ids to the associated 
   * metadata.  
   *
   * @return The treemap mapping ids to metadata 
   */
  public SortedMap scanMetadata() {
    return storage.scanMetadata();
  }
  
  /**
   * Returns the submapping of ids which have metadata less than the provided
   * value.
   *
   * @param value The maximal metadata value 
   * @return The submapping
   */
  public SortedMap scanMetadataValuesHead(Object value) {
    return storage.scanMetadataValuesHead(value);
  }
  
  /**
    * Returns the submapping of ids which have metadata null
   *
   * @return The submapping
   */
  public SortedMap scanMetadataValuesNull() {
    return storage.scanMetadataValuesNull();
  }

  /**
   * Returns the maximum size of the cache, in bytes. The result
   * is returned via the receiveResult method on the provided
   * Continuation with an Integer representing the size.
   *
   * @param c The command to run once the operation is complete
   */
  public long getMaximumSize() {
    return maximumSize;
  }
  
  /**
   * Returns the number of Ids currently stored in the catalog
   *
   * @return The number of ids in the catalog
   */
  public int getSize() {
    return storage.getSize();
  }

  /**
   * Returns the total size of the stored data in bytes. The result
   * is returned via the receiveResult method on the provided
   * Continuation with an Integer representing the size.
   *
   * @param c The command to run once the operation is complete
   * @return The total size, in bytes, of data stored.
   */
  public long getTotalSize() {
    return storage.getTotalSize();
  }

  /**
   * Sets the maximum size of the cache, in bytes. Setting this
   * value to a smaller value than the current value may result in
   * object being evicted from the cache.
   *
   * @param size The new maximum size, in bytes, of the cache.
   * @param c The command to run once the operation is complete
   * @return The success or failure of the setSize operation
   * (through receiveResult on c).
   */
  public void setMaximumSize(final int size, final Continuation c) {
    Continuation local = new Continuation() {
      public void receiveResult(Object o) {
        maximumSize = size;

        c.receiveResult(new Boolean(true));
      }

      public void receiveException(Exception e) {
        c.receiveException(e);
      }
    };

    if (size < maximumSize) {
      resize(size, local);
    } else {
      local.receiveResult(new Boolean(true));
    }
  }

  /**
   * Internal method which removes objects from the cache until the cache
   * is smaller than the specified size
   *
   * @param size The maximum number of bytes to make the cache
   * @param c The command to run once the operation is complete
   */
  private void resize(final int size, Continuation c) {
    final Continuation remove = new StandardContinuation(c) {
      private boolean waitingForSize = true;
      
      public void receiveResult(Object o) {
        if (storage.getTotalSize() > size) {
          uncache((Id) order.getLast(), this);
        } else {
          parent.receiveResult(new Boolean(true));
        }
      }
    };

    remove.receiveResult(Boolean.TRUE);
  }

  /**
   * Returns the size of the given object, in bytes.
   *
   * @param obj The object to determine the size of
   * @return The size, in bytes
   */
  private int getSize(Object obj) {
    try {
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      ObjectOutputStream oos = new XMLObjectOutputStream(new BufferedOutputStream(new GZIPOutputStream(baos)));

      oos.writeObject(obj);
      oos.close();

      return baos.toByteArray().length;
    } catch (IOException e) {
      Logger logger = environment.getLogManager().getLogger(LRUCache.class, null);
      if (logger.level <= Logger.WARNING) logger.logException("", e);
      // returns maximum value here, so it won't be cached
      return Integer.MAX_VALUE;
    }
  }
  
  /**
   * Method which is used to erase all data stored in the Cache.  
   * Use this method with care!
   *
   * @param c The command to run once done
   */
  public void flush(Continuation c) {
    storage.flush(new StandardContinuation(c) {
      public void receiveResult(Object o) {
        order.clear();
        parent.receiveResult(Boolean.TRUE);
      }
    });
  }
}
