/*
 * Copyright (c) 2001-2008 Caucho Technology, Inc.  All rights reserved.
 *
 * The Apache Software License, Version 1.1
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution, if
 *    any, must include the following acknowlegement:
 *       "This product includes software developed by the
 *        Caucho Technology (http://www.caucho.com/)."
 *    Alternately, this acknowlegement may appear in the software itself,
 *    if and wherever such third-party acknowlegements normally appear.
 *
 * 4. The names "Burlap", "Resin", and "Caucho" must not be used to
 *    endorse or promote products derived from this software without prior
 *    written permission. For written permission, please contact
 *    info@caucho.com.
 *
 * 5. Products derived from this software may not be called "Resin"
 *    nor may "Resin" appear in their names without prior written
 *    permission of Caucho Technology.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL CAUCHO TECHNOLOGY OR ITS CONTRIBUTORS
 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
 * OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
 * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
 * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN
 * IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * @author Scott Ferguson
 */

package com.caucho.hessian.util;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReferenceArray;

FreeList provides a simple class to manage free objects.  This is useful
for large data structures that otherwise would gobble up huge GC time.
The free list is bounded.  Freeing an object when the list is full will
do nothing.
/**
 * FreeList provides a simple class to manage free objects.  This is useful
 * for large data structures that otherwise would gobble up huge GC time.
 *
 * <p>The free list is bounded.  Freeing an object when the list is full will
 * do nothing.
 */
public final class HessianFreeList<T> {
  private final AtomicReferenceArray<T> _freeStack;
  private final AtomicInteger _top = new AtomicInteger();

  
Create a new free list.
Params:
initialSize – maximum number of free objects to store./**
   * Create a new free list.
   *
   * @param initialSize maximum number of free objects to store.
   */
  public HessianFreeList(int size)
  {
    _freeStack = new AtomicReferenceArray(size);
  }
  
  
Try to get an object from the free list.  Returns null if the free list
is empty.
Returns:
the new object or null./**
   * Try to get an object from the free list.  Returns null if the free list
   * is empty.
   *
   * @return the new object or null.
   */
  public T allocate()
  {
    int top = _top.get();

    if (top > 0 && _top.compareAndSet(top, top - 1))
      return _freeStack.getAndSet(top - 1, null);
    else
      return null;
  }
  
  
Frees the object.  If the free list is full, the object will be garbage
collected.
Params:
obj – the object to be freed./**
   * Frees the object.  If the free list is full, the object will be garbage
   * collected.
   *
   * @param obj the object to be freed.
   */
  public boolean free(T obj)
  {
    int top = _top.get();

    if (top < _freeStack.length()) {
      boolean isFree = _freeStack.compareAndSet(top, null, obj);
      
      _top.compareAndSet(top, top + 1);

      return isFree;
    }
    else
      return false;
  }

  public boolean allowFree(T obj)
  {
    return _top.get() < _freeStack.length();
  }

  
Frees the object.  If the free list is full, the object will be garbage
collected.
Params:
obj – the object to be freed./**
   * Frees the object.  If the free list is full, the object will be garbage
   * collected.
   *
   * @param obj the object to be freed.
   */
  public void freeCareful(T obj)
  {
    if (checkDuplicate(obj))
      throw new IllegalStateException("tried to free object twice: " + obj);

    free(obj);
  }

  
Debugging to see if the object has already been freed.
/**
   * Debugging to see if the object has already been freed.
   */
  public boolean checkDuplicate(T obj)
  {
    int top = _top.get();

    for (int i = top - 1; i >= 0; i--) {
      if (_freeStack.get(i) == obj)
        return true;
    }

    return false;
  }
}