/*
 * Copyright 2007 Jim Procter
 *
 * Licensed 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.
 */
package org.castor.core.util;

import java.util.IdentityHashMap;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * <p>
 * lightweight mechanism for thread-safe detection of cyclic calls to hashCode or equals in objects
 * created by the XML CodeGenerator.
 * </p>
 * <p>
 * Usage
 * </p>
 * <ol>
 * <li>startingToCycle is called on a particular object prior to recursing on it, and recursion
 * should only occur if this call returns false.</li>
 * <li>releaseCycleHandle is called after the recursive call returns in order to release the cycle
 * lock on the object.</li>
 * </ol>
 * <p>
 * <strong>Note :</strong> Do not use this cycle breaking mechanism on object comparisons where two
 * instances may share the same reference to some third object, such as a String constant.
 * </p>
 * 
 * @author <a href="mailto:jimp@compbio.dundee.ac.uk">Jim Procter</a>
 */
public class CycleBreaker {

  /**
   * Hash of threads and objects that we are keeping track of.
   */
  private static final IdentityHashMap<Thread, IdentityHashMap<Object, Object>> _threadHash =
      new IdentityHashMap<Thread, IdentityHashMap<Object, Object>>();

  /**
   * Lock used to isolate write accesses to <tt>_threadHash</tt>.
   * 
   * @since 1.3.1
   */
  private static final ReentrantReadWriteLock _lock = new ReentrantReadWriteLock();

  /**
   * Test to see if we are about to begin cycling on a method call to beingHashed.
   * 
   * @param beingHashed the object to check for a cycle.
   * @return true if a cycle is about to occur on this non-null object.
   */
  public static boolean startingToCycle(final Object beingHashed) {
    if (beingHashed == null) {
      return false;
    }

    // before accessing _threadHash acquire read lock first
    _lock.readLock().lock();
    IdentityHashMap<Object, Object> hthr = _threadHash.get(Thread.currentThread());
    _lock.readLock().unlock();
    if (hthr == null) {
      hthr = new IdentityHashMap<Object, Object>();
      hthr.put(beingHashed, beingHashed);

      // before accessing _threadHash acquire write lock first
      _lock.writeLock().lock();
      _threadHash.put(Thread.currentThread(), hthr);
      _lock.writeLock().unlock();
      return false; // first call. no cycle detected
    }

    // it's not necessary to lock here because every entry
    // read from _threadHash is accessed by the same thread which
    // added it to _threadHash (in _threadHash all values are keyed
    // by the threads hash)

    Object objhandle = hthr.get(beingHashed);
    if (objhandle == null) {

      // this is the default for a hash value currently being computed.
      hthr.put(beingHashed, beingHashed);
      return false; // first call. no cycle detected.
    }
    return true;
  }

  /**
   * Called to release Cycling lock for this object at the end of a routine where cycles are to be
   * detected.
   * 
   * @param beingHashed the object for which the cycle-lock will be released.
   */
  public static void releaseCycleHandle(final Object beingHashed) {
    if (beingHashed == null) {
      return;
    }

    Thread currentThread = Thread.currentThread();

    // before accessing _threadHash acquire read lock first
    _lock.readLock().lock();
    IdentityHashMap<Object, Object> hthr = _threadHash.get(currentThread);
    _lock.readLock().unlock();
    if (hthr != null) {

      // it's not necessary to lock here because every entry
      // read from _threadHash is accessed by the same thread which
      // added it to _threadHash (in _threadHash all values are keyed
      // by the threads hash)
      if (hthr.containsKey(beingHashed)) {
        hthr.remove(beingHashed);

        // release any references if we have no more CycleHandles
        if (hthr.isEmpty()) {

          // before removing the threads empty hash from _threadHash
          // acquire write lock first
          _lock.writeLock().lock();
          _threadHash.remove(currentThread);
          _lock.writeLock().unlock();
        }
      }
    }
  }
}
