/*
 * Copyright 2000 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code 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 General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 *  
 */

package sun.jvm.hotspot.utilities;

/** The backend for the message queue abstraction. This class is
    instantiated first and queried to provide the two "sides" of the
    message queue. */

import java.util.LinkedList;

public class MessageQueueBackend {
  // The two queues
  private MessageQueueImpl leftRightQueue;
  private MessageQueueImpl rightLeftQueue;

  public MessageQueueBackend() {
    LinkedList leftRightPipe = new LinkedList();
    LinkedList rightLeftPipe = new LinkedList();
    leftRightQueue = new MessageQueueImpl(rightLeftPipe, leftRightPipe);
    rightLeftQueue = new MessageQueueImpl(leftRightPipe, rightLeftPipe);
  }

  /** Get one of the two symmetric sides of this message queue. */
  public MessageQueue getFirstQueue() {
    return leftRightQueue;
  }

  /** Get second of the two symmetric sides of this message queue. */
  public MessageQueue getSecondQueue() {
    return rightLeftQueue;
  }

  private class MessageQueueImpl implements MessageQueue {
    private LinkedList readList;
    private LinkedList writeList;

    public MessageQueueImpl(LinkedList listToReadFrom, LinkedList listToWriteTo) {
      readList = listToReadFrom;
      writeList = listToWriteTo;
    }

    public Object readMessage() {
      synchronized(readList) {
	while (readList.isEmpty()) {
	  try {
	    readList.wait();
	  }
	  catch (InterruptedException e) {
	  }
	}
	return readList.removeFirst();
      }
    }

    public Object readMessageWithTimeout(long millis) {
      synchronized(readList) {
	if (readList.isEmpty()) {
	  if (millis == 0) {
	    return null;
	  }
	  try {
	    readList.wait(millis);
	  }
	  catch (InterruptedException e) {
	  }
	}
	// If list is still empty after wait, return null
	if (readList.isEmpty()) {
	  return null;
	}
	return readList.removeFirst();
      }
    }

    public void writeMessage(Object obj) {
      synchronized(writeList) {
	writeList.addLast(obj);
	writeList.notify();
      }
    }
  }
}
