//******************************************************************************
//
// File:    SharedObject.java
// Package: edu.rit.pj.reduction
// Unit:    Class edu.rit.pj.reduction.SharedObject
//
// This Java source file is copyright (C) 2007 by Alan Kaminsky. All rights
// reserved. For further information, contact the author, Alan Kaminsky, at
// ark@cs.rit.edu.
//
// This Java source file is part of the Parallel Java Library ("PJ"). PJ is free
// software; you can redistribute it and/or modify it under the terms of the GNU
// General Public License as published by the Free Software Foundation; either
// version 3 of the License, or (at your option) any later version.
//
// PJ 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 for more details.
//
// Linking this library statically or dynamically with other modules is making a
// combined work based on this library. Thus, the terms and conditions of the
// GNU General Public License cover the whole combination.
//
// As a special exception, the copyright holders of this library give you
// permission to link this library with independent modules to produce an
// executable, regardless of the license terms of these independent modules, and
// to copy and distribute the resulting executable under terms of your choice,
// provided that you also meet, for each linked independent module, the terms
// and conditions of the license of that module. An independent module is a
// module which is not derived from or based on this library. If you modify this
// library, you may extend this exception to your version of the library, but
// you are not obligated to do so. If you do not wish to do so, delete this
// exception statement from your version.
//
// A copy of the GNU General Public License is provided in the file gpl.txt. You
// may also obtain a copy of the GNU General Public License on the World Wide
// Web at http://www.gnu.org/licenses/gpl.html.
//
//******************************************************************************

package edu.rit.pj.reduction;

import java.util.concurrent.atomic.AtomicReference;

/**
 * Class SharedObject provides a reduction variable for a value of an object
 * type.
 * <P>
 * Class SharedObject is multiple thread safe. The methods use lock-free atomic
 * compare-and-set.
 * <P>
 * <I>Note:</I> Class SharedObject is implemented using class
 * java.util.concurrent.atomic.AtomicReference.
 *
 * @param  <T>  Object data type.
 *
 * @author  Alan Kaminsky
 * @version 20-Jun-2007
 */
public class SharedObject<T>
	{

// Hidden data members.

	private AtomicReference<T> myValue;

// Exported constructors.

	/**
	 * Construct a new object reduction variable with the initial value null.
	 */
	public SharedObject()
		{
		myValue = new AtomicReference<T>();
		}

	/**
	 * Construct a new object reduction variable with the given initial value.
	 *
	 * @param  initialValue  Initial value.
	 */
	public SharedObject
		(T initialValue)
		{
		myValue = new AtomicReference<T> (initialValue);
		}

// Exported operations.

	/**
	 * Returns this reduction variable's current value.
	 *
	 * @return  Current value.
	 */
	public T get()
		{
		return myValue.get();
		}

	/**
	 * Set this reduction variable to the given value.
	 *
	 * @param  value  New value.
	 */
	public void set
		(T value)
		{
		myValue.set (value);
		}

	/**
	 * Set this reduction variable to the given value and return the previous
	 * value.
	 *
	 * @param  value  New value.
	 *
	 * @return  Previous value.
	 */
	public T getAndSet
		(T value)
		{
		return myValue.getAndSet (value);
		}

	/**
	 * Atomically set this reduction variable to the given updated value if the
	 * current value equals the expected value.
	 *
	 * @param  expect  Expected value.
	 * @param  update  Updated value.
	 *
	 * @return  True if the update happened, false otherwise.
	 */
	public boolean compareAndSet
		(T expect,
		 T update)
		{
		return myValue.compareAndSet (expect, update);
		}

	/**
	 * Atomically set this reduction variable to the given updated value if the
	 * current value equals the expected value. May fail spuriously.
	 *
	 * @param  expect  Expected value.
	 * @param  update  Updated value.
	 *
	 * @return  True if the update happened, false otherwise.
	 */
	public boolean weakCompareAndSet
		(T expect,
		 T update)
		{
		return myValue.weakCompareAndSet (expect, update);
		}

	/**
	 * Combine this reduction variable with the given value using the given
	 * operation. The result is stored back into this reduction variable and is
	 * returned.
	 *
	 * @param  value  Value.
	 * @param  op     Binary operation.
	 *
	 * @return  (This variable) <I>op</I> (<TT>value</TT>).
	 */
	public T reduce
		(T value,
		 ObjectOp<T> op)
		{
		for (;;)
			{
			T oldvalue = myValue.get();
			T newvalue = op.op (oldvalue, value);
			if (myValue.compareAndSet (oldvalue, newvalue)) return newvalue;
			}
		}

	/**
	 * Returns a string version of this reduction variable.
	 *
	 * @return  String version.
	 */
	public String toString()
		{
		return myValue.toString();
		}

	}