//******************************************************************************
//
// File:    BarrierAction.java
// Package: edu.rit.pj
// Unit:    Class edu.rit.pj.BarrierAction
//
// 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.
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// A PARTICULAR PURPOSE. See the GNU General Public License for more details.
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// combined work based on this library. Thus, the terms and conditions of the
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// permission to link this library with independent modules to produce an
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// A copy of the GNU General Public License is provided in the file gpl.txt. You
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//******************************************************************************

package edu.rit.pj;

/**
 * Class BarrierAction is the abstract base class for an object containing code
 * that is executed as part of a barrier wait. A barrier wait occurs in these
 * situations within a {@linkplain ParallelRegion}:
 * <UL>
 * <LI>
 * When the {@linkplain ParallelTeam} threads finish executing all the
 * iterations of a parallel for loop.
 * <P><LI>
 * When the {@linkplain ParallelTeam} threads finish executing all the
 * {@linkplain ParallelSection}s in a group of parallel sections.
 * <P><LI>
 * When the {@linkplain ParallelTeam} threads call the {@linkplain
 * ParallelRegion}'s <TT>barrier()</TT> method explicitly.
 * </UL>
 * As each thread finishes executing one of the above constructs, each thread
 * encounters a barrier. What happens next depends on the barrier action
 * specified for that construct. There are three possibilities:
 * <UL>
 * <LI>
 * If the barrier action is omitted, or if the barrier action is {@link #WAIT
 * BarrierAction.WAIT}, each thread stops and waits at the barrier. When all
 * threads have arrived at the barrier, each thread resumes and proceeds to
 * execute whatever comes after the construct.
 * <P><LI>
 * If the barrier action is {@link #NO_WAIT BarrierAction.NO_WAIT}, nothing
 * happens. The threads do not wait for each other. Each thread immediately
 * proceeds to execute whatever comes after the construct.
 * <P><LI>
 * If the barrier action is an instance of class BarrierAction with the
 * <TT>run()</TT> method overridden, each thread stops and waits at the barrier.
 * When all threads have arrived at the barrier, <I>one</I> thread calls the
 * BarrierAction object's <TT>run()</TT> method. The particular thread that
 * calls the <TT>run()</TT> method is not specified. During this time the other
 * threads remain stopped. When the <TT>run()</TT> method returns, each thread
 * resumes and proceeds to execute whatever comes after the construct.
 * </UL>
 * <P>
 * Thus, the barrier serves to synchronize all the threads at the end of a
 * parallel construct and possibly to execute a section of code in a single
 * thread.
 *
 * @author  Alan Kaminsky
 * @version 11-Nov-2007
 */
public abstract class BarrierAction
	extends ParallelConstruct
	{

// Exported constructors.

	/**
	 * Construct a new barrier action.
	 */
	public BarrierAction()
		{
		}

// Exported operations.

	/**
	 * Execute this barrier action. The <TT>run()</TT> method is called by a
	 * single thread after all threads have arrived at the barrier.
	 * <P>
	 * The <TT>run()</TT> method must be implemented in a subclass.
	 *
	 * @exception  Exception
	 *     The <TT>run()</TT> method may throw any exception.
	 */
	public abstract void run()
		throws Exception;

// Hidden operations.

	/**
	 * Execute a barrier.
	 *
	 * @param  currentThread  Parallel team thread calling <TT>doBarrier()</TT>.
	 *
	 * @exception  Exception
	 *     The <TT>run()</TT> method may throw any exception.
	 */
	void doBarrier
		(ParallelTeamThread currentThread)
		throws Exception
		{
		// Default is to do a barrier wait with this as the barrier action.
		currentThread.barrier (this);
		}

// Exported constants.

	/**
	 * Do a barrier wait, without executing any code in a single thread.
	 */
	public static final BarrierAction WAIT = new BarrierAction()
		{
		public void run()
			{
			}
		void doBarrier
			(ParallelTeamThread currentThread)
			throws Exception
			{
			currentThread.barrier();
			}
		};

	/**
	 * Do not do a barrier wait.
	 */
	public static final BarrierAction NO_WAIT = new BarrierAction()
		{
		public void run()
			{
			}
		void doBarrier
			(ParallelTeamThread currentThread)
			throws Exception
			{
			}
		};

	}