/*
 *
 * Copyright (c) 1997-1999 Scott Oaks and Henry Wong. All Rights Reserved.
 *
 * Permission to use, copy, modify, and distribute this software
 * and its documentation for NON-COMMERCIAL purposes and
 * without fee is hereby granted.
 *
 * This sample source code is provided for example only,
 * on an unsupported, as-is basis. 
 *
 * AUTHOR MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF
 * THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE, OR NON-INFRINGEMENT. AUTHOR SHALL NOT BE LIABLE FOR
 * ANY DAMAGES SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR
 * DISTRIBUTING THIS SOFTWARE OR ITS DERIVATIVES.
 *
 * THIS SOFTWARE IS NOT DESIGNED OR INTENDED FOR USE OR RESALE AS ON-LINE
 * CONTROL EQUIPMENT IN HAZARDOUS ENVIRONMENTS REQUIRING FAIL-SAFE
 * PERFORMANCE, SUCH AS IN THE OPERATION OF NUCLEAR FACILITIES, AIRCRAFT
 * NAVIGATION OR COMMUNICATION SYSTEMS, AIR TRAFFIC CONTROL, DIRECT LIFE
 * SUPPORT MACHINES, OR WEAPONS SYSTEMS, IN WHICH THE FAILURE OF THE
 * SOFTWARE COULD LEAD DIRECTLY TO DEATH, PERSONAL INJURY, OR SEVERE
 * PHYSICAL OR ENVIRONMENTAL DAMAGE ("HIGH RISK ACTIVITIES").  AUTHOR
 * SPECIFICALLY DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR
 * HIGH RISK ACTIVITIES.
 */


public class CPUScheduler extends Thread {
	private CircularList threads;
	private Thread current;
	private int timeslice;
	private static boolean initialized = false;
	private boolean needThreads;

	private static synchronized boolean isInitialized() {
		if (initialized)
			return true;
		initialized = true;
		return false;
	}

	public CPUScheduler(int t) {
		if (isInitialized())
			throw new SecurityException("Already initialized");
		threads = new CircularList();
		timeslice = t;
		setDaemon(true);
	}

	public synchronized void addThread(Thread t) {
		t.setPriority(2);
		threads.insert(t);
		if (needThreads) {
			needThreads = false;
			notify();
		}
	}

	public void removeThread(Thread t) {
		threads.delete(t);
		synchronized(this) {
			if (t == current)
				current = null;
		}
	}

	public synchronized void run() {
		setPriority(6);
		while (true) {
			current = (Thread) threads.getNext();
			while (current == null) {
				needThreads = true;
				try {
					wait();
				} catch (Exception e) {}
				current = (Thread) threads.getNext();
			}
			try {
				current.setPriority(4);
			} catch (Exception e) {
				removeThread(current);
				continue;
			}
			try {
				wait(timeslice);
			} catch (InterruptedException ie) {};
			if (current != null) {
				try {
					current.setPriority(2);
				} catch (Exception e) {
					removeThread(current);
				}
			}
		}
	}
}