/**
 * Scheduler.java
 *
 * This class is a simple round-robin scheduler.
 * The idea for this scheduler came from "Java Threads"
 * by Oaks and Wong (Oreilly, 1999).
 *
 * @author Greg Gagne, Peter Galvin, Avi Silberschatz
 * @version 1.0 - July 15, 1999.
 * Copyright 2000 by Greg Gagne, Peter Galvin, Avi Silberschatz
 * Applied Operating Systems Concepts - John Wiley and Sons, Inc.
 */

 
public class Scheduler extends Thread
{
   private CircularList queue;
   private int timeSlice;
   private static final int DEFAULT_TIME_SLICE = 1000; // 1 second
   
   public Scheduler() {
      timeSlice = DEFAULT_TIME_SLICE;
      queue = new CircularList();
   }
   
   public Scheduler(int quantum) {
      timeSlice = quantum;
      queue = new CircularList();
   }
   
   /**
    * adds a thread to the queue
    * @return void
    */
   public void addThread(Thread t) {
      queue.addItem(t);   
   }
   
   /**
    * this method puts the scheduler to sleep for a time quantum
    * @return void
    */
   private void schedulerSleep() {
      try {
         Thread.sleep(timeSlice);
      } catch (InterruptedException e) { };
   }
   
   
   public void run() {
      Thread current;
     
      // set the priority of the scheduler to the highest priority
      this.setPriority(6);
      
      while (true) {
            try {
               current = (Thread)queue.getNext();
            
               if ( (current != null) && (current.isAlive()) ) {
                     current.setPriority(4);
               
                  schedulerSleep();

		System.out.println("* * * Context Switch * * * ");
               
                     current.setPriority(2);
               }
                     
            } catch (NullPointerException e3) { } ;
      }
   }
}