/** 
 * A class to hold the forks in the dining philosphers problem.
 * This class acts as a monitor to control the allocation of forks.
 * 
 * @author Scot Drysdale
 */
public class ForksMonitor {
  public boolean [] forks;     // True indicates that the fork is taken
  
  /**
   * Construct a object with num forks.
   * Mark all as not in use.
   * @param num number of forks (and philosophers)
   */
  public ForksMonitor(int num) {
  	forks = new boolean[num];
  	for(int i = 0; i < forks.length; i++)
  		forks[i] = false;
  }
  
  /**
   * Pick up forks numbered pNum and pNum+1 (wrapping).  
   * Block if either fork is in use.
   * pNum the number of the first fork
   */
  public synchronized void pickUpForks(int pNum) throws InterruptedException {
  	// If either fork is in use wait
	  while (forks[pNum] || forks[(pNum+1) % forks.length] )   
        wait();
		
	  // Take the forks
    forks[pNum] = true;
    forks[(pNum+1) % forks.length] = true;
  }
  
  /**
   * Put down forks numbered pNum and pNum+1 (wrapping).  
   * Assumes that this philosopher had been granted the forks.
   * pNum the number of the first fork
   */
  public synchronized void putDownForks(int pNum) throws InterruptedException {	
	  // Put down the forks
    forks[pNum] = false;
    forks[(pNum+1) % forks.length] = false;
    notifyAll();
  }
}