import java.util.Random;
import java.util.concurrent.Semaphore;

/**
 * Solves the dining philosophers problem
 * This version uses a semaphore to control access to 
 * each fork.
 * 
 * @author Scot Drysdale
 */
public class PhilosopherSemaphore extends Thread {
	private int myNum; // Number of this philosopher
	private static Semaphore [] forks;
	private Random generator;
	private static final int EAT_TIME = 1000;
	private static final int THINK_TIME = 1000;
	private static final int NUM_MEALS = 10;
	private static final int NUM_PHILOSOPHERS = 6;

	public PhilosopherSemaphore(int philNum) {
		myNum = philNum;
		generator = new Random();
	}

	/**
	 * Eat using semaphores to control the forks This version can (and probably
	 * will) deadlock
	 */
	public void starve(int meal) throws InterruptedException {

		System.out.println("Philosopher " + myNum + " tries to pick up fork "
				+ myNum);
		forks[myNum].acquire();
		System.out.println("Philosopher " + myNum + " has fork " + myNum
				+ " and tries to pick up fork " + ((myNum + 1) % NUM_PHILOSOPHERS));
		sleep(10);
		forks[(myNum + 1) % NUM_PHILOSOPHERS].acquire();
		System.out.println("Philosopher " + myNum + " eats meal " + meal);

		sleep(EAT_TIME + generator.nextInt(EAT_TIME));

		System.out.println("Philosopher " + myNum + " puts down forks and thinks");
		forks[myNum].release();;
		forks[(myNum + 1) % NUM_PHILOSOPHERS].release();

		sleep(THINK_TIME + generator.nextInt(THINK_TIME));
	}

	public void run() {
		try {
			for (int meal = 1; meal <= NUM_MEALS; meal++)
				starve(meal);
		}
		catch (InterruptedException e) {
		}
	}

	public static void main(String[] args) {
		forks = new Semaphore[NUM_PHILOSOPHERS];
		for(int i = 0; i < NUM_PHILOSOPHERS; i++)
			forks[i] = new Semaphore(1, true);
		
		for (int num = 0; num < NUM_PHILOSOPHERS; num++) {		
			PhilosopherSemaphore phil = new PhilosopherSemaphore(num);
			phil.start();
		}
	}

	/**
	 * Eat using semaphores to control the forks This version cannot deadlock
	 */
	public void eat(int meal) throws InterruptedException {

		if (myNum % 2 == 0) {
			System.out.println("Philosopher " + myNum + " tries to pick up fork "
					+ myNum);
			forks[myNum].acquire();;
			System.out.println("Philosopher " + myNum + " has fork " + myNum
					+ " and tries to pick up fork " + ((myNum + 1) % NUM_PHILOSOPHERS));
			sleep(10);
			forks[(myNum + 1) % NUM_PHILOSOPHERS].acquire();;
			System.out.println("Philosopher " + myNum + " eats meal " + meal);
		}
		else {
			System.out.println("Philosopher " + myNum + " tries to pick up fork "
					+ ((myNum + 1) % NUM_PHILOSOPHERS));
			forks[((myNum + 1) % NUM_PHILOSOPHERS)].acquire();
			System.out.println("Philosopher " + myNum + " has fork "
					+ ((myNum + 1) % NUM_PHILOSOPHERS) + " and tries to pick up fork "
					+ myNum);
			sleep(10);
			forks[myNum].acquire();;
			System.out.println("Philosopher " + myNum + " eats meal " + meal);
		}
		sleep(EAT_TIME + generator.nextInt(EAT_TIME));

		System.out.println("Philosopher " + myNum + " puts down forks and thinks");
		forks[myNum].release();;
		forks[(myNum + 1) % NUM_PHILOSOPHERS].release();;

		sleep(THINK_TIME + generator.nextInt(THINK_TIME));
	}
}