package tpsema;

public class S4 implements Semaphore {
	protected int n;
	protected int first;
	protected int last;
	protected Thread fifo[];
	
	public static final int MAX_FIFO = 100;
	
	S4(int pn) {
		n = pn;
		first=last=0;
		fifo = new Thread[MAX_FIFO];
	}
	
	public synchronized void P() {
		if(n<=0 || first != last) {
			last=(last+1)%MAX_FIFO;
			fifo[last] = Thread.currentThread();
			System.out.println("-> Thread " + Thread.currentThread() + " entering FIFO...");

			try {
				wait();
			} catch(InterruptedException e) {
				System.out.println("## Thread " + Thread.currentThread() + " woken up by exception..."); 
			}

			if(Thread.currentThread() != fifo[(first+1)%MAX_FIFO]) {
				System.out.println("!!! Serious error: Thread " + Thread.currentThread() + "is not the first in queue!!!");
				System.exit(1);
			}
			
			System.out.println("<- Thread " + Thread.currentThread() + " leaving FIFO...");
			first = (first+1)%MAX_FIFO;
		}
		n--;
	}
	
	public synchronized void V() {
		System.out.println("Thread " + Thread.currentThread() + " signaling " + this.toString() + "...");		
		if(first != last) {
			System.out.println("Waking up " + fifo[(first+1)%MAX_FIFO] + " with interrupt()...");
			fifo[(first+1)%MAX_FIFO].interrupt();
		}
		n++;
	}
}