/**
 * Created by Christophe De Troyer on 04.11.14.
 */
public class BarrierSafer {

    private int _threadcount;
    private int _releasecount;
    private final int _threadlimit;
    private boolean release = false;

    public BarrierSafer(int threadlimit)
    {
        this._threadcount = 0;
        this._threadlimit = threadlimit;
        this._releasecount = 0;
    }

    public synchronized void joinBarrier(int index) throws InterruptedException {

        // Wait until we can join. Happens when not all threads have yet left the previous barrier
        while(_threadcount >= _threadlimit)
            this.wait(); // will be notified by (A)

        // Thread got in, increment counter.
        _threadcount++;
        if(_threadcount < _threadlimit)
        {
            // Wait. Loop to avoid spurious wakeups.
            while(!release)
            {
                this.wait(); // will be notified by (B)
            }
            // At this point we have been released, so increment the release counter.
            _releasecount++;

            // If we are the last thread to leave, notify the releaseBarrier() that we are gone.
            if(_releasecount == (_threadlimit - 1))
                this.notifyAll(); // (C) why not simply notify?
        }
        else
        {
            releaseBarrier();
        }
    }

    public synchronized void releaseBarrier() throws InterruptedException {
        // Set the release flag to true, so thread can jump out of their wait loop.
        release = true;
        // Notify the waiting threads that they can leave.
        notifyAll(); // (B)

        // Wait untill the last leaving thread tells us that all threads are gone.
        while(_releasecount != (_threadlimit - 1))
        {
            this.wait(); // will be notified by (C)
        }
        // No more threads can leave.
        release = false;

        // Reset the counters.
        _threadcount = 0;
        _releasecount = 0;

        // If threads are waiting to come in, notify them.
        notifyAll(); // (A)
    }
}