import java.util.concurrent.atomic.AtomicLong;

public class ReentrantCASLock {
    private AtomicLong lockingThread;
    private int count;

    public ReentrantCASLock() {
        lockingThread = new AtomicLong(-1);
        count = 0;
    }

    public void lock() {
        while (!(lockingThread.compareAndSet(-1, Thread.currentThread().getId())
                || lockingThread.compareAndSet(Thread.currentThread().getId(), Thread.currentThread().getId())));

        count++;
    }

    public void unlock() throws IllegalMonitorStateException {
        if (lockingThread.get() != Thread.currentThread().getId()) {
            throw new IllegalMonitorStateException();
        }

        count--;

        if (count == 0) {
            while (lockingThread.compareAndSet(Thread.currentThread().getId(), -1));
        }
    }
}