public class ReadPriority {
    int activeReaders, waitingReaders, activeWriters;

    public ReadPriority (int N) {
	activeReaders = 0;     // |s.rdrs|
	waitingReaders = 0;    // |{e in q: e[2] = "enter_read"}|
	activeWriters = 0;     // |s.wrtrs|
    }

    public synchronized void StartRead () {
        waitingReaders++;
	while (activeWriters > 0)
            try { wait(); }
            catch (java.lang.InterruptedException e) { };
	waitingReaders--;
	activeReaders++;
    }

    public synchronized boolean EndRead () {
	if (activeReaders == 0) return false;
        activeReaders--;
	if (activeReaders == 0) notifyAll();
	return true;
    }

    public synchronized void StartWrite () {
	while (activeReaders > 0 || activeWriters > 0 || waitingReaders > 0)
            try { wait(); }
            catch (java.lang.InterruptedException e) { };
	activeWriters++;
    }

    public synchronized boolean EndWrite () {
	if (activeWriters == 0) return false;
        activeWriters--;
	notifyAll();
	return true;
    }
}