Lock rwLock;

// shared data
int numReaders = 0;
int numWriters = 0;

// before readerStart returns, we want
// numWriters = 0;
cv waitingReaders;

// before writerStart returns, we want
// numReaders + numWriters = 0;
cv waitingWriters;


void ReaderStart(){
	rwLock.lock();
	while (numWriters > 0){               // better to be defensive and do != 0?
		waitingReaders.wait(rwLock);
	}
	numReaders++;

	rwLock.unlock();
}

void ReaderFinish(){
	rwLock.lock();
	numReaders--;

	/* ------- BAD! ------- */
	// waitingReaders.signal();            // all readers looking to read were woken by the broadcast
	                                       // haven't changed anything that would make a writer reeady to go.

	if (numReaders == 0){                  // optimization -- avoids spurious signals
		waitingWriters.signal();
	}
	rwLock.unlock;
}

void WriterStart(){
	rwLock.lock();
	while(numReaders + numWriters > 0) {
		waitingWriters.wait(rwLock);
	}
	waitingWriters++;
	rwLock.unlock();
}

void WriterFinish(){
	rwLock.lock();
	numWriters--;
	waitingReaders.broadcast();             // want to wake many threads
	waitingWriters.signal();
	rwLock.unlock();
}

// a solution to the starvation problem:
// prevent new readers from getting in while there are waiting Writers;

int numWaitingWriters = 0;

void WriterStart(){
	rwLock.lock();
	numWaitingWriters++;
	while(numReaders + numWriters > 0) {
		waitingWriters.wait(rwLock);
	}
	numWaitingWriters--;
	waitingWriters++;
	rwLock.unlock();
}

void ReaderStart(){
	rwLock.lock();
	while (numWriters > 0 || numWaitingWriters > 0) {
		waitingReaders.wait(rwLock);
	}
	numReaders++;

	rwLock.unlock();
}

// ...but this just starves the readers!
// Another solution is to use a queue to manage readers & writers
// covered in discussion.