/*
 *
 * Copyright (c) 1997-1999 Scott Oaks and Henry Wong. All Rights Reserved.
 *
 * Permission to use, copy, modify, and distribute this software
 * and its documentation for NON-COMMERCIAL purposes and
 * without fee is hereby granted.
 *
 * This sample source code is provided for example only,
 * on an unsupported, as-is basis. 
 *
 * AUTHOR MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF
 * THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE, OR NON-INFRINGEMENT. AUTHOR SHALL NOT BE LIABLE FOR
 * ANY DAMAGES SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR
 * DISTRIBUTING THIS SOFTWARE OR ITS DERIVATIVES.
 *
 * THIS SOFTWARE IS NOT DESIGNED OR INTENDED FOR USE OR RESALE AS ON-LINE
 * CONTROL EQUIPMENT IN HAZARDOUS ENVIRONMENTS REQUIRING FAIL-SAFE
 * PERFORMANCE, SUCH AS IN THE OPERATION OF NUCLEAR FACILITIES, AIRCRAFT
 * NAVIGATION OR COMMUNICATION SYSTEMS, AIR TRAFFIC CONTROL, DIRECT LIFE
 * SUPPORT MACHINES, OR WEAPONS SYSTEMS, IN WHICH THE FAILURE OF THE
 * SOFTWARE COULD LEAD DIRECTLY TO DEATH, PERSONAL INJURY, OR SEVERE
 * PHYSICAL OR ENVIRONMENTAL DAMAGE ("HIGH RISK ACTIVITIES").  AUTHOR
 * SPECIFICALLY DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR
 * HIGH RISK ACTIVITIES.
 */


import java.util.*;

class RWNode {
	static final int READER = 0;
	static final int WRITER = 1;
	Thread t;
	int state;
	int nAcquires;
	RWNode(Thread t, int state) {
		this.t = t;
		this.state = state;
		nAcquires = 0;
	}
}

public class RWLock {
	private Vector waiters;

	private int firstWriter() {
		Enumeration e;
		int index;
		for (index = 0, e = waiters.elements();
								   e.hasMoreElements(); index++) {
			RWNode node = (RWNode) e.nextElement();
			if (node.state == RWNode.WRITER)
				return index;
		}
		return Integer.MAX_VALUE;
	}

	private int getIndex(Thread t) {
		Enumeration e;
		int index;
		for (index = 0, e = waiters.elements();
						 e.hasMoreElements(); index++) {
			RWNode node = (RWNode) e.nextElement();
			if (node.t == t)
				return index;
		}
		return -1;
	}

	public RWLock() {
		waiters = new Vector();
	}

	public synchronized void lockRead() {
		RWNode node;
		Thread me = Thread.currentThread();
		int index = getIndex(me);
		if (index == -1) {
			node = new RWNode(me, RWNode.READER);
			waiters.addElement(node);
		}
		else node = (RWNode) waiters.elementAt(index);
		while (getIndex(me) > firstWriter()) {
			try {
				wait();
			} catch (Exception e) {}
		}
		node.nAcquires++;
	}
	public synchronized void lockWrite() {
		RWNode node;
		Thread me = Thread.currentThread();
		int index = getIndex(me);
		if (index == -1) {
			node = new RWNode(me, RWNode.WRITER);
			waiters.addElement(node);
		}
		else {
			node = (RWNode) waiters.elementAt(index);
			if (node.state == RWNode.READER)
				throw new IllegalArgumentException("Upgrade lock");
			node.state = RWNode.WRITER;
		}
		while (getIndex(me) != 0) {
			try {
				wait();
			} catch (Exception e) {}
		}
		node.nAcquires++;
	}

	public synchronized void unlock() {
		RWNode node;
		Thread me = Thread.currentThread();
		int index;
		index = getIndex(me);
		if (index  > firstWriter())
			throw new IllegalArgumentException("Lock not held");
		node = (RWNode) waiters.elementAt(index);
		node.nAcquires--;
		if (node.nAcquires == 0) {
			waiters.removeElementAt(index);
			notifyAll();
		}
	}
}