// This example is from the book _Java Threads_ by Scott Oaks and Henry Wong. 
// Written by Scott Oaks and Henry Wong.
// Copyright (c) 1997 O'Reilly & Associates.
// You may study, use, modify, and distribute this example for any purpose.
// This example is provided WITHOUT WARRANTY either expressed or implied.

// Sample Reader-Writer Lock, Chapter 7, p. 182.

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();
    }
}