/*
 * Copyright 2005 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.wamblee.concurrency;

import java.util.HashSet;


/**
 * Read-write lock for allowing multiple concurrent readers or at most one
 * writer. This implementation does not aim for high performance but for
 * robustness and simplicity. Users of this class should not synchronize on
 * objects of this class.
 */
public class ReadWriteLock {
    /**
     * Sets containing the references to the threads that are currently
     * reading. This administration is useful to check that the lock has
     * already been  acquired before it is release. This check adds robustness
     * to the application.
     */
    private HashSet<Thread> readers;

    /**
     * The thread that has acquired the lock for writing or null if no such
     * thread exists currently.
     */
    private Thread writer;

    /**
     * Constructs read-write lock.
     */
    public ReadWriteLock() {
        readers = new HashSet<Thread>();
        writer = null;
    }

    /**
     * Acquires the lock for reading. This call will block until the lock can
     * be acquired.
     *
     * @throws IllegalStateException Thrown if the read or  write lock is
     *         already acquired.
     */
    public synchronized void acquireRead() {
        if (readers.contains(Thread.currentThread())) {
            throw new IllegalStateException(
                "Read lock already acquired by current thread: " +
                Thread.currentThread());
        }

        if (writer == Thread.currentThread()) {
            throw new IllegalStateException(
                "Trying to acquire the read lock while already holding a write lock: " +
                Thread.currentThread());
        }

        while (writer != null) {
            try {
                wait();
            } catch (InterruptedException e) {
                notifyAll();
            }
        }

        readers.add(Thread.currentThread());
    }

    /**
     * Releases the lock for reading. Note: This implementation assumes that
     * the lock has already been acquired for reading previously.
     *
     * @throws IllegalStateException Thrown when the lock was not acquired by
     *         this thread.
     */
    public synchronized void releaseRead() {
        if (!readers.remove(Thread.currentThread())) {
            throw new IllegalStateException(
                "Cannot release read lock because current thread has not acquired it.");
        }

        if (readers.size() == 0) {
            notifyAll();
        }
    }

    /**
     * Acquires the lock for writing. This call will block until the lock has
     * been acquired.
     *
     * @throws IllegalStateException Thrown if the read or write lock is
     *         already acquired.
     */
    public synchronized void acquireWrite() {
        if (writer == Thread.currentThread()) {
            throw new IllegalStateException(
                "Trying to acquire a write lock while already holding the write lock: " +
                Thread.currentThread());
        }

        if (readers.contains(Thread.currentThread())) {
            throw new IllegalStateException(
                "Trying to acquire a write lock while already holding the read lock: " +
                Thread.currentThread());
        }

        // wait until there are no more writers and no more
        // readers 
        while ((writer != null) || (readers.size() > 0)) {
            try {
                wait();
            } catch (InterruptedException e) {
                notifyAll();
            }
        }

        writer = Thread.currentThread();

        // notification not necessary since all writers and
        // readers are now blocked by this thread.
    }

    /**
     * Releases the lock for writing.
     *
     * @throws IllegalStateException Thrown when the lock was not acquired.
     */
    public synchronized void releaseWrite() {
        if (writer != Thread.currentThread()) {
            throw new IllegalStateException(
                "Cannot release write lock because it was not acquired. ");
        }

        writer = null;
        notifyAll();
    }
}