/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.activemq.transport.nio;

import java.io.IOException;
import java.nio.channels.spi.AbstractSelectableChannel;
import java.util.LinkedList;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * The SelectorManager will manage one Selector and the thread that checks the
 * selector.
 *
 * We may need to consider running more than one thread to check the selector if
 * servicing the selector takes too long.
 */
public final class SelectorManager {

    public static final SelectorManager SINGLETON = new SelectorManager();

    private Executor selectorExecutor = createDefaultExecutor();
    private Executor channelExecutor = selectorExecutor;
    private final LinkedList<SelectorWorker> freeWorkers = new LinkedList<SelectorWorker>();
    private int maxChannelsPerWorker = -1;

    protected ExecutorService createDefaultExecutor() {
        ThreadPoolExecutor rc = new ThreadPoolExecutor(getDefaultCorePoolSize(), getDefaultMaximumPoolSize(), getDefaultKeepAliveTime(), TimeUnit.SECONDS, newWorkQueue(),
            new ThreadFactory() {

                private long i = 0;

                @Override
                public Thread newThread(Runnable runnable) {
                    Thread t = new Thread(runnable, "ActiveMQ NIO Worker " + (i++));
                    t.setDaemon(true);
                    return t;
                }
            }, newRejectionHandler());

        return rc;
    }

    private RejectedExecutionHandler newRejectionHandler() {
        return canRejectWork() ? new ThreadPoolExecutor.AbortPolicy() : new ThreadPoolExecutor.CallerRunsPolicy();
    }

    private BlockingQueue<Runnable> newWorkQueue() {
        final int workQueueCapicity = getDefaultWorkQueueCapacity();
        return workQueueCapicity > 0 ? new LinkedBlockingQueue<Runnable>(workQueueCapicity) : new SynchronousQueue<Runnable>();
    }

    private static boolean canRejectWork() {
        return Boolean.getBoolean("org.apache.activemq.transport.nio.SelectorManager.rejectWork");
    }

    private static int getDefaultWorkQueueCapacity() {
        return Integer.getInteger("org.apache.activemq.transport.nio.SelectorManager.workQueueCapacity", 0);
    }

    private static int getDefaultCorePoolSize() {
        return Integer.getInteger("org.apache.activemq.transport.nio.SelectorManager.corePoolSize", 10);
    }

    private static int getDefaultMaximumPoolSize() {
        return Integer.getInteger("org.apache.activemq.transport.nio.SelectorManager.maximumPoolSize", 1024);
    }

    private static int getDefaultKeepAliveTime() {
        return Integer.getInteger("org.apache.activemq.transport.nio.SelectorManager.keepAliveTime", 30);
    }

    private static int getDefaultMaxChannelsPerWorker() {
        return Integer.getInteger("org.apache.activemq.transport.nio.SelectorManager.maxChannelsPerWorker", 1024);
    }

    public static SelectorManager getInstance() {
        return SINGLETON;
    }

    public interface Listener {
        void onSelect(SelectorSelection selector);

        void onError(SelectorSelection selection, Throwable error);
    }

    public synchronized SelectorSelection register(AbstractSelectableChannel selectableChannel, Listener listener) throws IOException {
        SelectorSelection selection = null;
        while (selection == null) {
            if (freeWorkers.size() > 0) {
                SelectorWorker worker = freeWorkers.getFirst();
                if (worker.isReleased()) {
                    freeWorkers.remove(worker);
                } else {
                    worker.retain();
                    selection = new SelectorSelection(worker, selectableChannel, listener);
                }
            } else {
                // Worker starts /w retain count of 1
                SelectorWorker worker = new SelectorWorker(this);
                freeWorkers.addFirst(worker);
                selection = new SelectorSelection(worker, selectableChannel, listener);
            }
        }

        return selection;
    }

    synchronized void onWorkerFullEvent(SelectorWorker worker) {
        freeWorkers.remove(worker);
    }

    public synchronized void onWorkerEmptyEvent(SelectorWorker worker) {
        freeWorkers.remove(worker);
    }

    public synchronized void onWorkerNotFullEvent(SelectorWorker worker) {
        freeWorkers.addFirst(worker);
    }

    public Executor getChannelExecutor() {
        return channelExecutor;
    }

    public void setChannelExecutor(Executor channelExecutor) {
        this.channelExecutor = channelExecutor;
    }

    public int getMaxChannelsPerWorker() {
        return maxChannelsPerWorker >= 0 ? maxChannelsPerWorker : getDefaultMaxChannelsPerWorker();
    }

    public void setMaxChannelsPerWorker(int maxChannelsPerWorker) {
        this.maxChannelsPerWorker = maxChannelsPerWorker;
    }

    public Executor getSelectorExecutor() {
        return selectorExecutor;
    }

    public void setSelectorExecutor(Executor selectorExecutor) {
        this.selectorExecutor = selectorExecutor;
    }
}