/*
 *
 * Copyright (c) 2006 The Apache Software Foundation
 *
 * 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.apache.qpid.server.queue;

import org.apache.log4j.Logger;
import java.util.List;
import java.util.ListIterator;
import java.util.concurrent.CopyOnWriteArrayList;

/**
 * Holds a set of subscriptions for a queue and manages the round
 * robin-ing of deliver etc.
 */
class SubscriptionSet implements WeightedSubscriptionManager
{
    private static final Logger _log = Logger.getLogger(SubscriptionSet.class);

    /**
     * List of registered subscribers
     */
    private List<Subscription> _subscriptions = new CopyOnWriteArrayList<Subscription>();

    /**
     * Used to control the round robin delivery of content
     */
    private int _currentSubscriber;

    /**
     * Accessor for unit tests.
     */
    int getCurrentSubscriber()
    {
        return _currentSubscriber;
    }

    public void addSubscriber(Subscription subscription)
    {
        _subscriptions.add(subscription);
    }

    /**
     * Remove the subscription, returning it if it was found
     * @param subscription
     * @return null if no match was found
     */
    public Subscription removeSubscriber(Subscription subscription)
    {
        boolean isRemoved = _subscriptions.remove(subscription); // TODO: possibly need O(1) operation here.
        if (isRemoved)
        {
            return subscription;
        }
        else
        {
            debugDumpSubscription(subscription);
            return null;
        }
    }

    private void debugDumpSubscription(Subscription subscription)
    {
        if (_log.isDebugEnabled())
        {
            _log.debug("Subscription " + subscription + " not found. Dumping subscriptions:");
            for (Subscription s : _subscriptions)
            {
                _log.debug("Subscription: " + s);
            }
            _log.debug("Subscription dump complete");
        }
    }

    /**
     * Return the next unsuspended subscription or null if not found.
     *
     * Performance note:
     * This method can scan all items twice when looking for a subscription that is not
     * suspended. The worst case occcurs when all subscriptions are suspended. However, it is does this
     * without synchronisation and subscriptions may be added and removed concurrently. Also note that because of
     * race conditions and when subscriptions are removed between calls to nextSubscriber, the
     * IndexOutOfBoundsException also causes the scan to start at the beginning.
     */
    public Subscription nextSubscriber(AMQMessage msg)
    {
        if (_subscriptions.isEmpty())
        {
            return null;
        }

        try {
            final Subscription result = nextSubscriber();
            if (result == null) {
                _currentSubscriber = 0;
                return nextSubscriber();
            } else {
                return result;
            }
        } catch (IndexOutOfBoundsException e) {
            _currentSubscriber = 0;
            return nextSubscriber();
        }
    }

    private Subscription nextSubscriber()
    {
        final ListIterator<Subscription> iterator = _subscriptions.listIterator(_currentSubscriber);
        while (iterator.hasNext()) {
            Subscription subscription = iterator.next();
            ++_currentSubscriber;
            subscriberScanned();
            if (!subscription.isSuspended()) {
                return subscription;
            }
        }
        return null;
    }

    /**
     * Overridden in test classes.
     */
    protected void subscriberScanned()
    {
    }

    public boolean isEmpty()
    {
        return _subscriptions.isEmpty();
    }

    public boolean hasActiveSubscribers()
    {
        for (Subscription s : _subscriptions)
        {
            if (!s.isSuspended()) return true;
        }
        return false;
    }

    public int getWeight()
    {
        int count = 0;
        for (Subscription s : _subscriptions)
        {
            if (!s.isSuspended()) count++;
        }
        return count;
    }

    /**
     * Notification that a queue has been deleted. This is called so that the subscription can inform the
     * channel, which in turn can update its list of unacknowledged messages.
     * @param queue
     */
    public void queueDeleted(AMQQueue queue)
    {
        for (Subscription s : _subscriptions)
        {
            s.queueDeleted(queue);
        }
    }

    int size() {
        return _subscriptions.size();
    }
}
