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/*
 * Copyright (c) 2008-2019 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.cometd.oort;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ScheduledExecutorService;

import org.cometd.bayeux.client.ClientSession;
import org.cometd.bayeux.client.ClientSessionChannel;
import org.cometd.client.BayeuxClient;
import org.cometd.client.transport.ClientTransport;

/**
 * The Oort comet client connects a local Oort comet server to a remote Oort comet server.
 */
public class OortComet extends BayeuxClient {
    private final ConcurrentMap _subscriptions = new ConcurrentHashMap<>();
    private final Oort _oort;
    private final String _cometURL;
    private volatile boolean _subscriptionsAllowed;

    public OortComet(Oort oort, String cometURL, ScheduledExecutorService scheduler, ClientTransport transport, ClientTransport... transports) {
        super(cometURL, scheduler, transport, transports);
        _oort = oort;
        _cometURL = cometURL;
    }

    protected void subscribe(Set observedChannels) {
        // Guard against concurrent subscription clearing from the handshake callback
        if (!_subscriptionsAllowed) {
            return;
        }

        for (String channel : observedChannels) {
            if (_subscriptions.containsKey(channel)) {
                continue;
            }

            ClientSessionChannel.MessageListener listener = (c, message) -> {
                if (logger.isDebugEnabled()) {
                    logger.debug("Republishing message {} from {}", message, _cometURL);
                }
                // BayeuxServer may sweep channels, so calling bayeux.getChannel(...)
                // may return null, and therefore we use the client to send the message.
                _oort.getOortSession().getChannel(message.getChannel()).publish(message);
            };

            ClientSessionChannel.MessageListener existing = _subscriptions.putIfAbsent(channel, listener);
            if (existing == null) {
                if (logger.isDebugEnabled()) {
                    logger.debug("Subscribing to messages on {} from {}", channel, _cometURL);
                }
                getChannel(channel).subscribe(listener);
            }
        }
        if (logger.isDebugEnabled()) {
            logger.debug("Subscriptions to messages on {} from {}", _subscriptions, _cometURL);
        }
    }

    protected void unsubscribe(String channel) {
        ClientSessionChannel.MessageListener listener = _subscriptions.remove(channel);
        if (listener != null) {
            if (logger.isDebugEnabled()) {
                logger.debug("Unsubscribing to messages on {} from {}", channel, _cometURL);
            }
            getChannel(channel).unsubscribe(listener);
        }
    }

    protected void clearSubscriptions() {
        for (String channel : _oort.getObservedChannels()) {
            unsubscribe(channel);
        }
    }

    void open(final ClientSession.MessageListener callback) {
        batch(() -> {
            // Subscribe to cloud notifications
            getChannel(Oort.OORT_CLOUD_CHANNEL).subscribe((channel, message) -> {
                if (message.isSuccessful()) {
                    _oort.joinComets(message);
                }
            });

            // It is possible that a call to Oort.observeChannel() (which triggers
            // the call to subscribe()) is performed concurrently with the handshake
            // of this OortComet with a remote comet.
            // For example, Seti calls Oort.observeChannel() on startup and this may
            // be called while the Oort cloud is connecting all the comets together.
            // In this case, below we will clear existing subscriptions, but we will
            // subscribe them again just afterwards, ensuring only one subscriber
            // (and not multiple ones) is subscribed.
            clearSubscriptions();
            _subscriptionsAllowed = true;

            Set channels = _oort.getObservedChannels();
            if (logger.isDebugEnabled()) {
                logger.debug("Observing channels {}", channels);
            }
            subscribe(channels);

            // Advertise our own network.
            getChannel(Oort.OORT_CLOUD_CHANNEL).publish(new ArrayList<>(_oort.getKnownComets()));

            // Advertise the remote node that we have joined.
            Map data = new HashMap<>(2);
            data.put(Oort.EXT_OORT_ID_FIELD, _oort.getId());
            data.put(Oort.EXT_OORT_URL_FIELD, _oort.getURL());
            if (logger.isDebugEnabled()) {
                logger.debug("Sending join data {}", data);
            }
            getChannel(Oort.OORT_SERVICE_CHANNEL).publish(data, callback);
        });
    }

    @Override
    public String toString() {
        return String.format("%s[%s]", super.toString(), _cometURL);
    }
}