using MQTTnet.Adapter;
using MQTTnet.Diagnostics;
using MQTTnet.Exceptions;
using MQTTnet.Formatter;
using MQTTnet.Internal;
using MQTTnet.Packets;
using MQTTnet.Protocol;
using MQTTnet.Server.Status;
using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace MQTTnet.Server
{
    public class MqttClientSessionsManager : Disposable
    {
        readonly AsyncQueue<MqttEnqueuedApplicationMessage> _messageQueue = new AsyncQueue<MqttEnqueuedApplicationMessage>();

        readonly AsyncLock _createConnectionGate = new AsyncLock();
        readonly ConcurrentDictionary<string, MqttClientConnection> _connections = new ConcurrentDictionary<string, MqttClientConnection>();
        readonly ConcurrentDictionary<string, MqttClientSession> _sessions = new ConcurrentDictionary<string, MqttClientSession>();
        readonly IDictionary<object, object> _serverSessionItems = new ConcurrentDictionary<object, object>();

        readonly CancellationToken _cancellationToken;
        readonly MqttServerEventDispatcher _eventDispatcher;

        readonly IMqttRetainedMessagesManager _retainedMessagesManager;
        readonly IMqttServerOptions _options;
        readonly IMqttNetLogger _logger;

        public MqttClientSessionsManager(
            IMqttServerOptions options,
            IMqttRetainedMessagesManager retainedMessagesManager,
            CancellationToken cancellationToken,
            MqttServerEventDispatcher eventDispatcher,
            IMqttNetLogger logger)
        {
            _cancellationToken = cancellationToken;

            if (logger == null) throw new ArgumentNullException(nameof(logger));
            _logger = logger.CreateChildLogger(nameof(MqttClientSessionsManager));

            _eventDispatcher = eventDispatcher ?? throw new ArgumentNullException(nameof(eventDispatcher));
            _options = options ?? throw new ArgumentNullException(nameof(options));
            _retainedMessagesManager = retainedMessagesManager ?? throw new ArgumentNullException(nameof(retainedMessagesManager));
        }

        public void Start()
        {
            Task.Run(() => TryProcessQueuedApplicationMessagesAsync(_cancellationToken), _cancellationToken).Forget(_logger);
        }

        public async Task StopAsync()
        {
            foreach (var connection in _connections.Values)
            {
                await connection.StopAsync().ConfigureAwait(false);
            }
        }

        public Task HandleClientConnectionAsync(IMqttChannelAdapter clientAdapter)
        {
            if (clientAdapter is null) throw new ArgumentNullException(nameof(clientAdapter));

            return HandleClientConnectionAsync(clientAdapter, _cancellationToken);
        }

        public Task<IList<IMqttClientStatus>> GetClientStatusAsync()
        {
            var result = new List<IMqttClientStatus>();

            foreach (var connection in _connections.Values)
            {
                var clientStatus = new MqttClientStatus(connection);
                connection.FillStatus(clientStatus);

                var sessionStatus = new MqttSessionStatus(connection.Session, this);
                connection.Session.FillStatus(sessionStatus);
                clientStatus.Session = sessionStatus;

                result.Add(clientStatus);
            }

            return Task.FromResult((IList<IMqttClientStatus>)result);
        }

        public Task<IList<IMqttSessionStatus>> GetSessionStatusAsync()
        {
            var result = new List<IMqttSessionStatus>();

            foreach (var session in _sessions.Values)
            {
                var sessionStatus = new MqttSessionStatus(session, this);
                session.FillStatus(sessionStatus);

                result.Add(sessionStatus);
            }

            return Task.FromResult((IList<IMqttSessionStatus>)result);
        }

        public void DispatchApplicationMessage(MqttApplicationMessage applicationMessage, MqttClientConnection sender)
        {
            if (applicationMessage == null) throw new ArgumentNullException(nameof(applicationMessage));

            _messageQueue.Enqueue(new MqttEnqueuedApplicationMessage(applicationMessage, sender));
        }

        public Task SubscribeAsync(string clientId, ICollection<MqttTopicFilter> topicFilters)
        {
            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
            if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));

            if (!_sessions.TryGetValue(clientId, out var session))
            {
                throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
            }

            return session.SubscribeAsync(topicFilters);
        }

        public Task UnsubscribeAsync(string clientId, IEnumerable<string> topicFilters)
        {
            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
            if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));

            if (!_sessions.TryGetValue(clientId, out var session))
            {
                throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
            }

            return session.UnsubscribeAsync(topicFilters);
        }

        public async Task DeleteSessionAsync(string clientId)
        {
            if (_connections.TryGetValue(clientId, out var connection))
            {
                await connection.StopAsync().ConfigureAwait(false);
            }

            if (_sessions.TryRemove(clientId, out _))
            {
            }

            _logger.Verbose("Session for client '{0}' deleted.", clientId);
        }

        protected override void Dispose(bool disposing)
        {
            if (disposing)
            {
                _messageQueue?.Dispose();
            }
            base.Dispose(disposing);
        }

        async Task TryProcessQueuedApplicationMessagesAsync(CancellationToken cancellationToken)
        {
            while (!cancellationToken.IsCancellationRequested)
            {
                try
                {
                    await TryProcessNextQueuedApplicationMessageAsync(cancellationToken).ConfigureAwait(false);
                }
                catch (OperationCanceledException)
                {
                }
                catch (Exception exception)
                {
                    _logger.Error(exception, "Unhandled exception while processing queued application messages.");
                }
            }
        }

        async Task TryProcessNextQueuedApplicationMessageAsync(CancellationToken cancellationToken)
        {
            try
            {
                if (cancellationToken.IsCancellationRequested)
                {
                    return;
                }

                var dequeueResult = await _messageQueue.TryDequeueAsync(cancellationToken).ConfigureAwait(false);

                if (!dequeueResult.IsSuccess)
                {
                    return;
                }

                var queuedApplicationMessage = dequeueResult.Item;

                var sender = queuedApplicationMessage.Sender;
                var applicationMessage = queuedApplicationMessage.ApplicationMessage;

                var interceptorContext = await InterceptApplicationMessageAsync(sender, applicationMessage).ConfigureAwait(false);
                if (interceptorContext != null)
                {
                    if (interceptorContext.CloseConnection)
                    {
                        if (sender != null)
                        {
                            await sender.StopAsync().ConfigureAwait(false);
                        }
                    }

                    if (interceptorContext.ApplicationMessage == null || !interceptorContext.AcceptPublish)
                    {
                        return;
                    }

                    applicationMessage = interceptorContext.ApplicationMessage;
                }

                await _eventDispatcher.SafeNotifyApplicationMessageReceivedAsync(sender?.ClientId, applicationMessage).ConfigureAwait(false);

                if (applicationMessage.Retain)
                {
                    await _retainedMessagesManager.HandleMessageAsync(sender?.ClientId, applicationMessage).ConfigureAwait(false);
                }

                foreach (var clientSession in _sessions.Values)
                {
                    clientSession.EnqueueApplicationMessage(
                        applicationMessage,
                        sender?.ClientId,
                        false);
                }
            }
            catch (OperationCanceledException)
            {
            }
            catch (Exception exception)
            {
                _logger.Error(exception, "Unhandled exception while processing next queued application message.");
            }
        }

        async Task HandleClientConnectionAsync(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
        {
            string clientId = null;

            MqttConnectPacket connectPacket;
            try
            {
                try
                {
                    var firstPacket = await channelAdapter.ReceivePacketAsync(_options.DefaultCommunicationTimeout, cancellationToken).ConfigureAwait(false);
                    connectPacket = firstPacket as MqttConnectPacket;
                    if (connectPacket == null)
                    {
                        _logger.Warning(null, "The first packet from client '{0}' was no 'CONNECT' packet [MQTT-3.1.0-1].", channelAdapter.Endpoint);
                        return;
                    }
                }
                catch (MqttCommunicationTimedOutException)
                {
                    _logger.Warning(null, "Client '{0}' connected but did not sent a CONNECT packet.", channelAdapter.Endpoint);
                    return;
                }

                var connectionValidatorContext = await ValidateConnectionAsync(connectPacket, channelAdapter).ConfigureAwait(false);

                if (connectionValidatorContext.ReasonCode != MqttConnectReasonCode.Success)
                {
                    // Send failure response here without preparing a session. The result for a successful connect
                    // will be sent from the session itself.
                    var connAckPacket = channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnAckPacket(connectionValidatorContext);
                    await channelAdapter.SendPacketAsync(connAckPacket, _options.DefaultCommunicationTimeout, cancellationToken).ConfigureAwait(false);

                    return;
                }

                clientId = connectPacket.ClientId;

                var connection = await CreateClientConnectionAsync(
                    connectPacket,
                    connectionValidatorContext,
                    channelAdapter,
                    async () => await _eventDispatcher.SafeNotifyClientConnectedAsync(clientId).ConfigureAwait(false),
                    async disconnectType => await CleanUpClient(clientId, channelAdapter, disconnectType)
                ).ConfigureAwait(false);

                await connection.RunAsync(connectionValidatorContext).ConfigureAwait(false);
            }
            catch (OperationCanceledException)
            {
            }
            catch (Exception exception)
            {
                _logger.Error(exception, exception.Message);
            }
        }

        private async Task CleanUpClient(string clientId, IMqttChannelAdapter channelAdapter, MqttClientDisconnectType disconnectType)
        {
            if (clientId != null)
            {
                _connections.TryRemove(clientId, out _);

                if (!_options.EnablePersistentSessions)
                {
                    await DeleteSessionAsync(clientId).ConfigureAwait(false);
                }
            }

            await SafeCleanupChannelAsync(channelAdapter).ConfigureAwait(false);

            if (clientId != null)
            {
                await _eventDispatcher.SafeNotifyClientDisconnectedAsync(clientId, disconnectType).ConfigureAwait(false);
            }
        }

        async Task<MqttConnectionValidatorContext> ValidateConnectionAsync(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter)
        {
            var context = new MqttConnectionValidatorContext(connectPacket, channelAdapter, new ConcurrentDictionary<object, object>());

            var connectionValidator = _options.ConnectionValidator;

            if (connectionValidator == null)
            {
                context.ReasonCode = MqttConnectReasonCode.Success;
                return context;
            }

            await connectionValidator.ValidateConnectionAsync(context).ConfigureAwait(false);

            // Check the client ID and set a random one if supported.
            if (string.IsNullOrEmpty(connectPacket.ClientId) && channelAdapter.PacketFormatterAdapter.ProtocolVersion == MqttProtocolVersion.V500)
            {
                connectPacket.ClientId = context.AssignedClientIdentifier;
            }

            if (string.IsNullOrEmpty(connectPacket.ClientId))
            {
                context.ReasonCode = MqttConnectReasonCode.ClientIdentifierNotValid;
            }

            return context;
        }

        async Task<MqttClientConnection> CreateClientConnectionAsync(MqttConnectPacket connectPacket, MqttConnectionValidatorContext connectionValidatorContext, IMqttChannelAdapter channelAdapter, Func<Task> onStart, Func<MqttClientDisconnectType, Task> onStop)
        {
            using (await _createConnectionGate.WaitAsync(_cancellationToken).ConfigureAwait(false))
            {
                var isSessionPresent = _sessions.TryGetValue(connectPacket.ClientId, out var session);

                var isConnectionPresent = _connections.TryGetValue(connectPacket.ClientId, out var existingConnection);
                if (isConnectionPresent)
                {
                    await existingConnection.StopAsync(true).ConfigureAwait(false);
                }

                if (isSessionPresent)
                {
                    if (connectPacket.CleanSession)
                    {
                        session = null;

                        _logger.Verbose("Deleting existing session of client '{0}'.", connectPacket.ClientId);
                    }
                    else
                    {
                        _logger.Verbose("Reusing existing session of client '{0}'.", connectPacket.ClientId);
                    }
                }

                if (session == null)
                {
                    session = new MqttClientSession(connectPacket.ClientId, connectionValidatorContext.SessionItems, _eventDispatcher, _options, _retainedMessagesManager, _logger);
                    _logger.Verbose("Created a new session for client '{0}'.", connectPacket.ClientId);
                }

                var connection = new MqttClientConnection(connectPacket, channelAdapter, session, _options, this, _retainedMessagesManager, onStart, onStop, _logger);

                _connections[connection.ClientId] = connection;
                _sessions[session.ClientId] = session;

                return connection;
            }
        }

        async Task<MqttApplicationMessageInterceptorContext> InterceptApplicationMessageAsync(MqttClientConnection senderConnection, MqttApplicationMessage applicationMessage)
        {
            var interceptor = _options.ApplicationMessageInterceptor;
            if (interceptor == null)
            {
                return null;
            }

            string senderClientId;
            IDictionary<object, object> sessionItems;

            var messageIsFromServer = senderConnection == null;
            if (messageIsFromServer)
            {
                senderClientId = _options.ClientId;
                sessionItems = _serverSessionItems;
            }
            else
            {
                senderClientId = senderConnection.ClientId;
                sessionItems = senderConnection.Session.Items;
            }

            var interceptorContext = new MqttApplicationMessageInterceptorContext(senderClientId, sessionItems, applicationMessage);
            await interceptor.InterceptApplicationMessagePublishAsync(interceptorContext).ConfigureAwait(false);
            return interceptorContext;
        }

        async Task SafeCleanupChannelAsync(IMqttChannelAdapter channelAdapter)
        {
            try
            {
                await channelAdapter.DisconnectAsync(_options.DefaultCommunicationTimeout, CancellationToken.None).ConfigureAwait(false);
            }
            catch (Exception exception)
            {
                _logger.Error(exception, "Error while disconnecting client channel.");
            }
        }
    }
}