using System;
using System.Collections.Concurrent;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Threading;

namespace Hazel.Udp
{
    /// <summary>
    ///     Listens for new UDP connections and creates UdpConnections for them.
    /// </summary>
    /// <inheritdoc />
    public class UdpConnectionListener : NetworkConnectionListener
    {
        private const int SendReceiveBufferSize = 1024 * 1024;
        private const int BufferSize = ushort.MaxValue;

        private Socket socket;
        private ILogger Logger;
        private Timer reliablePacketTimer;

        private ConcurrentDictionary<EndPoint, UdpServerConnection> allConnections = new ConcurrentDictionary<EndPoint, UdpServerConnection>();

        public override double AveragePing => this.allConnections.Values.Sum(c => c.AveragePingMs) / this.allConnections.Count;
        public override int ConnectionCount { get { return this.allConnections.Count; } }
        public override int ReceiveQueueLength => throw new NotImplementedException();
        public override int SendQueueLength => throw new NotImplementedException();

        /// <summary>
        ///     Creates a new UdpConnectionListener for the given <see cref="IPAddress"/>, port and <see cref="IPMode"/>.
        /// </summary>
        /// <param name="endPoint">The endpoint to listen on.</param>
        public UdpConnectionListener(IPEndPoint endPoint, IPMode ipMode = IPMode.IPv4, ILogger logger = null)
        {
            this.Logger = logger;
            this.EndPoint = endPoint;
            this.IPMode = ipMode;

            this.socket = UdpConnection.CreateSocket(this.IPMode);
            
            socket.ReceiveBufferSize = SendReceiveBufferSize;
            socket.SendBufferSize = SendReceiveBufferSize;
            
            reliablePacketTimer = new Timer(ManageReliablePackets, null, 100, Timeout.Infinite);
        }

        ~UdpConnectionListener()
        {
            this.Dispose(false);
        }
        
        private void ManageReliablePackets(object state)
        {
            foreach (var kvp in this.allConnections)
            {
                var sock = kvp.Value;
                sock.ManageReliablePackets();
            }

            try
            {
                this.reliablePacketTimer.Change(100, Timeout.Infinite);
            }
            catch { }
        }

        /// <inheritdoc />
        public override void Start()
        {
            try
            {
                socket.Bind(EndPoint);
            }
            catch (SocketException e)
            {
                throw new HazelException("Could not start listening as a SocketException occurred", e);
            }

            StartListeningForData();
        }

        /// <summary>
        ///     Instructs the listener to begin listening.
        /// </summary>
        private void StartListeningForData()
        {
            EndPoint remoteEP = EndPoint;

            MessageReader message = null;
            try
            {
                message = MessageReader.GetSized(this.ReceiveBufferSize);
                socket.BeginReceiveFrom(message.Buffer, 0, message.Buffer.Length, SocketFlags.None, ref remoteEP, ReadCallback, message);
            }
            catch (SocketException sx)
            {
                message?.Recycle();

                this.Logger?.WriteError("Socket Ex in StartListening: " + sx.Message);

                Thread.Sleep(10);
                StartListeningForData();
                return;
            }
            catch (Exception ex)
            {
                message.Recycle();
                this.Logger?.WriteError("Stopped due to: " + ex.Message);
                return;
            }
        }

        void ReadCallback(IAsyncResult result)
        {
            var message = (MessageReader)result.AsyncState;
            int bytesReceived;
            EndPoint remoteEndPoint = new IPEndPoint(this.EndPoint.Address, this.EndPoint.Port);

            //End the receive operation
            try
            {
                bytesReceived = socket.EndReceiveFrom(result, ref remoteEndPoint);

                message.Offset = 0;
                message.Length = bytesReceived;
            }
            catch (ObjectDisposedException)
            {
                message.Recycle();
                return;
            }
            catch (SocketException sx)
            {
                message.Recycle();
                if (sx.SocketErrorCode == SocketError.NotConnected)
                {
                    this.InvokeInternalError(HazelInternalErrors.ConnectionDisconnected);
                    return;
                }

                // Client no longer reachable, pretend it didn't happen
                // TODO should this not inform the connection this client is lost???

                // This thread suggests the IP is not passed out from WinSoc so maybe not possible
                // http://stackoverflow.com/questions/2576926/python-socket-error-on-udp-data-receive-10054
                this.Logger?.WriteError($"Socket Ex {sx.SocketErrorCode} in ReadCallback: {sx.Message}");

                Thread.Sleep(10);
                StartListeningForData();
                return;
            }
            catch (Exception ex)
            {
                // Idk, maybe a null ref after dispose?
                message.Recycle();
                this.Logger?.WriteError("Stopped due to: " + ex.Message);
                return;
            }

            // I'm a little concerned about a infinite loop here, but it seems like it's possible 
            // to get 0 bytes read on UDP without the socket being shut down.
            if (bytesReceived == 0)
            {
                message.Recycle();
                this.Logger?.WriteInfo("Received 0 bytes");
                Thread.Sleep(10);
                StartListeningForData();
                return;
            }

            //Begin receiving again
            StartListeningForData();

            bool aware = true;
            bool isHello = message.Buffer[0] == (byte)UdpSendOption.Hello;

            // If we're aware of this connection use the one already
            // If this is a new client then connect with them!
            UdpServerConnection connection;
            if (!this.allConnections.TryGetValue(remoteEndPoint, out connection))
            {
                lock (this.allConnections)
                {
                    if (!this.allConnections.TryGetValue(remoteEndPoint, out connection))
                    {
                        // Check for malformed connection attempts
                        if (!isHello)
                        {
                            message.Recycle();
                            return;
                        }

                        if (AcceptConnection != null)
                        {
                            if (!AcceptConnection((IPEndPoint)remoteEndPoint, message.Buffer, out var response))
                            {
                                message.Recycle();
                                if (response != null)
                                {
                                    SendData(response, response.Length, remoteEndPoint);
                                }

                                return;
                            }
                        }

                        aware = false;
                        connection = new UdpServerConnection(this, (IPEndPoint)remoteEndPoint, this.IPMode, this.Logger);
                        if (!this.allConnections.TryAdd(remoteEndPoint, connection))
                        {
                            throw new HazelException("Failed to add a connection. This should never happen.");
                        }
                    }
                }
            }

            // If it's a new connection invoke the NewConnection event.
            // This needs to happen before handling the message because in localhost scenarios, the ACK and
            // subsequent messages can happen before the NewConnection event sets up OnDataRecieved handlers
            if (!aware)
            {
                // Skip header and hello byte;
                message.Offset = 4;
                message.Length = bytesReceived - 4;
                message.Position = 0;
                InvokeNewConnection(message, connection);
            }

            // Inform the connection of the buffer (new connections need to send an ack back to client)
            connection.HandleReceive(message, bytesReceived);
        }

#if DEBUG
        public int TestDropRate = -1;
        private int dropCounter = 0;
#endif

        /// <summary>
        ///     Sends data from the listener socket.
        /// </summary>
        /// <param name="bytes">The bytes to send.</param>
        /// <param name="endPoint">The endpoint to send to.</param>
        internal void SendData(byte[] bytes, int length, EndPoint endPoint)
        {
            if (length > bytes.Length) return;

#if DEBUG
            if (TestDropRate > 0)
            {
                if (Interlocked.Increment(ref dropCounter) % TestDropRate == 0)
                {
                    return;
                }
            }
#endif

            try
            {
                socket.BeginSendTo(
                    bytes,
                    0,
                    length,
                    SocketFlags.None,
                    endPoint,
                    SendCallback,
                    null);

                this.Statistics.AddBytesSent(length);
            }
            catch (SocketException e)
            {
                this.Logger?.WriteError("Could not send data as a SocketException occurred: " + e);
            }
            catch (ObjectDisposedException)
            {
                //Keep alive timer probably ran, ignore
                return;
            }
        }

        private void SendCallback(IAsyncResult result)
        {
            try
            {
                socket.EndSendTo(result);
            }
            catch { }
        }

        /// <summary>
        ///     Sends data from the listener socket.
        /// </summary>
        /// <param name="bytes">The bytes to send.</param>
        /// <param name="endPoint">The endpoint to send to.</param>
        internal void SendDataSync(byte[] bytes, int length, EndPoint endPoint)
        {
            try
            {
                socket.SendTo(
                    bytes,
                    0,
                    length,
                    SocketFlags.None,
                    endPoint
                );

                this.Statistics.AddBytesSent(length);
            }
            catch { }
        }

        /// <summary>
        ///     Removes a virtual connection from the list.
        /// </summary>
        /// <param name="endPoint">The endpoint of the virtual connection.</param>
        internal void RemoveConnectionTo(EndPoint endPoint)
        {
            this.allConnections.TryRemove(endPoint, out var conn);
        }

        /// <inheritdoc />
        protected override void Dispose(bool disposing)
        {
            foreach (var kvp in this.allConnections)
            {
                kvp.Value.Dispose();
            }

            try { this.socket.Shutdown(SocketShutdown.Both); } catch { }
            try { this.socket.Close(); } catch { }
            try { this.socket.Dispose(); } catch { }

            this.reliablePacketTimer.Dispose();

            base.Dispose(disposing);
        }
    }
}