using System.Collections.Concurrent;
using System.Net;
using System.Net.Sockets;
using System.Net.WebSockets;
using System.Threading.Channels;
using LumaTunnel.Shared.Protocol;

namespace LumaTunnel.Server.Core.Runtime;

public sealed class TunnelConnection : IAsyncDisposable
{
    private readonly CancellationTokenSource _lifetime = new();
    private readonly Channel<TunnelFrame> _incoming;
    private readonly Channel<TunnelFrame> _outgoing;
    private readonly ConcurrentDictionary<uint, ServerTunnelStream> _streams = new();
    private readonly ServerRuntime _runtime;
    private readonly Action<TunnelConnection> _terminated;
    private Task? _incomingTask;
    private Task? _outgoingTask;
    private int _closed;

    public TunnelConnection(WebSocket webSocket, string deviceId, ServerRuntime runtime, Action<TunnelConnection> terminated)
    {
        WebSocket = webSocket;
        DeviceId = deviceId;
        _runtime = runtime;
        _terminated = terminated;
        _incoming = Channel.CreateBounded<TunnelFrame>(new BoundedChannelOptions(runtime.Settings.MaxConnectionsPerDevice * 2)
        {
            FullMode = BoundedChannelFullMode.Wait,
            SingleReader = true,
            SingleWriter = false
        });
        _outgoing = Channel.CreateBounded<TunnelFrame>(new BoundedChannelOptions(runtime.Settings.MaxConnectionsPerDevice * 4)
        {
            FullMode = BoundedChannelFullMode.Wait,
            SingleReader = true,
            SingleWriter = false
        });
    }

    public string DeviceId { get; }
    public WebSocket WebSocket { get; }
    public int ActiveStreams => _streams.Count;

    public void Start()
    {
        _incomingTask = ProcessIncomingAsync(_lifetime.Token);
        _outgoingTask = SendLoopAsync(_lifetime.Token);
    }

    public bool TryEnqueue(ReadOnlySpan<byte> bytes)
    {
        try
        {
            return _incoming.Writer.TryWrite(TunnelProtocol.Decode(bytes));
        }
        catch (TunnelProtocolException)
        {
            return false;
        }
    }

    public ValueTask SendAsync(TunnelFrame frame, CancellationToken cancellationToken = default) =>
        _outgoing.Writer.WriteAsync(frame, cancellationToken);

    public async Task CloseAsync(WebSocketCloseStatus status, string description)
    {
        if (Interlocked.Exchange(ref _closed, 1) != 0)
            return;

        _lifetime.Cancel();
        _incoming.Writer.TryComplete();
        _outgoing.Writer.TryComplete();
        foreach (var pair in _streams.ToArray())
        {
            if (_streams.TryRemove(pair.Key, out var stream))
            {
                _runtime.Traffic.ConnectionClosed(DeviceId);
                await stream.DisposeAsync().ConfigureAwait(false);
            }
        }

        try
        {
            if (WebSocket.State is WebSocketState.Open or WebSocketState.CloseReceived)
                await WebSocket.CloseAsync(status, description, CancellationToken.None).ConfigureAwait(false);
        }
        catch (WebSocketException)
        {
            WebSocket.Abort();
        }
        finally
        {
            _terminated(this);
        }
    }

    public async ValueTask DisposeAsync()
    {
        await CloseAsync(WebSocketCloseStatus.NormalClosure, "Tunnel connection closed.").ConfigureAwait(false);
        _lifetime.Dispose();
    }

    internal async Task RemoveStreamAsync(uint streamId)
    {
        if (_streams.TryRemove(streamId, out var stream))
        {
            _runtime.Traffic.ConnectionClosed(DeviceId);
            await stream.DisposeAsync().ConfigureAwait(false);
        }
    }

    private async Task ProcessIncomingAsync(CancellationToken cancellationToken)
    {
        try
        {
            await foreach (var frame in _incoming.Reader.ReadAllAsync(cancellationToken).ConfigureAwait(false))
            {
                switch (frame.Type)
                {
                    case TunnelFrameType.Open:
                        await OpenStreamAsync(frame, cancellationToken).ConfigureAwait(false);
                        break;
                    case TunnelFrameType.Data:
                        if (_streams.TryGetValue(frame.StreamId, out var dataStream))
                            await dataStream.WriteFromClientAsync(frame.Payload, cancellationToken).ConfigureAwait(false);
                        break;
                    case TunnelFrameType.HalfClose:
                        if (_streams.TryGetValue(frame.StreamId, out var halfCloseStream))
                            await halfCloseStream.ClientFinishedWritingAsync().ConfigureAwait(false);
                        break;
                    case TunnelFrameType.Close:
                    case TunnelFrameType.Reset:
                        await RemoveStreamAsync(frame.StreamId).ConfigureAwait(false);
                        break;
                    case TunnelFrameType.Ping:
                        await SendAsync(new TunnelFrame(TunnelFrameType.Pong, 0, frame.Payload), cancellationToken).ConfigureAwait(false);
                        break;
                }
            }
        }
        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
        {
        }
        catch (Exception)
        {
            await CloseAsync(WebSocketCloseStatus.InternalServerError, "Tunnel receive loop failed.").ConfigureAwait(false);
        }
    }

    private async Task OpenStreamAsync(TunnelFrame frame, CancellationToken cancellationToken)
    {
        if (frame.StreamId == 0 || (frame.StreamId & 1) == 0 || _streams.ContainsKey(frame.StreamId))
        {
            await SendOpenErrorAsync(frame.StreamId, TunnelErrorCode.MalformedFrame, "Stream id must be a new, non-zero odd number.", cancellationToken).ConfigureAwait(false);
            return;
        }
        if (_streams.Count >= _runtime.Settings.MaxConnectionsPerDevice || _runtime.Traffic.ActiveConnections >= _runtime.Settings.MaxConnections)
        {
            await SendOpenErrorAsync(frame.StreamId, TunnelErrorCode.TooManyConnections, "Connection limit reached.", cancellationToken).ConfigureAwait(false);
            return;
        }

        try
        {
            var request = TunnelMessageSerializer.Deserialize<TunnelOpenRequest>(frame.Payload);
            var address = await _runtime.TargetPolicy.ResolveAndValidateAsync(request.Host, request.Port, cancellationToken).ConfigureAwait(false);
            using var timeout = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
            timeout.CancelAfter(TimeSpan.FromSeconds(_runtime.Settings.ConnectTimeoutSeconds));
            var client = new TcpClient(address.AddressFamily) { NoDelay = true };
            try
            {
                await client.ConnectAsync(address, request.Port, timeout.Token).ConfigureAwait(false);
            }
            catch
            {
                client.Dispose();
                throw;
            }

            var stream = new ServerTunnelStream(frame.StreamId, client, this, _runtime);
            if (!_streams.TryAdd(frame.StreamId, stream))
            {
                await stream.DisposeAsync().ConfigureAwait(false);
                await SendOpenErrorAsync(frame.StreamId, TunnelErrorCode.MalformedFrame, "Stream id is already open.", cancellationToken).ConfigureAwait(false);
                return;
            }

            _runtime.Traffic.ConnectionOpened(DeviceId);
            stream.Start();
            await SendAsync(TunnelFrame.Empty(TunnelFrameType.OpenOk, frame.StreamId), cancellationToken).ConfigureAwait(false);
        }
        catch (TunnelProtocolException exception)
        {
            await SendOpenErrorAsync(frame.StreamId, exception.Code, exception.Message, cancellationToken).ConfigureAwait(false);
        }
        catch (OperationCanceledException) when (!cancellationToken.IsCancellationRequested)
        {
            await SendOpenErrorAsync(frame.StreamId, TunnelErrorCode.ConnectTimeout, "Target connection timed out.", cancellationToken).ConfigureAwait(false);
        }
        catch (SocketException exception)
        {
            await SendOpenErrorAsync(frame.StreamId, TunnelErrorCode.ConnectRefused, $"Target connection failed: {exception.SocketErrorCode}.", cancellationToken).ConfigureAwait(false);
        }
        catch (Exception)
        {
            await SendOpenErrorAsync(frame.StreamId, TunnelErrorCode.InternalError, "Target connection failed.", cancellationToken).ConfigureAwait(false);
        }
    }

    private ValueTask SendOpenErrorAsync(uint streamId, TunnelErrorCode code, string message, CancellationToken cancellationToken) =>
        SendAsync(new TunnelFrame(TunnelFrameType.OpenError, streamId, TunnelMessageSerializer.Serialize(new TunnelOpenError(code, message))), cancellationToken);

    private async Task SendLoopAsync(CancellationToken cancellationToken)
    {
        try
        {
            await foreach (var frame in _outgoing.Reader.ReadAllAsync(cancellationToken).ConfigureAwait(false))
            {
                var bytes = TunnelProtocol.Encode(frame);
                await WebSocket.SendAsync(bytes, WebSocketMessageType.Binary, true, cancellationToken).ConfigureAwait(false);
            }
        }
        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
        {
        }
        catch (Exception)
        {
            await CloseAsync(WebSocketCloseStatus.InternalServerError, "Tunnel send loop failed.").ConfigureAwait(false);
        }
    }
}