using System.Collections.Concurrent;
using System.Net;
using System.Net.Sockets;
using Microsoft.Extensions.Logging;
using NATS.Server.Configuration;
using NATS.Server.Subscriptions;

namespace NATS.Server.Gateways;

public sealed class GatewayManager : IAsyncDisposable
{
    private readonly GatewayOptions _options;
    private readonly ServerStats _stats;
    private readonly string _serverId;
    private readonly Action<RemoteSubscription> _remoteSubSink;
    private readonly Action<GatewayMessage> _messageSink;
    private readonly ILogger<GatewayManager> _logger;
    private readonly ConcurrentDictionary<string, GatewayConnection> _connections = new(StringComparer.Ordinal);
    private long _forwardedJetStreamClusterMessages;

    private CancellationTokenSource? _cts;
    private Socket? _listener;
    private Task? _acceptLoopTask;

    public string ListenEndpoint => $"{_options.Host}:{_options.Port}";
    public long ForwardedJetStreamClusterMessages => Interlocked.Read(ref _forwardedJetStreamClusterMessages);

    internal static bool ShouldForwardInterestOnly(SubList subList, string account, string subject)
        => subList.HasRemoteInterest(account, subject);

    public GatewayManager(
        GatewayOptions options,
        ServerStats stats,
        string serverId,
        Action<RemoteSubscription> remoteSubSink,
        Action<GatewayMessage> messageSink,
        ILogger<GatewayManager> logger)
    {
        _options = options;
        _stats = stats;
        _serverId = serverId;
        _remoteSubSink = remoteSubSink;
        _messageSink = messageSink;
        _logger = logger;
    }

    public Task StartAsync(CancellationToken ct)
    {
        _cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
        _listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        _listener.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        _listener.Bind(new IPEndPoint(IPAddress.Parse(_options.Host), _options.Port));
        _listener.Listen(128);

        if (_options.Port == 0)
            _options.Port = ((IPEndPoint)_listener.LocalEndPoint!).Port;

        _acceptLoopTask = Task.Run(() => AcceptLoopAsync(_cts.Token));
        foreach (var remote in _options.Remotes.Distinct(StringComparer.OrdinalIgnoreCase))
            _ = Task.Run(() => ConnectWithRetryAsync(remote, _cts.Token));

        _logger.LogDebug("Gateway manager started (name={Name}, listen={Host}:{Port})",
            _options.Name, _options.Host, _options.Port);
        return Task.CompletedTask;
    }

    public async Task ForwardMessageAsync(string account, string subject, string? replyTo, ReadOnlyMemory<byte> payload, CancellationToken ct)
    {
        foreach (var connection in _connections.Values)
            await connection.SendMessageAsync(account, subject, replyTo, payload, ct);
    }

    public async Task ForwardJetStreamClusterMessageAsync(GatewayMessage message, CancellationToken ct)
    {
        Interlocked.Increment(ref _forwardedJetStreamClusterMessages);
        await ForwardMessageAsync(message.Account, message.Subject, message.ReplyTo, message.Payload, ct);
    }

    public void PropagateLocalSubscription(string account, string subject, string? queue)
    {
        foreach (var connection in _connections.Values)
            _ = connection.SendAPlusAsync(account, subject, queue, _cts?.Token ?? CancellationToken.None);
    }

    public void PropagateLocalUnsubscription(string account, string subject, string? queue)
    {
        foreach (var connection in _connections.Values)
            _ = connection.SendAMinusAsync(account, subject, queue, _cts?.Token ?? CancellationToken.None);
    }

    public async ValueTask DisposeAsync()
    {
        if (_cts == null)
            return;

        await _cts.CancelAsync();
        _listener?.Dispose();
        if (_acceptLoopTask != null)
            await _acceptLoopTask.ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);

        foreach (var connection in _connections.Values)
            await connection.DisposeAsync();
        _connections.Clear();
        Interlocked.Exchange(ref _stats.Gateways, 0);
        _cts.Dispose();
        _cts = null;
        _logger.LogDebug("Gateway manager stopped");
    }

    private async Task AcceptLoopAsync(CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            Socket socket;
            try
            {
                socket = await _listener!.AcceptAsync(ct);
            }
            catch
            {
                break;
            }

            _ = Task.Run(() => HandleInboundAsync(socket, ct), ct);
        }
    }

    private async Task HandleInboundAsync(Socket socket, CancellationToken ct)
    {
        var connection = new GatewayConnection(socket);
        try
        {
            await connection.PerformInboundHandshakeAsync(_serverId, ct);
            Register(connection);
        }
        catch
        {
            await connection.DisposeAsync();
        }
    }

    private async Task ConnectWithRetryAsync(string remote, CancellationToken ct)
    {
        while (!ct.IsCancellationRequested)
        {
            try
            {
                var endPoint = ParseEndpoint(remote);
                var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
                await socket.ConnectAsync(endPoint.Address, endPoint.Port, ct);
                var connection = new GatewayConnection(socket);
                await connection.PerformOutboundHandshakeAsync(_serverId, ct);
                Register(connection);
                return;
            }
            catch (OperationCanceledException)
            {
                return;
            }
            catch (Exception ex)
            {
                _logger.LogDebug(ex, "Gateway connect retry for {Remote}", remote);
            }

            try
            {
                await Task.Delay(250, ct);
            }
            catch (OperationCanceledException)
            {
                return;
            }
        }
    }

    private void Register(GatewayConnection connection)
    {
        var key = $"{connection.RemoteId}:{connection.RemoteEndpoint}:{Guid.NewGuid():N}";
        if (!_connections.TryAdd(key, connection))
        {
            _ = connection.DisposeAsync();
            return;
        }

        connection.RemoteSubscriptionReceived = sub =>
        {
            _remoteSubSink(sub);
            return Task.CompletedTask;
        };
        connection.MessageReceived = msg =>
        {
            _messageSink(msg);
            return Task.CompletedTask;
        };
        connection.StartLoop(_cts!.Token);
        Interlocked.Increment(ref _stats.Gateways);
        _ = Task.Run(() => WatchConnectionAsync(key, connection, _cts!.Token));
    }

    private async Task WatchConnectionAsync(string key, GatewayConnection connection, CancellationToken ct)
    {
        try
        {
            await connection.WaitUntilClosedAsync(ct);
        }
        catch
        {
        }
        finally
        {
            if (_connections.TryRemove(key, out _))
                Interlocked.Decrement(ref _stats.Gateways);
            await connection.DisposeAsync();
        }
    }

    private static IPEndPoint ParseEndpoint(string endpoint)
    {
        var parts = endpoint.Split(':', 2, StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries);
        if (parts.Length != 2)
            throw new FormatException($"Invalid endpoint: {endpoint}");

        return new IPEndPoint(IPAddress.Parse(parts[0]), int.Parse(parts[1]));
    }
}