using System.Buffers;
using System.IO.Pipelines;
using System.Net.Sockets;
using System.Text;
using System.Text.Json;
using Microsoft.Extensions.Logging;
using NATS.Server.Protocol;
using NATS.Server.Subscriptions;

namespace NATS.Server;

public interface IMessageRouter
{
    void ProcessMessage(string subject, string? replyTo, ReadOnlyMemory<byte> headers,
        ReadOnlyMemory<byte> payload, NatsClient sender);
    void RemoveClient(NatsClient client);
}

public interface ISubListAccess
{
    SubList SubList { get; }
}

public sealed class NatsClient : IDisposable
{
    private readonly Socket _socket;
    private readonly NetworkStream _stream;
    private readonly NatsOptions _options;
    private readonly ServerInfo _serverInfo;
    private readonly NatsParser _parser;
    private readonly SemaphoreSlim _writeLock = new(1, 1);
    private readonly Dictionary<string, Subscription> _subs = new();
    private readonly ILogger _logger;

    public ulong Id { get; }
    public ClientOptions? ClientOpts { get; private set; }
    public IMessageRouter? Router { get; set; }
    public bool ConnectReceived { get; private set; }

    // Stats
    public long InMsgs;
    public long OutMsgs;
    public long InBytes;
    public long OutBytes;

    public IReadOnlyDictionary<string, Subscription> Subscriptions => _subs;

    public NatsClient(ulong id, Socket socket, NatsOptions options, ServerInfo serverInfo, ILogger logger)
    {
        Id = id;
        _socket = socket;
        _stream = new NetworkStream(socket, ownsSocket: false);
        _options = options;
        _serverInfo = serverInfo;
        _logger = logger;
        _parser = new NatsParser(options.MaxPayload);
    }

    public async Task RunAsync(CancellationToken ct)
    {
        var pipe = new Pipe();
        try
        {
            // Send INFO
            await SendInfoAsync(ct);

            // Start read pump and command processing in parallel
            var fillTask = FillPipeAsync(pipe.Writer, ct);
            var processTask = ProcessCommandsAsync(pipe.Reader, ct);

            await Task.WhenAny(fillTask, processTask);
        }
        catch (OperationCanceledException) { }
        catch (Exception ex)
        {
            _logger.LogDebug(ex, "Client {ClientId} connection error", Id);
        }
        finally
        {
            Router?.RemoveClient(this);
        }
    }

    private async Task FillPipeAsync(PipeWriter writer, CancellationToken ct)
    {
        try
        {
            while (!ct.IsCancellationRequested)
            {
                var memory = writer.GetMemory(4096);
                int bytesRead = await _stream.ReadAsync(memory, ct);
                if (bytesRead == 0)
                    break;

                writer.Advance(bytesRead);
                var result = await writer.FlushAsync(ct);
                if (result.IsCompleted)
                    break;
            }
        }
        finally
        {
            await writer.CompleteAsync();
        }
    }

    private async Task ProcessCommandsAsync(PipeReader reader, CancellationToken ct)
    {
        try
        {
            while (!ct.IsCancellationRequested)
            {
                var result = await reader.ReadAsync(ct);
                var buffer = result.Buffer;

                while (_parser.TryParse(ref buffer, out var cmd))
                {
                    await DispatchCommandAsync(cmd, ct);
                }

                reader.AdvanceTo(buffer.Start, buffer.End);

                if (result.IsCompleted)
                    break;
            }
        }
        finally
        {
            await reader.CompleteAsync();
        }
    }

    private async ValueTask DispatchCommandAsync(ParsedCommand cmd, CancellationToken ct)
    {
        switch (cmd.Type)
        {
            case CommandType.Connect:
                ProcessConnect(cmd);
                break;

            case CommandType.Ping:
                await WriteAsync(NatsProtocol.PongBytes, ct);
                break;

            case CommandType.Pong:
                // Update RTT tracking (placeholder)
                break;

            case CommandType.Sub:
                ProcessSub(cmd);
                break;

            case CommandType.Unsub:
                ProcessUnsub(cmd);
                break;

            case CommandType.Pub:
            case CommandType.HPub:
                ProcessPub(cmd);
                break;
        }
    }

    private void ProcessConnect(ParsedCommand cmd)
    {
        ClientOpts = JsonSerializer.Deserialize<ClientOptions>(cmd.Payload.Span)
                     ?? new ClientOptions();
        ConnectReceived = true;
        _logger.LogDebug("CONNECT received from client {ClientId}, name={ClientName}", Id, ClientOpts?.Name);
    }

    private void ProcessSub(ParsedCommand cmd)
    {
        var sub = new Subscription
        {
            Subject = cmd.Subject!,
            Queue = cmd.Queue,
            Sid = cmd.Sid!,
        };

        _subs[cmd.Sid!] = sub;
        sub.Client = this;

        _logger.LogDebug("SUB {Subject} {Sid} from client {ClientId}", cmd.Subject, cmd.Sid, Id);

        if (Router is ISubListAccess sl)
            sl.SubList.Insert(sub);
    }

    private void ProcessUnsub(ParsedCommand cmd)
    {
        _logger.LogDebug("UNSUB {Sid} from client {ClientId}", cmd.Sid, Id);

        if (!_subs.TryGetValue(cmd.Sid!, out var sub))
            return;

        if (cmd.MaxMessages > 0)
        {
            sub.MaxMessages = cmd.MaxMessages;
            // Will be cleaned up when MessageCount reaches MaxMessages
            return;
        }

        _subs.Remove(cmd.Sid!);

        if (Router is ISubListAccess sl)
            sl.SubList.Remove(sub);
    }

    private void ProcessPub(ParsedCommand cmd)
    {
        Interlocked.Increment(ref InMsgs);
        Interlocked.Add(ref InBytes, cmd.Payload.Length);

        ReadOnlyMemory<byte> headers = default;
        ReadOnlyMemory<byte> payload = cmd.Payload;

        if (cmd.Type == CommandType.HPub && cmd.HeaderSize > 0)
        {
            headers = cmd.Payload[..cmd.HeaderSize];
            payload = cmd.Payload[cmd.HeaderSize..];
        }

        Router?.ProcessMessage(cmd.Subject!, cmd.ReplyTo, headers, payload, this);
    }

    private async Task SendInfoAsync(CancellationToken ct)
    {
        var infoJson = JsonSerializer.Serialize(_serverInfo);
        var infoLine = Encoding.ASCII.GetBytes($"INFO {infoJson}\r\n");
        await WriteAsync(infoLine, ct);
    }

    public async Task SendMessageAsync(string subject, string sid, string? replyTo,
        ReadOnlyMemory<byte> headers, ReadOnlyMemory<byte> payload, CancellationToken ct)
    {
        Interlocked.Increment(ref OutMsgs);
        Interlocked.Add(ref OutBytes, payload.Length + headers.Length);

        byte[] line;
        if (headers.Length > 0)
        {
            int totalSize = headers.Length + payload.Length;
            line = Encoding.ASCII.GetBytes($"HMSG {subject} {sid} {(replyTo != null ? replyTo + " " : "")}{headers.Length} {totalSize}\r\n");
        }
        else
        {
            line = Encoding.ASCII.GetBytes($"MSG {subject} {sid} {(replyTo != null ? replyTo + " " : "")}{payload.Length}\r\n");
        }

        await _writeLock.WaitAsync(ct);
        try
        {
            await _stream.WriteAsync(line, ct);
            if (headers.Length > 0)
                await _stream.WriteAsync(headers, ct);
            if (payload.Length > 0)
                await _stream.WriteAsync(payload, ct);
            await _stream.WriteAsync(NatsProtocol.CrLf, ct);
            await _stream.FlushAsync(ct);
        }
        finally
        {
            _writeLock.Release();
        }
    }

    private async Task WriteAsync(byte[] data, CancellationToken ct)
    {
        await _writeLock.WaitAsync(ct);
        try
        {
            await _stream.WriteAsync(data, ct);
            await _stream.FlushAsync(ct);
        }
        finally
        {
            _writeLock.Release();
        }
    }

    public void RemoveAllSubscriptions(SubList subList)
    {
        foreach (var sub in _subs.Values)
            subList.Remove(sub);
        _subs.Clear();
    }

    public void Dispose()
    {
        _stream.Dispose();
        _socket.Dispose();
        _writeLock.Dispose();
    }
}