SVSimServer/SVSim.BattleNode/Sessions/Participants/RealParticipant.cs

using System.Net.WebSockets;
using System.Security.Cryptography;
using System.Text;
using Microsoft.Extensions.Logging;
using SVSim.BattleNode.Bridge;
using SVSim.BattleNode.Protocol;
using SVSim.BattleNode.Protocol.Bodies;
using SVSim.BattleNode.Reliability;
using SVSim.BattleNode.Wire;

namespace SVSim.BattleNode.Sessions.Participants;

/// <summary>
/// WS-backed participant. Owns the WS read loop, SIO encoding/decoding, per-WS
/// <see cref="OutboundSequencer"/> + <see cref="InboundTracker"/>. Fires
/// <see cref="FrameEmitted"/> on each deduplicated inbound <see cref="MsgEnvelope"/>.
/// PushAsync encodes + sends; ordered pushes get a playSeq from the sequencer,
/// no-stock control pushes bypass it.
/// </summary>
public sealed class RealParticipant : IBattleParticipant
{
    private readonly WebSocket _ws;
    private readonly ILogger<RealParticipant> _log;
    private CancellationToken _sessionCt;

    public long ViewerId { get; }
    public MatchContext Context { get; }
    public InboundTracker Inbound { get; } = new();
    public OutboundSequencer Outbound { get; } = new();

    public event Func<MsgEnvelope, CancellationToken, Task>? FrameEmitted;

    public RealParticipant(WebSocket ws, long viewerId, MatchContext context,
        ILogger<RealParticipant> log)
    {
        _ws = ws;
        _log = log;
        ViewerId = viewerId;
        Context = context;
    }

    public async Task RunAsync(CancellationToken cancellation)
    {
        _sessionCt = cancellation;
        await SendEioOpenAsync(cancellation);

        var buffer = new byte[8192];
        var pendingAttachments = new List<byte[]>();
        SocketIoFrame? pendingFrame = null;

        while (_ws.State == WebSocketState.Open && !cancellation.IsCancellationRequested)
        {
            var msg = await ReadCompleteMessageAsync(buffer, cancellation);
            if (msg is null) break;

            if (msg.Value.IsText)
            {
                var text = Encoding.UTF8.GetString(msg.Value.Bytes);
                if (text.Length == 0) continue;
                var eio = EngineIoFrame.Parse(text);
                if (eio.Type == EngineIoPacketType.Ping)
                {
                    await SendTextAsync("3", cancellation);
                    continue;
                }
                if (eio.Type != EngineIoPacketType.Message) continue;

                var sio = SocketIoFrame.Parse(eio.Payload);
                if (sio.AttachmentCount > 0)
                {
                    pendingFrame = sio;
                    pendingAttachments.Clear();
                    continue;
                }
                await DispatchSocketIo(sio);
            }
            else
            {
                var bin = msg.Value.Bytes;
                if (bin.Length > 0 && bin[0] == (byte)EngineIoPacketType.Message)
                {
                    bin = bin.AsSpan(1).ToArray();
                }
                pendingAttachments.Add(bin);
                if (pendingFrame is not null && pendingAttachments.Count == pendingFrame.AttachmentCount)
                {
                    var assembled = pendingFrame.WithAttachments(pendingAttachments.ToArray());
                    pendingFrame = null;
                    await DispatchSocketIo(assembled);
                }
            }
        }
    }

    public async Task PushAsync(MsgEnvelope envelope, bool noStock, CancellationToken ct)
    {
        var stamped = noStock ? Outbound.WrapNoStock(envelope) : Outbound.AssignAndArchive(envelope);
        await EncodeAndSendAsync(stamped, WireConstants.SynchronizeEvent, ct);
    }

    public Task TerminateAsync(BattleFinishReason reason)
    {
        // WS will close via the read loop exiting; nothing to do here.
        return Task.CompletedTask;
    }

    public ValueTask DisposeAsync()
    {
        if (_ws.State == WebSocketState.Open || _ws.State == WebSocketState.CloseReceived)
        {
            try { _ws.Abort(); } catch { /* best effort */ }
        }
        _ws.Dispose();
        return ValueTask.CompletedTask;
    }

    private async Task DispatchSocketIo(SocketIoFrame frame)
    {
        if (frame.Type is SocketIoPacketType.Event or SocketIoPacketType.BinaryEvent)
        {
            switch (frame.EventName)
            {
                case WireConstants.MsgEvent when frame.BinaryAttachments.Count == 1:
                    await HandleMsgEventAsync(frame);
                    return;
                case WireConstants.AliveEvent when frame.BinaryAttachments.Count == 1:
                    await HandleAliveEventAsync(frame);
                    return;
            }
        }
        _log.LogDebug("RealParticipant viewer={Vid}: dropping SIO event={Event}", ViewerId, frame.EventName);
    }

    private async Task HandleMsgEventAsync(SocketIoFrame frame)
    {
        try
        {
            MsgEnvelope env;
            try { env = MsgPayloadCodec.Decode(frame.BinaryAttachments[0]); }
            catch (Exception ex)
            {
                _log.LogWarning(ex, "RealParticipant viewer={Vid}: failed to decode msg envelope", ViewerId);
                return;
            }

            bool shouldDispatch = true;
            if (env.PubSeq.HasValue)
            {
                shouldDispatch = Inbound.Observe(env.PubSeq.Value);
                if (frame.AckId.HasValue)
                {
                    await SendSioAckAsync(frame.AckId.Value, env.PubSeq.Value);
                }
            }
            if (!shouldDispatch) return;

            if (FrameEmitted is not null)
            {
                await FrameEmitted.Invoke(env, _sessionCt);
            }
        }
        catch (Exception ex)
        {
            _log.LogError(ex, "RealParticipant viewer={Vid}: unhandled in HandleMsgEventAsync", ViewerId);
        }
    }

    private async Task HandleAliveEventAsync(SocketIoFrame frame)
    {
        try
        {
            if (frame.AckId.HasValue)
            {
                await SendSioAckAsync(frame.AckId.Value, 0);
            }
            var aliveEnv = new MsgEnvelope(
                Uri: NetworkBattleUri.Gungnir,
                ViewerId: SVSim.BattleNode.Lifecycle.ScriptedLifecycle.FakeOpponentViewerId,
                Uuid: WireConstants.ServerUuid,
                Bid: null,
                Try: 0,
                Cat: EmitCategory.General,
                PubSeq: null,
                PlaySeq: null,
                Body: new AlivePushBody(Scs: WireConstants.OnlineStatus, Ocs: WireConstants.OnlineStatus));
            var stamped = Outbound.WrapNoStock(aliveEnv);
            await EncodeAndSendAsync(stamped, WireConstants.AliveEvent, _sessionCt);
        }
        catch (Exception ex)
        {
            _log.LogError(ex, "RealParticipant viewer={Vid}: unhandled in HandleAliveEventAsync", ViewerId);
        }
    }

    private async Task EncodeAndSendAsync(MsgEnvelope env, string eventName, CancellationToken ct)
    {
        var key = NodeCrypto.GenerateKey(() => RandomNumberGenerator.GetInt32(0, 16));
        var bytes = MsgPayloadCodec.Encode(env, key);
        var sio = SocketIoFrame.BinaryEventWithAttachments(eventName, new[] { bytes });
        var (text, bins) = sio.Encode();
        var eioText = $"{(int)EngineIoPacketType.Message}{text}";
        await SendTextAsync(eioText, ct);
        foreach (var bin in bins)
        {
            var prefixed = new byte[bin.Length + 1];
            prefixed[0] = (byte)EngineIoPacketType.Message;
            Buffer.BlockCopy(bin, 0, prefixed, 1, bin.Length);
            await _ws.SendAsync(prefixed, WebSocketMessageType.Binary, endOfMessage: true, ct);
        }
    }

    internal static int ClipAckArg(long arg, ILogger log, long viewerId)
    {
        if (arg > int.MaxValue)
        {
            log.LogWarning("RealParticipant viewer={Vid}: pubSeq {Seq} exceeds int.MaxValue; clipping.", viewerId, arg);
            return int.MaxValue;
        }
        if (arg < int.MinValue)
        {
            log.LogWarning("RealParticipant viewer={Vid}: pubSeq {Seq} below int.MinValue; clipping.", viewerId, arg);
            return int.MinValue;
        }
        return (int)arg;
    }

    private async Task SendSioAckAsync(int ackId, long arg)
    {
        var ack = SocketIoFrame.AckResponse(ackId, ClipAckArg(arg, _log, ViewerId));
        var (text, _) = ack.Encode();
        var eioText = $"{(int)EngineIoPacketType.Message}{text}";
        await SendTextAsync(eioText, _sessionCt);
    }

    private async Task SendEioOpenAsync(CancellationToken ct)
    {
        var sid = Guid.NewGuid().ToString("N").Substring(0, 16);
        var handshake = new EngineIoHandshake(sid, Array.Empty<string>(), 25000, 60000).ToJson();
        await SendTextAsync($"0{handshake}", ct);
    }

    private Task SendTextAsync(string text, CancellationToken ct)
    {
        var bytes = Encoding.UTF8.GetBytes(text);
        return _ws.SendAsync(bytes, WebSocketMessageType.Text, endOfMessage: true, ct);
    }

    private async Task<(byte[] Bytes, bool IsText)?> ReadCompleteMessageAsync(byte[] buffer, CancellationToken ct)
    {
        using var ms = new MemoryStream();
        WebSocketReceiveResult result;
        do
        {
            try { result = await _ws.ReceiveAsync(buffer, ct); }
            catch (OperationCanceledException) { return null; }
            catch (WebSocketException) { return null; }
            if (result.MessageType == WebSocketMessageType.Close) return null;
            ms.Write(buffer, 0, result.Count);
        } while (!result.EndOfMessage);
        return (ms.ToArray(), result.MessageType == WebSocketMessageType.Text);
    }
}