SVSimServer/SVSim.BattleNode/Sessions/BattleSession.cs

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

namespace SVSim.BattleNode.Sessions;

/// <summary>
/// One per connected client. Owns the WebSocket + reliability ledgers + lifecycle phase.
/// </summary>
public sealed class BattleSession
{
    private readonly WebSocket _ws;
    private readonly ILogger<BattleSession> _log;

    public string BattleId { get; }
    public long ViewerId { get; }
    public BattleSessionPhase Phase { get; internal set; } = BattleSessionPhase.AwaitingInitNetwork;
    public InboundTracker Inbound { get; } = new();
    public OutboundSequencer Outbound { get; } = new();

    public BattleSession(WebSocket ws, string battleId, long viewerId, ILogger<BattleSession> log)
    {
        _ws = ws;
        _log = log;
        BattleId = battleId;
        ViewerId = viewerId;
    }

    /// <summary>
    /// Send the EIO3 open handshake then run the read loop until the WS closes.
    /// </summary>
    public async Task RunAsync(CancellationToken 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);  // EIO3 pong
                    continue;
                }
                if (eio.Type != EngineIoPacketType.Message) continue;

                // SIO inside the message payload.
                var sio = SocketIoFrame.Parse(eio.Payload);
                if (sio.AttachmentCount > 0)
                {
                    pendingFrame = sio;
                    pendingAttachments.Clear();
                    continue;
                }
                DispatchSocketIo(sio);
            }
            else
            {
                // Binary frame — an attachment for a pending binary event.
                // Engine.IO v3 prefixes binary WS frames with the packet-type byte
                // (0x04 = Message), analogous to the leading digit on text frames.
                // Strip it before treating the rest as the Socket.IO attachment payload.
                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;
                    DispatchSocketIo(assembled);
                }
            }
        }
    }

    private async void DispatchSocketIo(SocketIoFrame frame)
    {
        if (frame.Type is SocketIoPacketType.Event or SocketIoPacketType.BinaryEvent)
        {
            switch (frame.EventName)
            {
                case "msg" when frame.BinaryAttachments.Count == 1:
                    await HandleMsgEventAsync(frame);
                    return;
                case "alive" when frame.BinaryAttachments.Count == 1:
                    await HandleAliveEventAsync(frame);
                    return;
            }
        }
        // hand / unknown events: log and drop.
        _log.LogDebug("BattleSession {Bid}: dropping SIO event={Event}", BattleId, frame.EventName);
    }

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

            // Ack tracking + dedupe.
            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;

            // Run the pure-logic decision and drive sends.
            var responses = ComputeResponses(env);
            foreach (var (responseEnv, noStock) in responses)
            {
                if (noStock)
                    await PushNoStockAsync(responseEnv);
                else
                    await PushOrderedAsync(responseEnv);
            }
        }
        catch (Exception ex)
        {
            _log.LogError(ex, "BattleSession {Bid}: unhandled exception in HandleMsgEventAsync", BattleId);
        }
    }

    private async Task HandleAliveEventAsync(SocketIoFrame frame)
    {
        try
        {
            // Client emits Gungnir every 5s with an SIO ack callback expecting just liveness confirmation
            // (payload ignored). We ack immediately, then push our own alive back with scs/ocs ONLINE
            // placeholders — the only response the client uses to drive its scs/ocs state machine.
            if (frame.AckId.HasValue)
            {
                await SendSioAckAsync(frame.AckId.Value, 0);
            }
            var aliveEnv = new MsgEnvelope(
                Uri: NetworkBattleUri.Gungnir,
                ViewerId: ScriptedLifecycle.FakeOpponentViewerId,
                Uuid: "node-stub",
                Bid: null,
                Try: 0,
                Cat: EmitCategory.General,
                PubSeq: null,
                PlaySeq: null,
                Body: Gungnir.BuildAlivePushBody());
            await PushNoStockAsync(aliveEnv, eventName: "alive");
        }
        catch (Exception ex)
        {
            _log.LogError(ex, "BattleSession {Bid}: unhandled exception in HandleAliveEventAsync", BattleId);
        }
    }

    internal IReadOnlyList<(MsgEnvelope Envelope, bool NoStock)> ComputeResponses(MsgEnvelope env)
    {
        var result = new List<(MsgEnvelope Envelope, bool NoStock)>();
        switch (env.Uri)
        {
            case NetworkBattleUri.InitNetwork when Phase == BattleSessionPhase.AwaitingInitNetwork:
                result.Add((BuildAckedEnvelope(NetworkBattleUri.InitNetwork), NoStock: true));
                result.Add((ScriptedLifecycle.BuildMatched(ViewerId, ScriptedLifecycle.FakeOpponentViewerId, BattleId), NoStock: false));
                result.Add((ScriptedLifecycle.BuildBattleStart(ViewerId), NoStock: false));
                Phase = BattleSessionPhase.AwaitingLoaded;
                break;
            case NetworkBattleUri.Loaded when Phase == BattleSessionPhase.AwaitingLoaded:
                result.Add((ScriptedLifecycle.BuildDeal(), NoStock: false));
                Phase = BattleSessionPhase.AwaitingSwap;
                break;
            case NetworkBattleUri.Swap when Phase == BattleSessionPhase.AwaitingSwap:
                result.Add((ScriptedLifecycle.BuildSwapResponse(ExtractIdxList(env)), NoStock: false));
                result.Add((ScriptedLifecycle.BuildReady(), NoStock: false));
                Phase = BattleSessionPhase.AfterReady;
                break;
            case NetworkBattleUri.Retire:
            case NetworkBattleUri.Kill:
                // These always terminate, regardless of phase.
                result.Add((BuildBattleFinishNoContest(), NoStock: true));
                Phase = BattleSessionPhase.Terminal;
                break;
            default:
                // Out-of-order or unknown URI: log and drop (no response).
                _log.LogDebug("BattleSession {Bid}: dropping uri={Uri} in phase={Phase}", BattleId, env.Uri, Phase);
                break;
        }
        return result;
    }

    private MsgEnvelope BuildAckedEnvelope(NetworkBattleUri uri) => new(
        uri,
        ViewerId: ScriptedLifecycle.FakeOpponentViewerId,
        Uuid: "node-stub",
        Bid: null,
        Try: 0,
        Cat: EmitCategory.General,
        PubSeq: null,
        PlaySeq: null,
        Body: new Dictionary<string, object?> { ["resultCode"] = 1 });

    private MsgEnvelope BuildBattleFinishNoContest() => new(
        NetworkBattleUri.BattleFinish,
        ViewerId: ScriptedLifecycle.FakeOpponentViewerId,
        Uuid: "node-stub",
        Bid: null,
        Try: 0,
        Cat: EmitCategory.Battle,
        PubSeq: null,
        PlaySeq: null,
        Body: new Dictionary<string, object?> { ["result"] = 1, ["resultCode"] = 1 });

    private static IReadOnlyList<long> ExtractIdxList(MsgEnvelope env)
    {
        if (env.Body.TryGetValue("idxList", out var raw) && raw is List<object?> lst)
            return lst.OfType<long>().ToList();
        return Array.Empty<long>();
    }

    private Task PushOrderedAsync(MsgEnvelope env, string eventName = "synchronize") =>
        EncodeAndSendAsync(Outbound.AssignAndArchive(env), eventName);

    private Task PushNoStockAsync(MsgEnvelope env, string eventName = "synchronize") =>
        EncodeAndSendAsync(Outbound.WrapNoStock(env), eventName);

    private async Task EncodeAndSendAsync(MsgEnvelope env, string eventName)
    {
        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, CancellationToken.None);
        foreach (var bin in bins)
        {
            // Engine.IO v3 binary frames are prefixed with the packet-type byte
            // (0x04 = Message), the binary analog of the leading digit on text frames.
            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, CancellationToken.None);
        }
    }

    private async Task SendSioAckAsync(int ackId, long arg)
    {
        // checked() ensures we'd notice if pubSeq ever exceeded int.MaxValue (not realistic but defensive).
        var ack = SocketIoFrame.AckResponse(ackId, checked((int)arg));
        var (text, _) = ack.Encode();
        var eioText = $"{(int)EngineIoPacketType.Message}{text}";
        await SendTextAsync(eioText, CancellationToken.None);
    }

    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);
    }
}