Substation/Server.cs

using System.Net;
using System.Net.WebSockets;
using System.Text;
using System.Text.Json;

namespace Substation;

public class Server
{
    readonly CoyoteDevice _device;
    readonly State          _state;
    readonly HttpListener _listener;
    readonly string       _url;

    int _hasClient; // 0 = no, 1 = yes (Interlocked)

    WebSocket? _pushWs;
    readonly SemaphoreSlim _sendLock = new(1, 1);

    public bool HasClient => Interlocked.CompareExchange(ref _hasClient, 0, 0) == 1;

    public event Action<bool>? ClientChanged;

    static readonly JsonSerializerOptions JsonOpts = new()
    {
        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower
    };

    public Server(CoyoteDevice device, State state, int port)
    {
        _device  = device;
        _state   = state;
        _url     = $"http://127.0.0.1:{port}/";
        _listener = new HttpListener();
        _listener.Prefixes.Add(_url);
        _device.ConnectionChanged += OnDeviceConnectionChanged;
    }

    public void Stop()
    {
        try { _listener.Stop(); } catch { }
    }

    public async Task RunAsync(CancellationToken ct)
    {
        _listener.Start();
        Console.WriteLine($"[WS] listening on {_url}");

        using var reg = ct.Register(() =>
        {
            try { _listener.Stop(); } catch { }
        });

        while (!ct.IsCancellationRequested)
        {
            HttpListenerContext ctx;
            try
            {
                ctx = await _listener.GetContextAsync();
            }
            catch (HttpListenerException)   { break; }
            catch (ObjectDisposedException) { break; }

            if (!ctx.Request.IsWebSocketRequest)
            {
                ctx.Response.StatusCode = 400;
                ctx.Response.Close();
                continue;
            }

            if (HasClient)
            {
                // Reject: only one client at a time.
                try
                {
                    var rej = await ctx.AcceptWebSocketAsync(null);
                    var msg = Encoding.UTF8.GetBytes("{\"ok\":false,\"error\":\"another client is already connected\"}");
                    await rej.WebSocket.SendAsync(msg, WebSocketMessageType.Text, true, CancellationToken.None);
                    await rej.WebSocket.CloseAsync(WebSocketCloseStatus.PolicyViolation, "busy", CancellationToken.None);
                }
                catch { }
                continue;
            }

            _ = HandleClientAsync(ctx, ct);
        }
    }

    async Task HandleClientAsync(HttpListenerContext ctx, CancellationToken ct)
    {
        WebSocket ws;
        try
        {
            var wsCtx = await ctx.AcceptWebSocketAsync(null);
            ws = wsCtx.WebSocket;
        }
        catch (Exception ex)
        {
            Console.Error.WriteLine($"[WS] accept failed: {ex.Message}");
            return;
        }

        Interlocked.Exchange(ref _hasClient, 1);
        _pushWs = ws;
        ClientChanged?.Invoke(true);
        Console.WriteLine($"[WS] client connected ({ctx.Request.RemoteEndPoint})");

        // Push current status immediately so the client knows the BLE state
        await SendAsync(ws, BuildStatusPush(_device.IsConnected ? "hello" : null));

        var buf = new byte[8192];
        try
        {
            while (ws.State == WebSocketState.Open && !ct.IsCancellationRequested)
            {
                WebSocketReceiveResult result;
                var sb = new StringBuilder();

                do
                {
                    result = await ws.ReceiveAsync(buf, ct);
                    if (result.MessageType == WebSocketMessageType.Close)
                        goto done;

                    sb.Append(Encoding.UTF8.GetString(buf, 0, result.Count));
                }
                while (!result.EndOfMessage);

                var response = HandleCommand(sb.ToString());
                await SendAsync(ws, response);
            }
            done:;
        }
        catch (OperationCanceledException) { }
        catch (WebSocketException ex)
        {
            Console.Error.WriteLine($"[WS] socket error: {ex.Message}");
        }
        finally
        {
            _pushWs = null;
            if (ws.State == WebSocketState.Open)
                try { await ws.CloseAsync(WebSocketCloseStatus.NormalClosure, "bye", CancellationToken.None); } catch { }
            ws.Dispose();
            Interlocked.Exchange(ref _hasClient, 0);
            ClientChanged?.Invoke(false);
            Console.WriteLine("[WS] client disconnected");
        }
    }

    async Task SendAsync(WebSocket ws, string text)
    {
        await _sendLock.WaitAsync();
        try
        {
            if (ws.State == WebSocketState.Open)
            {
                var bytes = Encoding.UTF8.GetBytes(text);
                await ws.SendAsync(bytes, WebSocketMessageType.Text, endOfMessage: true, CancellationToken.None);
            }
        }
        finally { _sendLock.Release(); }
    }

    void OnDeviceConnectionChanged(bool connected)
    {
        var ws = _pushWs;
        if (ws == null) return;
        var msg = BuildStatusPush(connected ? "connected" : "disconnected");
        _ = SendAsync(ws, msg);
    }

    string BuildStatusPush(string? eventName)
    {
        return JsonSerializer.Serialize(new StatusResponse
        {
            Event = eventName,
            Connected = _device.IsConnected,
            StrengthA = _device.StrengthA,
            StrengthB = _device.StrengthB
        }, JsonOpts);
    }

    string HandleCommand(string json)
    {
        Command? cmd;
        try
        {
            cmd = JsonSerializer.Deserialize<Command>(json, JsonOpts);
        }
        catch (Exception ex)
        {
            return Err($"invalid JSON: {ex.Message}");
        }

        if (cmd == null || string.IsNullOrWhiteSpace(cmd.Op))
            return Err("missing 'op' field");

        try
        {
            return cmd.Op switch
            {
                "connect"    => DoConnect(),
                "status"     => DoStatus(),
                "ping"       => Ok("pong"),
                _ when !_device.IsConnected => Err("device not connected"),
                "strength"   => DoStrength(cmd),
                "wave"       => DoWave(cmd),
                "stream"     => DoStream(cmd),
                "stop"       => DoStop(cmd),
                "config"     => DoConfig(cmd),
                "disconnect" => DoDisconnect(),
                _            => Err($"unknown op: '{cmd.Op}'")
            };
        }
        catch (Exception ex)
        {
            return Err(ex.Message);
        }
    }

    string DoConnect()
    {
        if (_device.IsConnected)
            return Ok("already connected");

        Task.Run(async () =>
        {
            try
            {
                await _device.ConnectAsync();
                Console.WriteLine($"[BLE] connected to {_device.DeviceName}");
            }
            catch (Exception ex)
            {
                Console.Error.WriteLine($"[BLE] connect failed: {ex.Message}");
            }
        });

        return Ok("connecting...");
    }

    string DoStatus()
    {
        return JsonSerializer.Serialize(new StatusResponse
        {
            Connected = _device.IsConnected,
            StrengthA = _device.StrengthA,
            StrengthB = _device.StrengthB
        }, JsonOpts);
    }

    string DoStrength(Command cmd)
    {
        if (cmd.Channel is not ("A" or "B"))
            return Err("channel must be 'A' or 'B'");
        if (!cmd.Value.HasValue)
            return Err("value required (0-200)");

        var ch = cmd.Channel[0];
        _state.SetStrength(ch, cmd.Value.Value);
        return Ok($"strength {ch} -> {cmd.Value.Value}");
    }

    string DoWave(Command cmd)
    {
        if (cmd.Channel is not ("A" or "B"))
            return Err("channel must be 'A' or 'B'");
        if (cmd.Freq == null || cmd.Intensity == null)
            return Err("freq[] and intensity[] required (4 values each)");

        var freq  = Freq.Compress4(cmd.Freq);
        var intensity = Intensity.Clamp4(cmd.Intensity);
        var ch = cmd.Channel[0];

        _state.SetLoop(ch, freq, intensity);
        return Ok($"wave {ch} looped: freq=[{string.Join(',', freq)}] int=[{string.Join(',', intensity)}]");
    }

    string DoStream(Command cmd)
    {
        if (cmd.Channel is not ("A" or "B"))
            return Err("channel must be 'A' or 'B'");
        if (cmd.Frames == null || cmd.Frames.Length == 0)
            return Err("frames[] required");

        var ch = cmd.Channel[0];
        var frames = cmd.Frames.Select(f =>
        {
            if (f.Freq == null || f.Intensity == null || f.Freq.Length != 4 || f.Intensity.Length != 4)
                throw new ArgumentException("each frame needs freq[4] and intensity[4]");
            return new WaveFrame(Freq.Compress4(f.Freq), Intensity.Clamp4(f.Intensity));
        }).ToList();

        _state.EnqueueStream(ch, frames);
        return Ok($"streamed {frames.Count} frames to {ch}");
    }

    string DoStop(Command cmd)
    {
        if (cmd.Channel is not ("A" or "B"))
            return Err("channel must be 'A' or 'B'");

        var ch = cmd.Channel[0];
        _state.Stop(ch);
        return Ok($"stopped {ch}");
    }

    string DoConfig(Command cmd)
    {
        var capA     = (byte)Math.Clamp(cmd.SoftCapA ?? 200, 0, 200);
        var capB     = (byte)Math.Clamp(cmd.SoftCapB ?? 200, 0, 200);
        var freqBalA = (byte)Math.Clamp(cmd.FreqBalA ?? 128, 0, 255);
        var freqBalB = (byte)Math.Clamp(cmd.FreqBalB ?? 128, 0, 255);
        var intBalA  = (byte)Math.Clamp(cmd.IntBalA  ?? 128, 0, 255);
        var intBalB  = (byte)Math.Clamp(cmd.IntBalB  ?? 128, 0, 255);

        Task.Run(() => _device.SendBF(capA, capB, freqBalA, freqBalB, intBalA, intBalB));
        return Ok($"config sent: caps={capA}/{capB} freqBal={freqBalA}/{freqBalB} intBal={intBalA}/{intBalB}");
    }

    string DoDisconnect()
    {
        _state.Stop('A');
        _state.Stop('B');
        return Ok("waveforms stopped (BLE stays connected; restart app to fully disconnect)");
    }

    static string Ok(string? msg = null) =>
        JsonSerializer.Serialize(new OkResponse { Msg = msg }, JsonOpts);

    static string Err(string error) =>
        JsonSerializer.Serialize(new ErrResponse { Error = error }, JsonOpts);
}