feat(backend): GameProcess subprocess wrapper with async step/reset

Backend/GameProcess.cs

@@ -0,0 +1,166 @@
+using System.Diagnostics;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Backend;
+
+/// <summary>
+/// One instance == one <c>GameCli</c> subprocess. Not thread-safe.
+/// Callers must sequence <see cref="StepAsync"/> and <see cref="ResetAsync"/>.
+/// </summary>
+public sealed class GameProcess : IAsyncDisposable
+{
+    private readonly string _binaryPath;
+    private Process? _proc;
+    private StreamReader? _stdout;
+    private StreamWriter? _stdin;
+
+    public float Dt { get; private set; }
+    public int ObsDim { get; private set; }
+    public int NActions { get; private set; }
+
+    public GameProcess(string binaryPath)
+    {
+        _binaryPath = binaryPath;
+    }
+
+    public async Task StartAsync()
+    {
+        if (_proc is not null) throw new InvalidOperationException("already started");
+        if (!File.Exists(_binaryPath))
+            throw new FileNotFoundException($"GameCli binary not found at {_binaryPath}");
+
+        var psi = new ProcessStartInfo
+        {
+            FileName = _binaryPath,
+            RedirectStandardInput = true,
+            RedirectStandardOutput = true,
+            RedirectStandardError = true,
+            UseShellExecute = false,
+        };
+        _proc = Process.Start(psi)
+                ?? throw new InvalidOperationException("failed to start GameCli");
+        _stdout = _proc.StandardOutput;
+        _stdin  = _proc.StandardInput;
+
+        var line = await _stdout.ReadLineAsync()
+                   ?? throw new IOException("GameCli died before init handshake");
+        var init = JsonSerializer.Deserialize(line, ProcessJsonContext.Default.InitEnvelope)
+                   ?? throw new IOException($"malformed init handshake: {line}");
+        Dt       = init.Init.Dt;
+        ObsDim   = init.Init.ObsDim;
+        NActions = init.Init.NActions;
+    }
+
+    public async Task<StepResult> StepAsync(int action, (float X, float Y) target)
+    {
+        var input = new StepIn
+        {
+            Action = action,
+            Target = new[] { target.X, target.Y },
+        };
+        await WriteAsync(input, ProcessJsonContext.Default.StepIn);
+        return await ReadStepResultAsync();
+    }
+
+    public async Task<StepResult> ResetAsync(int? seed = null)
+    {
+        var input = new StepIn { Cmd = "reset", Seed = seed };
+        await WriteAsync(input, ProcessJsonContext.Default.StepIn);
+        return await ReadStepResultAsync();
+    }
+
+    private async Task WriteAsync<T>(T value, System.Text.Json.Serialization.Metadata.JsonTypeInfo<T> ctx)
+    {
+        if (_stdin is null) throw new InvalidOperationException("not started");
+        var json = JsonSerializer.Serialize(value, ctx);
+        await _stdin.WriteLineAsync(json);
+        await _stdin.FlushAsync();
+    }
+
+    private async Task<StepResult> ReadStepResultAsync()
+    {
+        if (_stdout is null) throw new InvalidOperationException("not started");
+        var line = await _stdout.ReadLineAsync()
+                   ?? throw new IOException("GameCli closed stdout unexpectedly");
+        var msg = JsonSerializer.Deserialize(line, ProcessJsonContext.Default.StepOut)
+                  ?? throw new IOException($"malformed step output: {line}");
+        return new StepResult(msg.Obs, msg.State, msg.Reward, msg.Done, msg.Step);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        try { _stdin?.Close(); } catch { }
+        if (_proc is not null)
+        {
+            try
+            {
+                var exited = _proc.WaitForExit(2000);
+                if (!exited) _proc.Kill(entireProcessTree: true);
+            }
+            catch { }
+            _proc.Dispose();
+            _proc = null;
+        }
+        await ValueTask.CompletedTask;
+    }
+}
+
+// -------- Result --------
+
+public readonly record struct StepResult(
+    float[] Observation,
+    ShipStatePayload State,
+    float Reward,
+    bool Done,
+    int Step);
+
+// -------- Wire DTOs (local mirror of GameCli's Protocol.cs) --------
+
+public sealed class StepIn
+{
+    [JsonPropertyName("action")] public int? Action { get; set; }
+    [JsonPropertyName("target")] public float[]? Target { get; set; }
+    [JsonPropertyName("cmd")]    public string? Cmd { get; set; }
+    [JsonPropertyName("seed")]   public int? Seed { get; set; }
+}
+
+public sealed class StepOut
+{
+    [JsonPropertyName("obs")]    public float[] Obs { get; set; } = Array.Empty<float>();
+    [JsonPropertyName("state")]  public ShipStatePayload State { get; set; } = new();
+    [JsonPropertyName("reward")] public float Reward { get; set; }
+    [JsonPropertyName("done")]   public bool Done { get; set; }
+    [JsonPropertyName("step")]   public int Step { get; set; }
+}
+
+public sealed class ShipStatePayload
+{
+    [JsonPropertyName("x")]      public float X { get; set; }
+    [JsonPropertyName("y")]      public float Y { get; set; }
+    [JsonPropertyName("angle")]  public float Angle { get; set; }
+    [JsonPropertyName("engine")] public int Engine { get; set; }
+}
+
+public sealed class InitEnvelope
+{
+    [JsonPropertyName("init")] public InitPayload Init { get; set; } = new();
+}
+
+public sealed class InitPayload
+{
+    [JsonPropertyName("world")]     public float[] World { get; set; } = new[] { 1f, 1f };
+    [JsonPropertyName("dt")]        public float Dt { get; set; }
+    [JsonPropertyName("obs_dim")]   public int ObsDim { get; set; }
+    [JsonPropertyName("n_actions")] public int NActions { get; set; }
+}
+
+[JsonSourceGenerationOptions(WriteIndented = false)]
+[JsonSerializable(typeof(StepIn))]
+[JsonSerializable(typeof(StepOut))]
+[JsonSerializable(typeof(ShipStatePayload))]
+[JsonSerializable(typeof(InitEnvelope))]
+[JsonSerializable(typeof(InitPayload))]
+internal partial class ProcessJsonContext : JsonSerializerContext
+{
+}