feat(gamecli): implement pure Reward.Compute with unit tests

GameCli.Tests/RewardTests.cs

@@ -0,0 +1,56 @@
+using GameCli;
+using Xunit;
+
+namespace GameCli.Tests;
+
+public class RewardTests
+{
+    [Fact]
+    public void AtTarget_ZeroVel_ZeroAngle_Noop_HasZeroReward()
+    {
+        var ship = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        float r = Reward.Compute(ship, target: (0.5f, 0.5f), action: 0);
+        Assert.Equal(0f, r, precision: 5);
+    }
+
+    [Fact]
+    public void FarFromTarget_HasLargerNegativeThanNear()
+    {
+        var near = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var far  = new ShipState(0.9f, 0.9f, 0f, 0f, 0f, 0f);
+        float rNear = Reward.Compute(near, target: (0.5f, 0.5f), action: 0);
+        float rFar  = Reward.Compute(far,  target: (0.5f, 0.5f), action: 0);
+        Assert.True(rFar < rNear, $"far reward {rFar} should be < near reward {rNear}");
+    }
+
+    [Fact]
+    public void HighVelocity_PenalizesReward()
+    {
+        var stopped = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var moving  = new ShipState(0.5f, 0.5f, 1f, 1f, 0f, 0f);
+        float rStop = Reward.Compute(stopped, target: (0.5f, 0.5f), action: 0);
+        float rMove = Reward.Compute(moving,  target: (0.5f, 0.5f), action: 0);
+        Assert.True(rMove < rStop, $"moving reward {rMove} should be < stopped reward {rStop}");
+    }
+
+    [Fact]
+    public void Tilted_PenalizesReward()
+    {
+        var upright = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        var tilted  = new ShipState(0.5f, 0.5f, 0f, 0f, 0.8f, 0f);
+        float rUp = Reward.Compute(upright, target: (0.5f, 0.5f), action: 0);
+        float rTi = Reward.Compute(tilted,  target: (0.5f, 0.5f), action: 0);
+        Assert.True(rTi < rUp, $"tilted reward {rTi} should be < upright reward {rUp}");
+    }
+
+    [Fact]
+    public void FiringEngine_PenalizesReward()
+    {
+        var ship = new ShipState(0.5f, 0.5f, 0f, 0f, 0f, 0f);
+        float rNoop  = Reward.Compute(ship, target: (0.5f, 0.5f), action: 0);
+        float rMain  = Reward.Compute(ship, target: (0.5f, 0.5f), action: 2);
+        float rLeft  = Reward.Compute(ship, target: (0.5f, 0.5f), action: 1);
+        Assert.True(rMain < rNoop, "firing main should be worse than noop");
+        Assert.True(rLeft < rNoop, "firing left should be worse than noop");
+    }
+}

GameCli/Reward.cs

@@ -0,0 +1,29 @@
+namespace GameCli;
+
+/// <summary>
+/// Reward = -distance_to_target
+///        - λ_v · speed
+///        - λ_θ · |angle|
+///        - λ_fuel · engine_on
+/// Coefficients are public so training scripts can log/inspect them.
+/// </summary>
+public static class Reward
+{
+    public const float LambdaVelocity = 0.10f;
+    public const float LambdaAngle    = 0.10f;
+    public const float LambdaFuel     = 0.03f;
+
+    public static float Compute(ShipState s, (float X, float Y) target, int action)
+    {
+        float dx = target.X - s.X;
+        float dy = target.Y - s.Y;
+        float distance = MathF.Sqrt(dx * dx + dy * dy);
+        float speed    = MathF.Sqrt(s.VX * s.VX + s.VY * s.VY);
+        float engineOn = action == Physics.ActionNoop ? 0f : 1f;
+
+        return -distance
+             - LambdaVelocity * speed
+             - LambdaAngle    * MathF.Abs(s.Angle)
+             - LambdaFuel     * engineOn;
+    }
+}