Implement deterministic simulation spine

tests/SideScrollerGame.Sim.Tests/SimulationSerializationTests.cs

@@ -0,0 +1,38 @@
+using SideScrollerGame.Sim.Input;
+
+namespace SideScrollerGame.Sim.Tests;
+
+public sealed class SimulationSerializationTests
+{
+    [Fact]
+    public void SaveStateLoadState_PreservesStateAndNextStepHash()
+    {
+        var definition = SimulationTestFactory.CreateGameDefinition();
+        var config = SimulationTestFactory.CreateConfig();
+
+        Simulation original = new(definition, config, 17);
+        original.Step(SimulationTestFactory.CreateTick(1, new MoveAxisChanged(new(1), 1, 1)));
+
+        var bytes = original.SaveState();
+        var loaded = Simulation.LoadState(bytes, definition, config);
+
+        Assert.Equal(original.CurrentTick, loaded.CurrentTick);
+        Assert.Equal(original.CurrentSnapshot.StateHash, loaded.CurrentSnapshot.StateHash);
+
+        var nextBatch = SimulationTestFactory.CreateTick(2, new ButtonChanged(new(1), InputButton.FirePrimary, true));
+        var originalHash = original.Step(nextBatch).StateHash;
+        var loadedHash = loaded.Step(nextBatch).StateHash;
+
+        Assert.Equal(originalHash, loadedHash);
+    }
+
+    [Fact]
+    public void Constructor_NormalizesZeroSeed()
+    {
+        Simulation simulation = new(SimulationTestFactory.CreateGameDefinition(), SimulationTestFactory.CreateConfig(), 0);
+
+        simulation.Step(TickActionBatch.Empty(1));
+
+        Assert.NotEqual(0UL, simulation.CurrentState.RandomState);
+    }
+}