Implement deterministic simulation spine

src/SideScrollerGame.Sim/Replay/ReplayPlayer.cs

@@ -0,0 +1,30 @@
+using System.Collections.Immutable;
+using SideScrollerGame.Sim.Definitions;
+using SideScrollerGame.Sim.Serialization;
+
+namespace SideScrollerGame.Sim.Replay;
+
+public static class ReplayPlayer
+{
+    public static ImmutableArray<int> Play(ReplayRecord replay, GameDefinition gameDefinition, SimulationConfig config)
+    {
+        if (replay.ContentHash != GameDefinitionHasher.Compute(gameDefinition))
+            throw new InvalidOperationException("Replay content hash does not match the supplied game definition.");
+
+        if (replay.TicksPerSecond != config.TicksPerSecond)
+            throw new InvalidOperationException("Replay tick rate does not match the supplied simulation config.");
+
+        var hashes = ImmutableArray.CreateBuilder<int>(replay.Ticks.Length);
+        Simulation simulation = new(gameDefinition, config, replay.Seed);
+        foreach (var recordedTick in replay.Ticks)
+        {
+            var result = simulation.Step(recordedTick.ActionBatch);
+            if (result.StateHash != recordedTick.ExpectedStateHash)
+                throw new InvalidOperationException($"Replay diverged at tick {recordedTick.ActionBatch.Tick}.");
+
+            hashes.Add(result.StateHash);
+        }
+
+        return hashes.MoveToImmutable();
+    }
+}