feat: add IChunkModificationStore and in-memory implementation

VoxelEngine.Tests/Persistence/InMemoryChunkModificationStoreTests.cs

@@ -0,0 +1,65 @@
+using OpenTK.Mathematics;
+using VoxelEngine.World.Persistence;
+using Xunit;
+
+namespace VoxelEngine.Tests.Persistence;
+
+public class InMemoryChunkModificationStoreTests
+{
+    [Fact]
+    public void NewStore_HasNoDiffs()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Assert.False(s.HasDiff(new Vector3i(0, 0, 0)));
+        Assert.False(s.TryGetDiff(new Vector3i(0, 0, 0), out _));
+    }
+
+    [Fact]
+    public void RecordEdit_StoresAndIsRetrievable()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Vector3i pos = new(1, 2, 3);
+        s.RecordEdit(pos, 42, 7);
+
+        Assert.True(s.HasDiff(pos));
+        Assert.True(s.TryGetDiff(pos, out var diff));
+        Assert.Single(diff!);
+        Assert.Equal((ushort)7, diff![42]);
+    }
+
+    [Fact]
+    public void RecordEdit_OverwritesSameLocalIndex()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Vector3i pos = new(0, 0, 0);
+        s.RecordEdit(pos, 10, 1);
+        s.RecordEdit(pos, 10, 2);
+
+        Assert.True(s.TryGetDiff(pos, out var diff));
+        Assert.Single(diff!);
+        Assert.Equal((ushort)2, diff![10]);
+    }
+
+    [Fact]
+    public void RecordEdit_MultipleChunks_AreIndependent()
+    {
+        var s = new InMemoryChunkModificationStore();
+        s.RecordEdit(new Vector3i(0, 0, 0), 0, 1);
+        s.RecordEdit(new Vector3i(1, 0, 0), 0, 2);
+
+        Assert.True(s.TryGetDiff(new Vector3i(0, 0, 0), out var a));
+        Assert.True(s.TryGetDiff(new Vector3i(1, 0, 0), out var b));
+        Assert.Equal((ushort)1, a![0]);
+        Assert.Equal((ushort)2, b![0]);
+    }
+
+    [Fact]
+    public void ClearDiff_RemovesChunk()
+    {
+        var s = new InMemoryChunkModificationStore();
+        Vector3i pos = new(5, 5, 5);
+        s.RecordEdit(pos, 0, 1);
+        s.ClearDiff(pos);
+        Assert.False(s.HasDiff(pos));
+    }
+}