test: add relationship and reordering tests

OECS.Tests/ReactivityTests.cs

@@ -9,6 +9,8 @@ public class ReactivityTests
     private struct Position { public float X; public float Y; }
     private struct Health { public int Value; }
     private struct Frozen { }
+    private struct ChildOf { }
+    private struct ParentOf { }
 
     /// <summary>
     /// Helper that collects changes into a list and exposes an R3 Observer.
@@ -264,6 +266,31 @@ public class ReactivityTests
         collector.Changes.Should().Contain(c => c.Kind == ChangeKind.EntityAdded);
     }
 
+    [Fact]
+    public void ReorderSources_PostsRelationshipReordered()
+    {
+        var world = new World();
+        var parent = world.CreateEntity();
+        var a = world.CreateEntity();
+        var b = world.CreateEntity();
+
+        world.AddComponent(a, new Relationship<ChildOf, ParentOf> { Target = parent });
+        world.AddComponent(b, new Relationship<ChildOf, ParentOf> { Target = parent });
+        world.PostChanges(); // Clear pending
+
+        var collector = new ChangeCollector();
+        using var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());
+
+        world.ReorderSources<Relationship<ChildOf, ParentOf>>(parent, [b, a]);
+        world.PostChanges();
+
+        collector.Changes.Should().ContainSingle()
+            .Which.Should().Match<EntityChange>(c =>
+                c.Entity == parent &&
+                c.Kind == ChangeKind.RelationshipReordered &&
+                c.ComponentType == typeof(Relationship<ChildOf, ParentOf>));
+    }
+
     private class EntityCreatorSystem : ISystem
     {
         private readonly World _world;

OECS.Tests/RelationshipTests.cs

@@ -197,6 +197,80 @@ public class RelationshipTests
         results.Should().BeEquivalentTo([child]);
     }
 
+    [Fact]
+    public void GetSources_PreservesInsertionOrder()
+    {
+        var world = new World();
+        var parent = world.CreateEntity();
+
+        // Add sources in a specific order.
+        var sources = new Entity[5];
+        for (int i = 0; i < 5; i++)
+        {
+            sources[i] = world.CreateEntity();
+            world.AddComponent(sources[i], new Relationship<ChildOf, ParentOf>
+            {
+                Target = parent
+            });
+        }
+
+        var result = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
+        result.Should().Equal(sources);
+    }
+
+    [Fact]
+    public void GetSources_OrderIsStableAfterMiddleRemove()
+    {
+        var world = new World();
+        var parent = world.CreateEntity();
+
+        var a = world.CreateEntity();
+        var b = world.CreateEntity();
+        var c = world.CreateEntity();
+
+        world.AddComponent(a, new Relationship<ChildOf, ParentOf> { Target = parent });
+        world.AddComponent(b, new Relationship<ChildOf, ParentOf> { Target = parent });
+        world.AddComponent(c, new Relationship<ChildOf, ParentOf> { Target = parent });
+
+        // Remove the middle source.
+        world.RemoveComponent<Relationship<ChildOf, ParentOf>>(b);
+
+        // Remaining sources should preserve insertion order.
+        var result = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
+        result.Should().Equal([a, c]);
+    }
+
+    [Fact]
+    public void ReorderSources_ChangesIterationOrder()
+    {
+        var world = new World();
+        var parent = world.CreateEntity();
+
+        var a = world.CreateEntity();
+        var b = world.CreateEntity();
+        var c = world.CreateEntity();
+
+        world.AddComponent(a, new Relationship<ChildOf, ParentOf> { Target = parent });
+        world.AddComponent(b, new Relationship<ChildOf, ParentOf> { Target = parent });
+        world.AddComponent(c, new Relationship<ChildOf, ParentOf> { Target = parent });
+
+        // Reverse the order.
+        world.ReorderSources<Relationship<ChildOf, ParentOf>>(parent, [c, b, a]);
+
+        var result = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
+        result.Should().Equal([c, b, a]);
+    }
+
+    [Fact]
+    public void ReorderSources_NoopsWhenTargetHasNoRelationships()
+    {
+        var world = new World();
+        var entity = world.CreateEntity();
+
+        // Should not throw.
+        world.ReorderSources<Relationship<ChildOf, ParentOf>>(entity, []);
+    }
+
     [Fact]
     public void DestroyEntity_WithBothIncomingAndOutgoingRelationships()
     {