Tests: add Test_GitHub_44 for discussions question

src/Tests/ECS/Github/Test_GitHub_44.cs

@@ -0,0 +1,70 @@
+/*
+using System.Numerics;
+using Friflo.Engine.ECS;
+using NUnit.Framework;
+
+
+// ReSharper disable InconsistentNaming
+namespace Tests.ECS.Github
+{
+    public struct LocalTransform : IComponent
+    {
+        public Matrix4x4 matrix;
+    }
+
+    // Small test to check performance of GitHub discussion: "Best practice for update hierarchy's transform?"
+    // https://github.com/friflo/Friflo.Engine.ECS/discussions/44
+    public static class Test_GitHub_44
+    {
+        [Test]
+        public static void TraverseHierarchy()
+        {
+            var store = new EntityStore();
+            // create a hierarchy of entities 
+            var root = store.CreateEntity(new Transform(), new LocalTransform());
+            for (int n0 = 0; n0 < 10; n0++) {
+                var entity0 = store.CreateEntity(new Transform(), new LocalTransform());
+                root.AddChild(entity0);
+                for (int n1 = 0; n1 < 10; n1++) {
+                    var entity1 = store.CreateEntity(new Transform(), new LocalTransform());
+                    entity0.AddChild(entity1);
+                     for (int n2 = 0; n2 < 10; n2++) {
+                        var entity2 = store.CreateEntity(new Transform(), new LocalTransform());
+                        entity1.AddChild(entity2);
+                        for (int n3 = 0; n3 < 5; n3++) {
+                            var entity3 = store.CreateEntity(new Transform(), new LocalTransform());
+                            entity2.AddChild(entity3);
+                        }
+                    }
+                }
+            }
+            Assert.AreEqual(6111, store.Entities.Count);
+            
+            // 10,000 iterations traversing 6111 entities: 1.2 sec
+            for (int n = 0; n < 10_000; n++) {
+                int calls = 0;
+                Traverse(root, Matrix4x4.Identity, ref calls);
+                Assert.AreEqual(6111, calls);
+            }
+        }
+        
+        private static void Traverse (Entity entity, Matrix4x4 parentLocalToWorld, ref int calls)
+        {
+            calls++;
+            if (entity.HasComponent<Transform>()) {
+                ref var transform = ref entity.GetComponent<Transform>();
+                if (entity.TryGetComponent<LocalTransform>(out var localTransform))
+                {
+                    transform.value = parentLocalToWorld * localTransform.matrix;
+                    parentLocalToWorld = transform.value;
+                } else {
+                    transform.value = parentLocalToWorld;
+                }    
+            }
+            foreach (var child in entity.ChildEntities) {
+                Traverse(child, parentLocalToWorld, ref calls);        
+            }
+        }
+    }
+}
+*/