Fixed Relations<> indexer. See #70 (comment)

Author: friflo

src/ECS/Base/Types/ComponentType.cs

@@ -154,8 +154,7 @@ internal override void WriteRelations(ComponentWriter writer, Entity entity)
             } else {
                 writer.writer.json.AppendChar(',');
             }
-            var index = relations.start + n;
-            var position = relations.GetPosition(index);
+            var position = relations.GetPosition(n);
             var bytes = heap!.Write(writer.componentWriter, position);
             writer.writer.json.AppendBytes(bytes);
         }

src/ECS/Relations/Relations.cs

@@ -78,7 +78,7 @@ private static MethodInfo MakeGetRelationKey(out bool isEntity)
     }
 
     internal int GetPosition(int index) {
-        return positions != null ? positions[index] : position;
+        return positions != null ? positions[start + index] : position;
     }
 
     /// <summary>
@@ -111,7 +111,7 @@ public ref TRelation this[int index] {
         get {
             if (version != entityRelations.version) throw RelationsModifiedException();
             if (index >= 0 && index < length) {
-                return ref components[positions != null ? positions[index] : position];
+                return ref components[positions != null ? positions[start + index] : position];
             }
             throw IndexOutOfRangeException(index);
         }

src/Tests/ECS/Relations/Test_Relations.cs

@@ -557,6 +557,45 @@ public static void Test_Relations_outdated_DeleteEntity_source()
         });
         AreEqual("Relations<AttackRelation> outdated. Added / Removed relations after calling GetRelations<AttackRelation>().", e1!.Message);
     }
+    
+    [Test]
+    // Regression test for issue in Relations<> indexer (missed to use start)
+    // Issue detected at:   [Relations<TRelation> [index] operator sometimes returns the wrong TRelation]
+    //                      https://github.com/friflo/Friflo.Engine.ECS/issues/70#issuecomment-2896790850
+    public static void Test_Relations_indexer()
+    {
+        var store = new EntityStore();
+
+        var entity1 = store.CreateEntity();
+        entity1.AddRelation(new IntRelation { value = 10 });
+        entity1.AddRelation(new IntRelation { value = 11 });
+        entity1.AddRelation(new IntRelation { value = 12 });
+        entity1.AddRelation(new IntRelation { value = 13 });
+        
+        var entity2 = store.CreateEntity();
+        entity2.AddRelation(new IntRelation { value = 20 });
+        entity2.AddRelation(new IntRelation { value = 21 });
+        entity2.AddRelation(new IntRelation { value = 22 });
+        entity2.AddRelation(new IntRelation { value = 23 });
+
+        var relations1 = entity1.GetRelations<IntRelation>();
+        var relations2 = entity2.GetRelations<IntRelation>();
+        
+        // Both relations must have same length for this test.
+        // So they are using the same Relations<TRelation>.positions with different Relations<TRelation>.start values
+        AreEqual(relations1.Length, relations2.Length); 
+        {
+            int index = 0;
+            foreach (var relation in relations1) {
+                AreEqual(relations1[index++].value, relation.value);
+            }
+        } {
+            int index = 0;
+            foreach (var relation in relations2) {
+                AreEqual(relations2[index++].value, relation.value);
+            }
+        }
+    }
 }
 
 }