Implemented component data per Austin Morlan's example

javaecs/src/main/java/nz/ac/massey/programming_project_159333_s1_2021/ECS.java

@@ -32,7 +32,71 @@ public class ECS {
     // Unfortunately, given Java's constraints on dynamic typing (run-time type determination), the returned data must be
     // explicitly cast. Therefore, all components must implement the IComponent interface - which mandates the getType()
     // function
-    List<List<Object>> componentDataArrays = new ArrayList<>();
+
+    // As discussed by Austin Morlan, this array must be packed, otherwise we waste time iterating over non-subscribed entities
+    // This also means we cannot use the location implicitly - we must keep track of exactly which data instance associates with
+    // which entity.
+    // Therefore, we need a custom data structure that keeps reversable references to component instances and entities:
+    interface IComponentArray{
+        void entityDestroyed(int entity);
+    }
+    class ComponentArray<E> implements IComponentArray{
+        List<E> componentArray = new ArrayList<>();
+        Map<Integer, Integer> entityComponentDataMap = new HashMap<>();
+        Map<Integer, Integer> componentDataEntityMap = new HashMap<>();
+
+        public void entityDestroyed(int entity){
+            Optional<Integer> pos = entityComponentDataMap.get(entity);
+            if (pos.isEmpty()){
+                removeData(entity);
+            }
+        }
+
+        void removeData(int entity){
+            if (!entityComponentDataMap.containsKey(entity)){
+                System.err.println("Attempted to remove non-existent entity");
+                return;
+            }
+            // Get the componentData index of the entity
+            int removedComponentDataIndex = entityComponentDataMap.get(entity);
+            // Replace the removed component with the last component in the array
+            componentArray.set(removedComponentDataIndex, componentArray.get(componentArray.size() -1));
+            // update the data positions in the map
+            int lastEntity = componentDataEntityMap.get(componentArray.size()-1);
+            entityComponentDataMap.replace(lastEntity, removedComponentDataIndex);
+            componentDataEntityMap.replace(removedComponentDataIndex, lastEntity);
+            // Finally, remomve the last elements
+            entityComponentDataMap.remove(entity);
+            componentDataEntityMap.remove(componentArray.size() -1);
+
+            componentArray.remove(componentArray.size() -1);
+        }
+
+        void insertData(int entity, E component){
+            Optional<Integer> pos = entityComponentDataMap.get(entity);
+            if (!pos.isEmpty()){
+                System.err.println("Entity is already subscribed to the component");
+                return;
+            }
+            int index = componentArray.size();
+            entityComponentDataMap.put(entity, index);
+            componentDataEntityMap.put(index, entity);
+
+            componentArray.add(component);
+        }
+
+        E getData(int entity){
+            if (!entityComponentDataMap.containsKey(entity)){
+                System.err.println("Attempted to retrieve non-existent data");
+                return null;
+            }
+
+            return componentArray.get(entityComponentDataMap.get(entity));
+        }
+    }
+    // The actual list of data arrays
+    List<IComponentArray> componentDataArrays = new ArrayList<>();
+    
 
     // Components are either primitive types or class/struct definitions, with an additional list associating
     // the entities the component is assigned to