Add entity grouping.

core/src/com/me/common/ecs/Engine.java

@@ -1,6 +1,7 @@
 package com.me.common.ecs;
 
 import com.badlogic.gdx.utils.Array;
+import com.badlogic.gdx.utils.ObjectSet;
 import com.me.common.ecs.event.Event;
 import com.me.common.ecs.event.Listener;
 
@@ -89,6 +90,23 @@ public class Engine {
         listenerRegistry.callEvent(event);
     }
 
+    /**
+     * Return the list of entities that are a part of the given group.
+     * @param group the group to look up
+     * @return a list of entities
+     */
+    public Array<Entity> getEntitiesInGroup(String group) {
+        return entityManager.getGroupEntities(group);
+    }
+
+    void addEntityGroup(Entity entity, String group) {
+        entityManager.addEntityToGroup(entity, group);
+    }
+
+    void removeEntityGroup(Entity entity, String group) {
+        entityManager.removeEntityFromGroup(entity, group);
+    }
+
     void refreshEntity(Entity entity) {
         entityManager.queueRefresh(entity);
     }

core/src/com/me/common/ecs/Entity.java

@@ -1,5 +1,7 @@
 package com.me.common.ecs;
 
+import com.badlogic.gdx.utils.ObjectSet;
+
 public final class Entity {
 
     private static int nextId = 0;
@@ -15,11 +17,14 @@ public final class Entity {
     boolean active;
     boolean removed;
 
+    ObjectSet<String> groups;
+
     private long uniqueId;
 
     protected Entity(Engine engine) {
         this.engine = engine;
         this.id = nextId++;
+        this.groups = new ObjectSet<>();
     }
 
     public int getId() {
@@ -89,6 +94,22 @@ public final class Entity {
         return engine.getEntityComponent(this, clazz);
     }
 
+    public ObjectSet<String> getGroups() {
+        return groups;
+    }
+
+    public void addGroup(String group) {
+        engine.addEntityGroup(this, group);
+    }
+
+    public void removeGroup(String group) {
+        engine.removeEntityGroup(this, group);
+    }
+
+    public boolean inGroup(String group) {
+        return groups.contains(group);
+    }
+
     // Internal getComponent method accessed only by ComponentMapper
     Component getComponent(ComponentType type) {
         return engine.getEntityComponent(this, type);

core/src/com/me/common/ecs/EntityManager.java

@@ -1,6 +1,7 @@
 package com.me.common.ecs;
 
 import com.badlogic.gdx.utils.Array;
+import com.badlogic.gdx.utils.ObjectMap;
 
 final class EntityManager {
 
@@ -10,12 +11,16 @@ final class EntityManager {
     private final Array<Entity> removedEntities;
     private final Array<Entity> toRefresh;
 
+    private final ObjectMap<String, Array<Entity>> groupEntities;
+
     EntityManager(Engine engine) {
         this.engine = engine;
 
         this.entities = new Array<>(false, 16);
         this.removedEntities = new Array<>(false, 16);
         this.toRefresh = new Array<>(false, 16);
+
+        this.groupEntities = new ObjectMap<>();
     }
 
     Array<Entity> getEntities() {
@@ -47,6 +52,27 @@ final class EntityManager {
         toRefresh.clear();
     }
 
+    Array<Entity> getGroupEntities(String group) {
+        Array<Entity> entities = groupEntities.get(group);
+        if (entities == null) {
+            entities = new Array<>();
+            groupEntities.put(group, entities);
+        }
+        return entities;
+    }
+
+    void addEntityToGroup(Entity entity, String group) {
+        if (!entity.groups.contains(group)) {
+            getGroupEntities(group).add(entity);
+            entity.groups.add(group);
+        }
+    }
+
+    void removeEntityFromGroup(Entity entity, String group) {
+        getGroupEntities(group).removeValue(entity, true);
+        entity.groups.remove(group);
+    }
+
     void queueRefresh(Entity entity) {
         toRefresh.add(entity);
     }
@@ -64,7 +90,14 @@ final class EntityManager {
             engine.removeAllEntityComponents(entity.id);
             entities.removeValue(entity, true);
             removedEntities.add(entity);
+
+            // remove the entity from any groups it is in
+            for (String group : entity.groups) {
+                getGroupEntities(group).removeValue(entity, true);
+            }
+            entity.groups.clear();
         }
     }
 
+
 }