Reuse removed entities - prevents massive entity component arrays.

Assign each entity a uniqueId as well as their regular ID. The uniqueID
of an entity is always difference from past or present active entities.

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

@@ -13,6 +13,7 @@ public class Engine {
     private Array<Entity> toActivate;
     private Array<Entity> toDeactivate;
     private Array<Entity> toRemove;
+    private Array<Entity> removedEntities;
 
     private ComponentBag[] components;
     private Array<BaseSystem> systems;
@@ -26,6 +27,7 @@ public class Engine {
         this.toActivate = new Array<>();
         this.toDeactivate = new Array<>();
         this.toRemove = new Array<>();
+        this.removedEntities = new Array<>(false, 16);
 
         this.systems = new Array<>();
         this.listenerRegistry = new ListenerRegistry();
@@ -72,13 +74,19 @@ public class Engine {
     }
 
     public Entity createEntity() {
-        Entity entity = new Entity(this);
+        Entity entity;
+        if (!removedEntities.isEmpty()) {
+            entity = removedEntities.removeIndex(0);
+            entity.reset();
+        } else {
+            entity = new Entity(this);
+        }
+        entity.updateUniqueId();
         entities.add(entity);
         return entity;
     }
 
     protected void removeEntity(Entity entity) {
-        entity.deactivate();
         toRemove.add(entity);
     }
 
@@ -117,6 +125,7 @@ public class Engine {
             removeAllEntityComponents(entity.getId());
             entities.removeValue(entity, true);
             removeEntityFromSystems(entity);
+            removedEntities.add(entity);
         }
 
         toRemove.clear();

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

@@ -3,10 +3,12 @@ package com.me.common.ecs;
 public final class Entity {
 
     private static int nextId = 0;
+    private static long nextUniqueId = 0;
 
     private Engine engine;
 
     private int id;
+    private long uniqueId;
     private boolean active;
 
     private long componentBits;
@@ -21,6 +23,10 @@ public final class Entity {
         return id;
     }
 
+    public long getUniqueId() {
+        return uniqueId;
+    }
+
     public boolean isActive() {
         return active;
     }
@@ -71,4 +77,13 @@ public final class Entity {
         return componentBits;
     }
 
+    protected void reset() {
+        componentBits = 0;
+        active = false;
+    }
+
+    protected void updateUniqueId() {
+        this.uniqueId = nextUniqueId++;
+    }
+
 }