From f24ee21de01633fe2ea3c81256fd204483989674 Mon Sep 17 00:00:00 2001
From: Evan Hemsley <evan@moonside.games>
Date: Tue, 17 Mar 2020 17:40:11 -0700
Subject: [PATCH] pruning empty entities

---
 encompass-cs/ComponentManager.cs |  6 ++++++
 encompass-cs/Engine.cs           |  8 ++++++++
 encompass-cs/EntityManager.cs    | 12 ++++++++++++
 encompass-cs/World.cs            |  1 +
 test/EngineTest.cs               | 31 +++++++++++++++++++++++++++++++
 5 files changed, 58 insertions(+)

diff --git a/encompass-cs/ComponentManager.cs b/encompass-cs/ComponentManager.cs
index 8c45186..046704e 100644
--- a/encompass-cs/ComponentManager.cs
+++ b/encompass-cs/ComponentManager.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using MoonTools.FastCollections;
 
 namespace Encompass
 {
@@ -262,5 +263,10 @@ namespace Encompass
                 drawLayerManager.UnRegisterComponentWithLayer<TComponent>(entityID);
             }
         }
+
+        public bool UpToDateEntityIsEmpty(int entityID)
+        {
+            return upToDateComponentStore.EntityBitArray(entityID).AllFalse();
+        }
     }
 }
diff --git a/encompass-cs/Engine.cs b/encompass-cs/Engine.cs
index 8be53dd..a7ba186 100644
--- a/encompass-cs/Engine.cs
+++ b/encompass-cs/Engine.cs
@@ -202,6 +202,14 @@ namespace Encompass
             return entityManager.EntityExists(entity.ID);
         }
 
+        /// <summary>
+        /// Returns true if an Entity with the specified ID exists.
+        /// </summary>
+        protected bool EntityExists(int entityID)
+        {
+            return entityManager.EntityExists(entityID);
+        }
+
         /// <summary>
         /// Returns an Entity containing the specified Component type.
         /// </summary>
diff --git a/encompass-cs/EntityManager.cs b/encompass-cs/EntityManager.cs
index 483b36b..c800281 100644
--- a/encompass-cs/EntityManager.cs
+++ b/encompass-cs/EntityManager.cs
@@ -76,5 +76,17 @@ namespace Encompass
 
             entitiesMarkedForDestroy.Clear();
         }
+
+        // NOTE: this is very suboptimal
+        public void PruneEmptyEntities()
+        {
+            foreach (var id in EntityIDs)
+            {
+                if (componentManager.UpToDateEntityIsEmpty(id))
+                {
+                    MarkForDestroy(id);
+                }
+            }
+        }
     }
 }
diff --git a/encompass-cs/World.cs b/encompass-cs/World.cs
index fcc3384..b611d96 100644
--- a/encompass-cs/World.cs
+++ b/encompass-cs/World.cs
@@ -59,6 +59,7 @@ namespace Encompass
             }
 
             messageManager.ClearMessages();
+            entityManager.PruneEmptyEntities();
             entityManager.DestroyMarkedEntities(enginesInOrder);
 
             componentManager.RemoveMarkedComponents();
diff --git a/test/EngineTest.cs b/test/EngineTest.cs
index 29da00c..77ceeb1 100644
--- a/test/EngineTest.cs
+++ b/test/EngineTest.cs
@@ -1387,6 +1387,37 @@ namespace Tests
             getComponentResult.Should().Be(new MockComponentB(5));
         }
 
+        static bool entityExistsResult;
+
+        class EntityExistsEngine : Engine
+        {
+            private int _id;
+
+            public EntityExistsEngine(int id)
+            {
+                _id = id;
+            }
+
+            public override void Update(double dt)
+            {
+                entityExistsResult = EntityExists(_id);
+            }
+        }
+
+        [Test]
+        public void PruneEmptyEntities()
+        {
+            var worldBuilder = new WorldBuilder();
+            var entity = worldBuilder.CreateEntity();
+            var id = entity.ID;
+
+            var world = worldBuilder.Build();
+
+            world.Update(0.01);
+
+            entityExistsResult.Should().BeFalse();
+        }
+
         public class QueryTests
         {
             struct MockComponentB : IComponent { }