From 61845d752a00c1fdd10b1b1cc34d9f4d266ef475 Mon Sep 17 00:00:00 2001
From: Evan Hemsley <evan@moonside.games>
Date: Sun, 22 Mar 2020 17:03:18 -0700
Subject: [PATCH] contiguous component storage

---
 .../Collections/TypedComponentStore.cs        | 61 +++++++++++++------
 1 file changed, 41 insertions(+), 20 deletions(-)

diff --git a/encompass-cs/Collections/TypedComponentStore.cs b/encompass-cs/Collections/TypedComponentStore.cs
index 536b536..9cb2788 100644
--- a/encompass-cs/Collections/TypedComponentStore.cs
+++ b/encompass-cs/Collections/TypedComponentStore.cs
@@ -15,17 +15,18 @@ namespace Encompass
 
     internal class TypedComponentStore<TComponent> : TypedComponentStore where TComponent : struct
     {
-        private readonly Dictionary<int, int> _indices = new Dictionary<int, int>(512);
-        private readonly Dictionary<int, int> _priorities = new Dictionary<int, int>(512);
-        private readonly TComponent[] _components = new TComponent[512]; // TODO: allow specify size
-        private readonly IDManager _idManager = new IDManager();
+        private int _nextID = 0;
+        private readonly Dictionary<int, int> _entityIDToStorageIndex = new Dictionary<int, int>(128);
+        private readonly Dictionary<int, int> _storageIndexToEntityID = new Dictionary<int, int>(128);
+        private readonly Dictionary<int, int> _priorities = new Dictionary<int, int>(128);
+        private TComponent[] _components = new TComponent[128];
 
-        public override int Count { get => _indices.Count; }
+        public override int Count { get => _entityIDToStorageIndex.Count; }
 
         public ref TComponent Get(int entityID)
         {
-            if (!_indices.ContainsKey(entityID)) { throw new Exceptions.NoComponentOfTypeOnEntityException("No component of type {0} exists on Entity with ID {1}", typeof(TComponent), entityID); }
-            return ref _components[_indices[entityID]];
+            if (!_entityIDToStorageIndex.ContainsKey(entityID)) { throw new Exceptions.NoComponentOfTypeOnEntityException("No component of type {0} exists on Entity with ID {1}", typeof(TComponent), entityID); }
+            return ref _components[_entityIDToStorageIndex[entityID]];
         }
 
         public void Set(int entityID, in TComponent component)
@@ -47,12 +48,18 @@ namespace Encompass
 
         private void InternalSet(int entityID, in TComponent component)
         {
-            if (!_indices.ContainsKey(entityID))
+            if (!_entityIDToStorageIndex.ContainsKey(entityID))
             {
-                _indices[entityID] = _idManager.NextID();
+                var index = _nextID++;
+                if (index >= _components.Length)
+                {
+                    System.Array.Resize(ref _components, _components.Length * 2);
+                }
+                _entityIDToStorageIndex[entityID] = index;
+                _storageIndexToEntityID[index] = entityID;
             }
 
-            _components[_indices[entityID]] = component;
+            _components[_entityIDToStorageIndex[entityID]] = component;
         }
 
         public override bool Remove(int entityID, int priority)
@@ -69,26 +76,40 @@ namespace Encompass
 
         public override void ForceRemove(int entityID)
         {
-            if (_indices.ContainsKey(entityID))
+            if (_entityIDToStorageIndex.ContainsKey(entityID))
             {
-                _idManager.Free(_indices[entityID]);
-                _indices.Remove(entityID);
+                var storageIndex = _entityIDToStorageIndex[entityID];
+                _entityIDToStorageIndex.Remove(entityID);
+                _storageIndexToEntityID.Remove(storageIndex);
                 _priorities.Remove(entityID);
+
+                // move a component into the hole to maintain contiguous memory
+                if (_nextID > 1 && storageIndex != _nextID - 1)
+                {
+                    var lastStorageIndex = _nextID - 1;
+                    var lastEntityID = _storageIndexToEntityID[lastStorageIndex];
+                    _storageIndexToEntityID.Remove(lastStorageIndex);
+
+                    _entityIDToStorageIndex[lastEntityID] = storageIndex;
+                    _storageIndexToEntityID[storageIndex] = lastEntityID;
+                    _components[storageIndex] = _components[lastStorageIndex];
+
+                }
+
+                _nextID--;
             }
         }
 
         public override bool Has(int entityID)
         {
-            return _indices.ContainsKey(entityID);
+            return _entityIDToStorageIndex.ContainsKey(entityID);
         }
 
         public override void Clear()
         {
-            foreach (var mappedID in _indices.Values)
-            {
-                _idManager.Free(mappedID);
-            }
-            _indices.Clear();
+            _nextID = 0;
+            _entityIDToStorageIndex.Clear();
+            _storageIndexToEntityID.Clear();
             _priorities.Clear();
         }
 
@@ -99,7 +120,7 @@ namespace Encompass
 
         public IEnumerable<(TComponent, int)> All()
         {
-            foreach (var kvp in _indices)
+            foreach (var kvp in _entityIDToStorageIndex)
             {
                 yield return (_components[kvp.Value], kvp.Key);
             }