Engine - ECS: add struct Entities & EntityEnumerator skeletons

Engine/src/ECS/Archetype/Entities.cs

@@ -0,0 +1,81 @@
+// Copyright (c) Ullrich Praetz - https://github.com/friflo. All rights reserved.
+// See LICENSE file in the project root for full license information.
+
+using System.Collections;
+using System.Collections.Generic;
+
+// ReSharper disable once CheckNamespace
+namespace Friflo.Engine.ECS;
+
+public readonly struct Entities : IReadOnlyList<Entity>
+{
+#region properties
+    public          int     Count => count;
+    #endregion
+
+#region interal fields
+    internal    readonly int[]          ids;    //  8
+    internal    readonly EntityStore    store;  //  8
+    internal    readonly int            start;  //  4
+    public      readonly int            count;  //  4
+    #endregion
+
+#region general
+    internal Entities(int[] ids, EntityStore store, int start, int count) {
+        this.ids    = ids;
+        this.store  = store;
+        this.start  = start;
+        this.count  = count;
+    }
+
+    public Entity this[int index] => throw new System.NotImplementedException();
+    #endregion
+
+
+#region IEnumerator
+    public EntityEnumerator                 GetEnumerator() => new EntityEnumerator (this);
+
+    // --- IEnumerable
+    IEnumerator                 IEnumerable.GetEnumerator() => new EntityEnumerator (this);
+
+    // --- IEnumerable<>
+    IEnumerator<Entity> IEnumerable<Entity>.GetEnumerator() => new EntityEnumerator (this);
+    #endregion
+}
+
+
+public struct EntityEnumerator : IEnumerator<Entity>
+{
+    private readonly    int[]       ids;        //  8
+    private readonly    EntityStore store;      //  8
+    private readonly    int         start;      //  4
+    private readonly    int         last;       //  8
+    private             int         index;      //  4
+
+    internal EntityEnumerator(in Entities entities) {
+        ids     = entities.ids;
+        store   = entities.store;
+        start   = entities.start;
+        last    = start + entities.count;
+        index   = start;
+    }
+
+    // --- IEnumerator
+    public          void         Reset()    => index = start;
+
+    readonly object  IEnumerator.Current    => new Entity(store, ids[index]);
+
+    public   Entity              Current    => new Entity(store, ids[index]);
+
+    // --- IEnumerator
+    public bool MoveNext()
+    {
+        if (index < last) {
+            index++;
+            return true;
+        }
+        return false;
+    }
+
+    public readonly void Dispose() { }
+}

Engine/src/ECS/Entity/Store/Entities.cs

@@ -246,9 +246,3 @@ internal void DeleteEntityEvent(Entity entity)
     }
 }
 
-/// <summary>
-/// Reserved symbol name.
-/// If exposing public it needs to store an array of <see cref="Entity"/>'s.<br/>
-/// Similar to <see cref="Archetypes"/>.
-/// </summary>
-internal struct Entities;