Finishes implementation of EntitySystem

src/Aurora/Core/EC/Entity.cs

@@ -6,7 +6,10 @@ public struct Entity : IEntity
 {
 	public int Id { get; }
 
-	public bool Active { get; set; }
+	public bool Active {
+		get => EntitySystem.GetActive(Id);
+		set => EntitySystem.SetActive(Id, value);
+	}
 
 	internal Entity(int id) {
 		Id = id;

src/Aurora/Core/EC/EntitySystem.cs

@@ -1,33 +1,75 @@
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using Microsoft.Xna.Framework.Input;
 
 namespace Aurora.Core.EC;
 
 public sealed class EntitySystem : ModSystem
 {
-	private static readonly List<Entity> Entities = [];
+	private static readonly List<int> ActiveEntityIds = [];
+	private static readonly List<int> InactiveEntityIds = [];
+
+	private static readonly ConcurrentBag<int> FreeEntityIds = [];
+
+	private static int NextEntityId;
 
 	/// <summary>
 	///		Creates a new instance of an entity.
 	/// </summary>
 	/// <param name="activate">Whether to activate the entity instance or not.</param>
 	/// <returns>The created entity instance.</returns>
-	public static Entity Create(bool activate = true) {
-		var entity = new Entity();
+	public static Entity Create(bool activate) {
+		int id;
+
+		if (!FreeEntityIds.TryTake(out id)) {
+			id = NextEntityId++;
+		}
 
 		if (activate) {
-			Entities.Add(entity);
+			ActiveEntityIds.Add(id);
 		}
 
-		return entity;
+		return new Entity(id);
 	}
 
 	/// <summary>
-	///		Removes an instance of an entity.
+	///		Removes an instance of an entity from its unique identifier.
 	/// </summary>
-	/// <param name="entity">The entity to remove.</param>
+	/// <param name="entityId">The identity of the entity to remove.</param>
 	/// <returns><c>true</c> if the entity was successfully removed; otherwise, <c>false</c>.</returns>
-	public static bool Remove(Entity entity) {
-		return Entities.Remove(entity);
+	public static bool Remove(int id) {
+		if (id < 0) {
+			return false;
+		}
+
+		ActiveEntityIds.Remove(id);
+		InactiveEntityIds.Remove(id);
+
+		FreeEntityIds.Add(id);
+
+		return true;
+	}
+
+	internal static bool GetActive(int entityId) {
+		if (entityId < 0) {
+			return false;
+		}
+
+		return ActiveEntityIds.Contains(entityId);
+	}
+
+	internal static void SetActive(int entityId, bool value) {
+		if (entityId < 0) {
+			return;
+		}
+
+		if (value) {
+			ActiveEntityIds.Add(entityId);
+			InactiveEntityIds.Remove(entityId);
+			return;
+		}
+
+		ActiveEntityIds.Remove(entityId);
+		InactiveEntityIds.Add(entityId);
 	}
 }