Add EntityRef.GetTarget, .Parent and .GetChildren

src/gaemstone/ECS/EntityRef.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using gaemstone.Utility;
@@ -57,6 +58,17 @@ public unsafe sealed class EntityRef
 	private static EntityPath? EnsureRelativePath(EntityPath? path)
 		{ if (path?.IsAbsolute == true) throw new ArgumentException("path must not be absolute", nameof(path)); return path; }
 
+
+	public EntityRef? Parent => GetTarget(Universe.ChildOf);
+
+	public IEnumerable<EntityRef> GetChildren()
+	{
+		foreach (var iter in Iterator.FromTerm(Universe, new(Universe.ChildOf, this)))
+			for (var i = 0; i < iter.Count; i++)
+				yield return iter.Entity(i);
+	}
+
+
 	public override EntityRef Add(Identifier id) { ecs_add_id(Universe, this, id); return this; }
 	public override EntityRef Remove(Identifier id) { ecs_remove_id(Universe, this, id); return this; }
 	public override bool Has(Identifier id) => ecs_has_id(Universe, this, id);
@@ -133,6 +145,13 @@ public unsafe sealed class EntityRef
 		return this;
 	}
 
+	public EntityRef? GetTarget(Entity relation, int index = 0)
+		=> CreateOrNull(Universe, new(ecs_get_target(Universe, this, relation, index)));
+	public EntityRef? GetTarget(string symbol, int index = 0)
+		=> GetTarget(Universe.LookupSymbolOrThrow(symbol), index);
+	public EntityRef? GetTarget<T>(int index = 0)
+		=> GetTarget(Universe.LookupOrThrow(typeof(T)), index);
+
 	public bool Equals(EntityRef? other) => (other is not null) && Universe == other.Universe && Entity == other.Entity;
 	public override bool Equals(object? obj) => Equals(obj as EntityRef);
 	public override int GetHashCode() => HashCode.Combine(Universe, Entity);