garbage collection optimizations

src/ComponentStorage.cs

@@ -83,9 +83,8 @@ namespace MoonTools.ECS
 		// Returns true if the entity had this component.
 		public override bool Remove(int entityID)
 		{
-			if (entityIDToStorageIndex.ContainsKey(entityID))
+			if (entityIDToStorageIndex.TryGetValue(entityID, out int storageIndex))
 			{
-				var storageIndex = entityIDToStorageIndex[entityID];
 				entityIDToStorageIndex.Remove(entityID);
 
 				var lastElementIndex = nextID - 1;

src/DynamicArray.cs

@@ -0,0 +1,40 @@
+using System;
+
+namespace MoonTools.ECS
+{
+	public class DynamicArray<T> where T : unmanaged
+	{
+		private T[] Array;
+		public int Count { get; private set; }
+
+		public Span<T> ToSpan() => new Span<T>(Array, 0, Count);
+		public ReverseSpanEnumerator<T> GetEnumerator() => new ReverseSpanEnumerator<T>(new Span<T>(Array, 0, Count));
+
+		public DynamicArray(int capacity = 16)
+		{
+			Array = new T[capacity];
+			Count = 0;
+		}
+
+		public ref T this[int i]
+		{
+			get { return ref Array[i]; }
+		}
+
+		public void Add(T item)
+		{
+			if (Count >= Array.Length)
+			{
+				global::System.Array.Resize(ref Array, Array.Length * 2);
+			}
+
+			Array[Count] = item;
+			Count += 1;
+		}
+
+		public void Clear()
+		{
+			Count = 0;
+		}
+	}
+}

src/Entity.cs

@@ -35,5 +35,15 @@ namespace MoonTools.ECS
 		{
 			return !a.Equals(b);
 		}
+
+		public static implicit operator int(Entity e)
+		{
+			return e.ID;
+		}
+
+		public static implicit operator Entity(int i)
+		{
+			return new Entity(i);
+		}
 	}
 }

src/EntityComponentReader.cs

@@ -62,13 +62,18 @@ namespace MoonTools.ECS
 			return RelationDepot.Related<TRelationKind>(a.ID, b.ID);
 		}
 
-		// relations go A->B, so given A, will give all outgoing B relations.
+		protected TRelationKind GetRelationData<TRelationKind>(in Entity a, in Entity b) where TRelationKind : unmanaged
-		protected IEnumerable<(Entity, TRelationKind)> OutRelations<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
+		{
+			return RelationDepot.Get<TRelationKind>(new Relation(a.ID, b.ID));
+		}
+
+		// relations go A->B, so given A, will give all entities in outgoing relations of this kind.
+		protected ReverseSpanEnumerator<Entity> OutRelations<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
 		{
 			return RelationDepot.OutRelations<TRelationKind>(entity.ID);
 		}
 
-		protected (Entity, TRelationKind) OutRelationSingleton<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
+		protected Entity OutRelationSingleton<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
 		{
 			return RelationDepot.OutRelationSingleton<TRelationKind>(entity.ID);
 		}
@@ -83,13 +88,13 @@ namespace MoonTools.ECS
 			return RelationDepot.OutRelationCount<TRelationKind>(entity.ID);
 		}
 
-		// Relations go A->B, so given B, will give all incoming A relations.
+		// Relations go A->B, so given B, will give all entities in incoming A relations of this kind.
-		protected IEnumerable<(Entity, TRelationKind)> InRelations<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
+		protected ReverseSpanEnumerator<Entity> InRelations<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
 		{
 			return RelationDepot.InRelations<TRelationKind>(entity.ID);
 		}
 
-		protected (Entity, TRelationKind) InRelationSingleton<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
+		protected Entity InRelationSingleton<TRelationKind>(in Entity entity) where TRelationKind : unmanaged
 		{
 			return RelationDepot.InRelationSingleton<TRelationKind>(entity.ID);
 		}

src/EntityStorage.cs

@@ -62,13 +62,12 @@ namespace MoonTools.ECS
 			EntityToRelationTypeIndices[entityId].Remove(relationIndex);
 		}
 
-		// TODO: should these ints be ID types?
+		public HashSet<int> ComponentTypeIndices(int entityID)
-		public IEnumerable<int> ComponentTypeIndices(int entityID)
 		{
 			return EntityToComponentTypeIndices[entityID];
 		}
 
-		public IEnumerable<int> RelationTypeIndices(int entityID)
+		public HashSet<int> RelationTypeIndices(int entityID)
 		{
 			return EntityToRelationTypeIndices[entityID];
 		}

src/Enumerators/ReverseSpanEnumerator.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Runtime.CompilerServices;
+
+namespace MoonTools.ECS
+{
+	public ref struct ReverseSpanEnumerator<T>
+	{
+		private ReadOnlySpan<T> Span;
+		private int index;
+
+		public ReverseSpanEnumerator<T> GetEnumerator() => this;
+
+		public T Current => Span[index];
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public bool MoveNext()
+		{
+			if (index > 0)
+			{
+				index -= 1;
+				return true;
+			}
+
+			return false;
+		}
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public ReverseSpanEnumerator(Span<T> span)
+		{
+			Span = span;
+			index = span.Length;
+		}
+
+		public static ReverseSpanEnumerator<T> Empty => new ReverseSpanEnumerator<T>();
+	}
+}

src/Filter.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 
 namespace MoonTools.ECS
 {
@@ -13,8 +14,8 @@ namespace MoonTools.ECS
 			Signature = new FilterSignature(included, excluded);
 		}
 
-		public IEnumerable<Entity> Entities => FilterStorage.FilterEntities(Signature);
+		public ReverseSpanEnumerator<Entity> Entities => FilterStorage.FilterEntities(Signature);
-		public IEnumerable<Entity> EntitiesInRandomOrder => FilterStorage.FilterEntitiesRandom(Signature);
+		public LinearCongruentialEnumerator EntitiesInRandomOrder => FilterStorage.FilterEntitiesRandom(Signature);
 		public Entity RandomEntity => FilterStorage.FilterRandomEntity(Signature);
 
 		public int Count => FilterStorage.FilterCount(Signature);

src/FilterStorage.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 
 namespace MoonTools.ECS
@@ -7,7 +7,7 @@ namespace MoonTools.ECS
 	{
 		private EntityStorage EntityStorage;
 		private TypeIndices ComponentTypeIndices;
-		private Dictionary<FilterSignature, IndexableSet<int>> filterSignatureToEntityIDs = new Dictionary<FilterSignature, IndexableSet<int>>();
+		private Dictionary<FilterSignature, IndexableSet<Entity>> filterSignatureToEntityIDs = new Dictionary<FilterSignature, IndexableSet<Entity>>();
 		private Dictionary<int, HashSet<FilterSignature>> typeToFilterSignatures = new Dictionary<int, HashSet<FilterSignature>>();
 
 		public FilterStorage(EntityStorage entityStorage, TypeIndices componentTypeIndices)
@@ -21,7 +21,7 @@ namespace MoonTools.ECS
 			var filterSignature = new FilterSignature(included, excluded);
 			if (!filterSignatureToEntityIDs.ContainsKey(filterSignature))
 			{
-				filterSignatureToEntityIDs.Add(filterSignature, new IndexableSet<int>());
+				filterSignatureToEntityIDs.Add(filterSignature, new IndexableSet<Entity>());
 
 				foreach (var type in included)
 				{
@@ -46,20 +46,14 @@ namespace MoonTools.ECS
 			return new Filter(this, included, excluded);
 		}
 
-		public IEnumerable<Entity> FilterEntities(FilterSignature filterSignature)
+		public ReverseSpanEnumerator<Entity> FilterEntities(FilterSignature filterSignature)
 		{
-			foreach (var id in filterSignatureToEntityIDs[filterSignature])
+			return filterSignatureToEntityIDs[filterSignature].GetEnumerator();
-			{
-				yield return new Entity(id);
-			}
 		}
 
-		public IEnumerable<Entity> FilterEntitiesRandom(FilterSignature filterSignature)
+		public LinearCongruentialEnumerator FilterEntitiesRandom(FilterSignature filterSignature)
 		{
-			foreach (var index in RandomGenerator.LinearCongruentialGenerator(FilterCount(filterSignature)))
+			return RandomGenerator.LinearCongruentialGenerator(FilterCount(filterSignature));
-			{
-				yield return new Entity(filterSignatureToEntityIDs[filterSignature][index]);
-			}
 		}
 
 		public Entity FilterNthEntity(FilterSignature filterSignature, int index)
@@ -80,9 +74,9 @@ namespace MoonTools.ECS
 
 		public void Check(int entityID, int componentTypeIndex)
 		{
-			if (typeToFilterSignatures.ContainsKey(componentTypeIndex))
+			if (typeToFilterSignatures.TryGetValue(componentTypeIndex, out var filterSignatures))
 			{
-				foreach (var filterSignature in typeToFilterSignatures[componentTypeIndex])
+				foreach (var filterSignature in filterSignatures)
 				{
 					CheckFilter(entityID, filterSignature);
 				}
@@ -140,9 +134,9 @@ namespace MoonTools.ECS
 
 		public void RemoveEntity(int entityID, int componentTypeIndex)
 		{
-			if (typeToFilterSignatures.ContainsKey(componentTypeIndex))
+			if (typeToFilterSignatures.TryGetValue(componentTypeIndex, out var filterSignatures))
 			{
-				foreach (var filterSignature in typeToFilterSignatures[componentTypeIndex])
+				foreach (var filterSignature in filterSignatures)
 				{
 					filterSignatureToEntityIDs[filterSignature].Remove(entityID);
 				}

src/IndexableSet.cs

@@ -9,7 +9,7 @@ namespace MoonTools.ECS
 		private Dictionary<T, int> indices;
 		private T[] array;
 		public int Count { get; private set; }
-		public Enumerator GetEnumerator() => new Enumerator(this);
+		public ReverseSpanEnumerator<T> GetEnumerator() => new ReverseSpanEnumerator<T>(new Span<T>(array, 0, Count));
 
 		public IndexableSet(int size = 32)
 		{

src/MessageDepot.cs

@@ -42,7 +42,7 @@ namespace MoonTools.ECS
 			return Lookup<TMessage>().First();
 		}
 
-		public IEnumerable<TMessage> WithEntity<TMessage>(int entityID) where TMessage : unmanaged
+		public ReverseSpanEnumerator<TMessage> WithEntity<TMessage>(int entityID) where TMessage : unmanaged
 		{
 			return Lookup<TMessage>().WithEntity(entityID);
 		}

src/MessageStorage.cs

@@ -13,7 +13,8 @@ namespace MoonTools.ECS
 		private int count = 0;
 		private int capacity = 128;
 		private TMessage[] messages;
-		private Dictionary<int, List<int>> entityToIndices = new Dictionary<int, List<int>>();
+		// duplicating storage here for fast iteration
+		private Dictionary<int, DynamicArray<TMessage>> entityToMessages = new Dictionary<int, DynamicArray<TMessage>>();
 
 		public MessageStorage()
 		{
@@ -34,11 +35,11 @@ namespace MoonTools.ECS
 
 		public void Add(int entityID, in TMessage message)
 		{
-			if (!entityToIndices.ContainsKey(entityID))
+			if (!entityToMessages.ContainsKey(entityID))
 			{
-				entityToIndices.Add(entityID, new List<int>());
+				entityToMessages.Add(entityID, new DynamicArray<TMessage>());
 			}
-			entityToIndices[entityID].Add(count);
+			entityToMessages[entityID].Add(message);
 
 			Add(message);
 		}
@@ -58,31 +59,32 @@ namespace MoonTools.ECS
 			return messages[0];
 		}
 
-		public IEnumerable<TMessage> WithEntity(int entityID)
+		public ReverseSpanEnumerator<TMessage> WithEntity(int entityID)
 		{
-			if (entityToIndices.ContainsKey(entityID))
+			if (entityToMessages.TryGetValue(entityID, out var messages))
 			{
-				foreach (var index in entityToIndices[entityID])
+				return messages.GetEnumerator();
-				{
-					yield return messages[index];
 			}
+			else
+			{
+				return ReverseSpanEnumerator<TMessage>.Empty;
 			}
 		}
 
 		public ref readonly TMessage FirstWithEntity(int entityID)
 		{
-			return ref messages[entityToIndices[entityID][0]];
+			return ref entityToMessages[entityID][0];
 		}
 
 		public bool SomeWithEntity(int entityID)
 		{
-			return entityToIndices.ContainsKey(entityID) && entityToIndices[entityID].Count > 0;
+			return entityToMessages.ContainsKey(entityID) && entityToMessages[entityID].Count > 0;
 		}
 
 		public override void Clear()
 		{
 			count = 0;
-			foreach (var set in entityToIndices.Values)
+			foreach (var set in entityToMessages.Values)
 			{
 				set.Clear();
 			}

src/Random.cs

@@ -1,5 +1,5 @@
 using System;
-using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 
 namespace MoonTools.ECS
 {
@@ -20,7 +20,7 @@ namespace MoonTools.ECS
 		/// <summary>
 		/// A psuedorandom nonrepeating sequence of integers from 0 to n.
 		/// </summary>
-		public static IEnumerable<int> LinearCongruentialGenerator(int n)
+		public static LinearCongruentialEnumerator LinearCongruentialGenerator(int n)
 		{
 			var x = Primes[random.Next(Primes.Length - 1)];
 			while (x % n == 0)
@@ -29,12 +29,44 @@ namespace MoonTools.ECS
 				x = Primes[random.Next(Primes.Length - 1)];
 			}
 
-			var start = random.Next(n);
+			return new LinearCongruentialEnumerator(random.Next(n), x, n);
-
-			for (var i = start; i < start + n; i++)
-			{
-				yield return (i * x) % n;
 		}
 	}
+
+	public struct LinearCongruentialEnumerator
+	{
+		private readonly int start;
+		private readonly int count;
+		private readonly int prime;
+		private int current;
+
+		public LinearCongruentialEnumerator GetEnumerator() => this;
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		internal LinearCongruentialEnumerator(int start, int prime, int count)
+		{
+			current = start;
+			this.start = start;
+			this.prime = prime;
+			this.count = count;
+		}
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public bool MoveNext()
+		{
+			current += 1;
+			if (current < start + count)
+			{
+				return true;
+			}
+
+			return false;
+		}
+
+		public int Current
+		{
+			[MethodImpl(MethodImplOptions.AggressiveInlining)]
+			get => (current * prime) % count;
+		}
 	}
 }

src/Relation.cs

@@ -13,12 +13,6 @@ namespace MoonTools.ECS
 			B = entityB;
 		}
 
-		internal Relation(int idA, int idB)
-		{
-			A = new Entity(idA);
-			B = new Entity(idB);
-		}
-
 		public override bool Equals(object? obj)
 		{
 			return obj is Relation relation && Equals(relation);

src/RelationDepot.cs

@@ -41,6 +41,11 @@ namespace MoonTools.ECS
 			Lookup<TRelationKind>().Set(relation, relationData);
 		}
 
+		public TRelationKind Get<TRelationKind>(Relation relation) where TRelationKind : unmanaged
+		{
+			return Lookup<TRelationKind>().Get(relation);
+		}
+
 		public (bool, bool) Remove<TRelationKind>(Relation relation) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().Remove(relation);
@@ -61,12 +66,12 @@ namespace MoonTools.ECS
 			return Lookup<TRelationKind>().Has(new Relation(idA, idB));
 		}
 
-		public IEnumerable<(Entity, TRelationKind)> OutRelations<TRelationKind>(int entityID) where TRelationKind : unmanaged
+		public ReverseSpanEnumerator<Entity> OutRelations<TRelationKind>(int entityID) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().OutRelations(entityID);
 		}
 
-		public (Entity, TRelationKind) OutRelationSingleton<TRelationKind>(int entityID) where TRelationKind : unmanaged
+		public Entity OutRelationSingleton<TRelationKind>(int entityID) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().OutFirst(entityID);
 		}
@@ -81,12 +86,12 @@ namespace MoonTools.ECS
 			return Lookup<TRelationKind>().HasOutRelation(entityID);
 		}
 
-		public IEnumerable<(Entity, TRelationKind)> InRelations<TRelationKind>(int entityID) where TRelationKind : unmanaged
+		public ReverseSpanEnumerator<Entity> InRelations<TRelationKind>(int entityID) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().InRelations(entityID);
 		}
 
-		public (Entity, TRelationKind) InRelationSingleton<TRelationKind>(int entityID) where TRelationKind : unmanaged
+		public Entity InRelationSingleton<TRelationKind>(int entityID) where TRelationKind : unmanaged
 		{
 			return Lookup<TRelationKind>().InFirst(entityID);
 		}
@@ -108,6 +113,11 @@ namespace MoonTools.ECS
 			storages[relationTypeIndex].Set(entityA, entityB, relationData);
 		}
 
+		public int GetStorageIndex(int relationTypeIndex, int entityA, int entityB)
+		{
+			return storages[relationTypeIndex].GetStorageIndex(entityA, entityB);
+		}
+
 		public unsafe void* Get(int relationTypeIndex, int relationStorageIndex)
 		{
 			return storages[relationTypeIndex].Get(relationStorageIndex);
@@ -118,9 +128,9 @@ namespace MoonTools.ECS
 			storages[relationTypeIndex].UnrelateAll(entityID);
 		}
 
-		public IEnumerable<(int, int)> OutRelationIndices(int entityID, int relationTypeIndex)
+		public ReverseSpanEnumerator<Entity> OutRelations(int entityID, int relationTypeIndex)
 		{
-			return storages[relationTypeIndex].OutRelationIndices(entityID);
+			return storages[relationTypeIndex].OutRelations(entityID);
 		}
 
 		public void Clear()

src/RelationStorage.cs

@@ -6,9 +6,10 @@ namespace MoonTools.ECS
 	internal abstract class RelationStorage
 	{
 		public abstract unsafe void Set(int entityA, int entityB, void* relationData);
+		public abstract int GetStorageIndex(int entityA, int entityB);
 		public abstract unsafe void* Get(int relationStorageIndex);
 		public abstract void UnrelateAll(int entityID);
-		public abstract IEnumerable<(int, int)> OutRelationIndices(int entityID);
+		public abstract ReverseSpanEnumerator<Entity> OutRelations(int entityID);
 		public abstract RelationStorage CreateStorage();
 		public abstract void Clear();
 	}
@@ -21,9 +22,9 @@ namespace MoonTools.ECS
 		private Dictionary<Relation, int> indices = new Dictionary<Relation, int>(16);
 		private Relation[] relations = new Relation[16];
 		private TRelation[] relationDatas = new TRelation[16];
-		private Dictionary<int, IndexableSet<int>> outRelations = new Dictionary<int, IndexableSet<int>>(16);
+		private Dictionary<int, IndexableSet<Entity>> outRelations = new Dictionary<int, IndexableSet<Entity>>(16);
-		private Dictionary<int, IndexableSet<int>> inRelations = new Dictionary<int, IndexableSet<int>>(16);
+		private Dictionary<int, IndexableSet<Entity>> inRelations = new Dictionary<int, IndexableSet<Entity>>(16);
-		private Stack<IndexableSet<int>> listPool = new Stack<IndexableSet<int>>();
+		private Stack<IndexableSet<Entity>> listPool = new Stack<IndexableSet<Entity>>();
 
 		public IEnumerable<(Entity, Entity, TRelation)> All()
 		{
@@ -36,9 +37,8 @@ namespace MoonTools.ECS
 
 		public void Set(Relation relation, TRelation relationData)
 		{
-			if (indices.ContainsKey(relation))
+			if (indices.TryGetValue(relation, out var index))
 			{
-				var index = indices[relation];
 				relationDatas[index] = relationData;
 				return;
 			}
@@ -70,34 +70,37 @@ namespace MoonTools.ECS
 			count += 1;
 		}
 
+		public TRelation Get(Relation relation)
+		{
+			return relationDatas[indices[relation]];
+		}
+
 		public bool Has(Relation relation)
 		{
 			return indices.ContainsKey(relation);
 		}
 
-		// FIXME: creating the new Relation in here is slightly deranged
+		public override ReverseSpanEnumerator<Entity> OutRelations(int entityID)
-		public IEnumerable<(Entity, TRelation)> OutRelations(int entityID)
 		{
-			if (outRelations.ContainsKey(entityID))
+			if (outRelations.TryGetValue(entityID, out var entityOutRelations))
 			{
-				foreach (var id in outRelations[entityID])
+				return entityOutRelations.GetEnumerator();
-				{
-					var relation = new Relation(entityID, id);
-					yield return (relation.B, relationDatas[indices[relation]]);
 			}
+			else
+			{
+				return ReverseSpanEnumerator<Entity>.Empty;
 			}
 		}
 
-		public (Entity, TRelation) OutFirst(int entityID)
+		public Entity OutFirst(int entityID)
 		{
 #if DEBUG
-			if (!outRelations.ContainsKey(entityID))
+			if (!outRelations.ContainsKey(entityID) || outRelations[entityID].Count == 0)
 			{
 				throw new KeyNotFoundException("No out relations to this entity!");
 			}
 #endif
-			var relation = new Relation(entityID, outRelations[entityID][0]);
+			return outRelations[entityID][0];
-			return (relation.B, relationDatas[indices[relation]]);
 		}
 
 		public bool HasOutRelation(int entityID)
@@ -107,32 +110,31 @@ namespace MoonTools.ECS
 
 		public int OutRelationCount(int entityID)
 		{
-			return outRelations.ContainsKey(entityID) ? outRelations[entityID].Count : 0;
+			return outRelations.TryGetValue(entityID, out var entityOutRelations) ? entityOutRelations.Count : 0;
 		}
 
-		public IEnumerable<(Entity, TRelation)> InRelations(int entityID)
+		public ReverseSpanEnumerator<Entity> InRelations(int entityID)
 		{
-			if (inRelations.ContainsKey(entityID))
+			if (inRelations.TryGetValue(entityID, out var entityInRelations))
 			{
-				foreach (var id in inRelations[entityID])
+				return entityInRelations.GetEnumerator();
-				{
-					var relation = new Relation(id, entityID);
-					yield return (relation.A, relationDatas[indices[relation]]);
 			}
+			else
+			{
+				return ReverseSpanEnumerator<Entity>.Empty;
 			}
 		}
 
-		public (Entity, TRelation) InFirst(int entityID)
+		public Entity InFirst(int entityID)
 		{
 #if DEBUG
-			if (!inRelations.ContainsKey(entityID))
+			if (!inRelations.ContainsKey(entityID) || inRelations[entityID].Count == 0)
 			{
 				throw new KeyNotFoundException("No out relations to this entity!");
 			}
 #endif
 
-			var relation = new Relation(inRelations[entityID][0], entityID);
+			return inRelations[entityID][0];
-			return (relation.A, relationDatas[indices[relation]]);
 		}
 
 		public bool HasInRelation(int entityID)
@@ -142,7 +144,7 @@ namespace MoonTools.ECS
 
 		public int InRelationCount(int entityID)
 		{
-			return inRelations.ContainsKey(entityID) ? inRelations[entityID].Count : 0;
+			return inRelations.TryGetValue(entityID, out var entityInRelations) ? entityInRelations.Count : 0;
 		}
 
 		public (bool, bool) Remove(Relation relation)
@@ -150,27 +152,26 @@ namespace MoonTools.ECS
 			var aEmpty = false;
 			var bEmpty = false;
 
-			if (outRelations.ContainsKey(relation.A.ID))
+			if (outRelations.TryGetValue(relation.A.ID, out var entityOutRelations))
 			{
-				outRelations[relation.A.ID].Remove(relation.B.ID);
+				entityOutRelations.Remove(relation.B.ID);
 				if (outRelations[relation.A.ID].Count == 0)
 				{
 					aEmpty = true;
 				}
 			}
 
-			if (inRelations.ContainsKey(relation.B.ID))
+			if (inRelations.TryGetValue(relation.B.ID, out var entityInRelations))
 			{
-				inRelations[relation.B.ID].Remove(relation.A.ID);
+				entityInRelations.Remove(relation.A.ID);
 				if (inRelations[relation.B.ID].Count == 0)
 				{
 					bEmpty = true;
 				}
 			}
 
-			if (indices.ContainsKey(relation))
+			if (indices.TryGetValue(relation, out var index))
 			{
-				var index = indices[relation];
 				var lastElementIndex = count - 1;
 
 				// move an element into the hole
@@ -189,17 +190,17 @@ namespace MoonTools.ECS
 			return (aEmpty, bEmpty);
 		}
 
-		private IndexableSet<int> AcquireHashSetFromPool()
+		private IndexableSet<Entity> AcquireHashSetFromPool()
 		{
 			if (listPool.Count == 0)
 			{
-				listPool.Push(new IndexableSet<int>());
+				listPool.Push(new IndexableSet<Entity>());
 			}
 
 			return listPool.Pop();
 		}
 
-		private void ReturnHashSetToPool(IndexableSet<int> hashSet)
+		private void ReturnHashSetToPool(IndexableSet<Entity> hashSet)
 		{
 			hashSet.Clear();
 			listPool.Push(hashSet);
@@ -212,6 +213,11 @@ namespace MoonTools.ECS
 			Set(new Relation(entityA, entityB), *((TRelation*) relationData));
 		}
 
+		public override int GetStorageIndex(int entityA, int entityB)
+		{
+			return indices[new Relation(entityA, entityB)];
+		}
+
 		public override unsafe void* Get(int relationStorageIndex)
 		{
 			fixed (void* p = &relations[relationStorageIndex])
@@ -222,41 +228,29 @@ namespace MoonTools.ECS
 
 		public override void UnrelateAll(int entityID)
 		{
-			if (outRelations.ContainsKey(entityID))
+			if (outRelations.TryGetValue(entityID, out var entityOutRelations))
 			{
-				foreach (var entityB in outRelations[entityID])
+				foreach (var entityB in entityOutRelations)
 				{
 					Remove(new Relation(entityID, entityB));
 				}
 
-				ReturnHashSetToPool(outRelations[entityID]);
+				ReturnHashSetToPool(entityOutRelations);
 				outRelations.Remove(entityID);
 			}
 
-			if (inRelations.ContainsKey(entityID))
+			if (inRelations.TryGetValue(entityID, out var entityInRelations))
 			{
-				foreach (var entityA in inRelations[entityID])
+				foreach (var entityA in entityInRelations)
 				{
 					Remove(new Relation(entityA, entityID));
 				}
 
-				ReturnHashSetToPool(inRelations[entityID]);
+				ReturnHashSetToPool(entityInRelations);
 				inRelations.Remove(entityID);
 			}
 		}
 
-		public override IEnumerable<(int, int)> OutRelationIndices(int entityID)
-		{
-			if (outRelations.ContainsKey(entityID))
-			{
-				foreach (var id in outRelations[entityID])
-				{
-					var relation = new Relation(entityID, id);
-					yield return (id, indices[relation]);
-				}
-			}
-		}
-
 		public override RelationStorage<TRelation> CreateStorage()
 		{
 			return new RelationStorage<TRelation>();

src/Snapshot.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 
 namespace MoonTools.ECS
@@ -50,7 +50,7 @@ namespace MoonTools.ECS
 				{
 					SnapshotEntityStorage.AddRelationKind(snapshotEntityID, relationTypeIndex);
 
-					foreach (var (otherEntityID, relationStorageIndex) in World.RelationDepot.OutRelationIndices(worldEntity.ID, relationTypeIndex))
+					foreach (var otherEntityID in World.RelationDepot.OutRelations(worldEntity.ID, relationTypeIndex))
 					{
 #if DEBUG
 						if (!World.FilterStorage.CheckSatisfied(otherEntityID, Filter.Signature))
@@ -58,6 +58,7 @@ namespace MoonTools.ECS
 							throw new InvalidOperationException($"Snapshot entity {worldEntity.ID} is related to non-snapshot entity {otherEntityID}!");
 						}
 #endif
+						var relationStorageIndex = World.RelationDepot.GetStorageIndex(relationTypeIndex, worldEntity, otherEntityID);
 						var otherSnapshotID = WorldToSnapshotID[otherEntityID];
 						SnapshotEntityStorage.AddRelationKind(otherSnapshotID, relationTypeIndex);
 						SnapshotRelationDepot.Set(snapshotEntityID, otherSnapshotID, relationTypeIndex, World.RelationDepot.Get(relationTypeIndex, relationStorageIndex));
@@ -99,8 +100,9 @@ namespace MoonTools.ECS
 				{
 					World.EntityStorage.AddRelationKind(worldID, relationTypeIndex);
 
-					foreach (var (otherEntityID, relationStorageIndex) in SnapshotRelationDepot.OutRelationIndices(i, relationTypeIndex))
+					foreach (var otherEntityID in SnapshotRelationDepot.OutRelations(i, relationTypeIndex))
 					{
+						var relationStorageIndex = SnapshotRelationDepot.GetStorageIndex(relationTypeIndex, i, otherEntityID);
 						var otherEntityWorldID = SnapshotToWorldID[otherEntityID];
 						World.RelationDepot.Set(worldID, otherEntityWorldID, relationTypeIndex, SnapshotRelationDepot.Get(relationTypeIndex, relationStorageIndex));
 					}

src/System.cs

@@ -43,7 +43,7 @@ namespace MoonTools.ECS
 			return MessageDepot.Some<TMessage>();
 		}
 
-		protected IEnumerable<TMessage> ReadMessagesWithEntity<TMessage>(in Entity entity) where TMessage : unmanaged
+		protected ReverseSpanEnumerator<TMessage> ReadMessagesWithEntity<TMessage>(in Entity entity) where TMessage : unmanaged
 		{
 			return MessageDepot.WithEntity<TMessage>(entity.ID);
 		}

src/TypeIndices.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 
 namespace MoonTools.ECS
@@ -27,7 +27,7 @@ namespace MoonTools.ECS
 
 
 #if DEBUG
-		public IEnumerable<Type> Types => TypeToIndex.Keys;
+		public Dictionary<Type, int>.KeyCollection Types => TypeToIndex.Keys;
 #endif
 	}
 }