id-preserving snapshots

src/NativeArray.cs

@@ -9,17 +9,19 @@ namespace MoonTools.ECS.Collections
 		private T* Array;
 		private int count;
 		private int capacity;
+		private int elementSize;
 
 		public int Count => count;
 
-		public ReverseSpanEnumerator<T> GetEnumerator() => new ReverseSpanEnumerator<T>(new Span<T>(Array, Count));
+		public Span<T>.Enumerator GetEnumerator() => new Span<T>(Array, count).GetEnumerator();
 
 		private bool disposed;
 
 		public NativeArray(int capacity = 16)
 		{
 			this.capacity = capacity;
-			Array = (T*) NativeMemory.Alloc((nuint) (capacity * Unsafe.SizeOf<T>()));
+			elementSize = Unsafe.SizeOf<T>();
+			Array = (T*) NativeMemory.Alloc((nuint) (capacity * elementSize));
 			count = 0;
 		}
 
@@ -37,11 +39,51 @@ namespace MoonTools.ECS.Collections
 			count += 1;
 		}
 
+		public void RemoveLastElement()
+		{
+			count -= 1;
+		}
+
+		public bool TryPop(out T element)
+		{
+			if (count > 0)
+			{
+				element = Array[count - 1];
+				count -= 1;
+				return true;
+			}
+
+			element = default;
+			return false;
+		}
+
 		public void Clear()
 		{
 			count = 0;
 		}
 
+		private void ResizeTo(int size)
+		{
+			capacity = size;
+			Array = (T*) NativeMemory.Realloc((void*) Array, (nuint) (elementSize * capacity));
+		}
+
+		public void CopyTo(NativeArray<T> other)
+		{
+			if (count >= other.capacity)
+			{
+				other.ResizeTo(Count);
+			}
+
+			NativeMemory.Copy(
+				(void*) Array,
+				(void*) other.Array,
+				(nuint) (elementSize * Count)
+			);
+
+			other.count = count;
+		}
+
 		protected virtual void Dispose(bool disposing)
 		{
 			if (!disposed)

src/Rev2/Archetype.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using MoonTools.ECS.Collections;
 
 namespace MoonTools.ECS.Rev2;
 
@@ -6,8 +7,8 @@ internal class Archetype
 {
 	public World World;
 	public ArchetypeSignature Signature;
-	public List<Column> ComponentColumns = new List<Column>();
+	public Column[] ComponentColumns;
-	public List<Id> RowToEntity = new List<Id>();
+	public NativeArray<Id> RowToEntity = new NativeArray<Id>();
 
 	public Dictionary<Id, int> ComponentToColumnIndex =
 		new Dictionary<Id, int>();
@@ -19,11 +20,12 @@ internal class Archetype
 	{
 		World = world;
 		Signature = signature;
+		ComponentColumns = new Column[signature.Count];
 	}
 
 	public void ClearAll()
 	{
-		for (int i = 0; i < ComponentColumns.Count; i += 1)
+		for (int i = 0; i < ComponentColumns.Length; i += 1)
 		{
 			ComponentColumns[i].Count = 0;
 		}

src/Rev2/Filter.cs

@@ -83,16 +83,6 @@ namespace MoonTools.ECS.Rev2
 			}
 		}
 
-		public void TakeSnapshot(Snapshot snapshot)
-		{
-			snapshot.Reset();
-
-			foreach (var archetype in Archetypes)
-			{
-				snapshot.TakeArchetypeSnapshot(archetype);
-			}
-		}
-
 		internal Filter(Archetype emptyArchetype, HashSet<Id> included, HashSet<Id> excluded)
 		{
 			EmptyArchetype = emptyArchetype;

src/Rev2/IdAssigner.cs

@@ -1,15 +1,15 @@
-using System.Collections.Generic;
+using MoonTools.ECS.Collections;
 
-namespace MoonTools.ECS.Rev2
+namespace MoonTools.ECS.Rev2;
+
+internal class IdAssigner
 {
-	internal class IdAssigner
-	{
 	ulong Next;
-		Queue<ulong> AvailableIds = new Queue<ulong>();
+	NativeArray<ulong> AvailableIds = new NativeArray<ulong>();
 
 	public Id Assign()
 	{
-			if (!AvailableIds.TryDequeue(out var id))
+		if (!AvailableIds.TryPop(out var id))
 		{
 			id = Next;
 			Next += 1;
@@ -20,7 +20,12 @@ namespace MoonTools.ECS.Rev2
 
 	public void Unassign(Id id)
 	{
-			AvailableIds.Enqueue(id.Value);
+		AvailableIds.Add(id.Value);
 	}
+
+	public void CopyTo(IdAssigner other)
+	{
+		AvailableIds.CopyTo(other.AvailableIds);
+		other.Next = Next;
 	}
 }

src/Rev2/Snapshot.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using MoonTools.ECS.Collections;
 
 namespace MoonTools.ECS.Rev2;
 
@@ -7,6 +8,9 @@ public class Snapshot
 	private Dictionary<ArchetypeSignature, ArchetypeSnapshot> ArchetypeSnapshots =
 		new Dictionary<ArchetypeSignature, ArchetypeSnapshot>();
 
+	private Dictionary<Id, Record> EntityIndex = new Dictionary<Id, Record>();
+	private IdAssigner IdAssigner = new IdAssigner();
+
 	public int Count
 	{
 		get
@@ -24,18 +28,40 @@ public class Snapshot
 
 	public void Restore(World world)
 	{
+		// restore archetype storage
 		foreach (var (archetypeSignature, archetypeSnapshot) in ArchetypeSnapshots)
 		{
 			var archetype = world.ArchetypeIndex[archetypeSignature];
 			RestoreArchetypeSnapshot(archetype);
 		}
+
+		// restore entity index
+		world.EntityIndex.Clear();
+		foreach (var (id, record) in EntityIndex)
+		{
+			world.EntityIndex[id] = record;
 		}
 
-	internal void Reset()
+		// restore id assigner state
+		IdAssigner.CopyTo(world.IdAssigner);
+	}
+
+	public void Take(World world)
 	{
-		foreach (var archetypeSnapshot in ArchetypeSnapshots.Values)
+		// copy id assigner state
+		world.IdAssigner.CopyTo(IdAssigner);
+
+		// copy entity index
+		EntityIndex.Clear();
+		foreach (var (id, record) in world.EntityIndex)
 		{
-			archetypeSnapshot.Count = 0;
+			EntityIndex[id] = record;
+		}
+
+		// copy archetypes
+		foreach (var archetype in world.ArchetypeIndex.Values)
+		{
+			TakeArchetypeSnapshot(archetype);
 		}
 	}
 
@@ -59,59 +85,48 @@ public class Snapshot
 	private class ArchetypeSnapshot
 	{
 		public ArchetypeSignature Signature;
-		public readonly List<Column> ComponentColumns;
+		private readonly Column[] ComponentColumns;
+		private readonly NativeArray<Id> RowToEntity;
 
-		public int Count;
+		public int Count => RowToEntity.Count;
 
 		public ArchetypeSnapshot(ArchetypeSignature signature)
 		{
 			Signature = signature;
-			ComponentColumns = new List<Column>(signature.Count);
+			ComponentColumns = new Column[signature.Count];
+			RowToEntity = new NativeArray<Id>();
 
 			for (int i = 0; i < signature.Count; i += 1)
 			{
 				var componentId = signature[i];
-				ComponentColumns.Add(new Column(World.ElementSizes[componentId]));
+				ComponentColumns[i] = new Column(World.ElementSizes[componentId]);
 			}
 		}
 
+		public void Clear()
+		{
+			RowToEntity.Clear();
+		}
+
 		public void Take(Archetype archetype)
 		{
-			for (int i = 0; i < ComponentColumns.Count; i += 1)
+			for (int i = 0; i < ComponentColumns.Length; i += 1)
 			{
 				archetype.ComponentColumns[i].CopyAllTo(ComponentColumns[i]);
 			}
 
-			Count = archetype.Count;
+			archetype.RowToEntity.CopyTo(RowToEntity);
 		}
 
 		public void Restore(Archetype archetype)
 		{
 			// Copy all component data
-			for (int i = 0; i < ComponentColumns.Count; i += 1)
+			for (int i = 0; i < ComponentColumns.Length; i += 1)
 			{
 				ComponentColumns[i].CopyAllTo(archetype.ComponentColumns[i]);
 			}
 
-			var archetypeCount = archetype.Count;
+			RowToEntity.CopyTo(archetype.RowToEntity);
-
-			if (Count < archetypeCount)
-			{
-				// if snapshot has fewer entities than archetype, remove extra entities
-				for (int i = archetypeCount - 1; i >= Count; i -= 1)
-				{
-					archetype.World.FreeEntity(archetype.RowToEntity[i]);
-					archetype.RowToEntity.RemoveAt(i);
-				}
-			}
-			else if (Count > archetypeCount)
-			{
-				// if snapshot has more entities than archetype, add entities
-				for (int i = archetypeCount; i < Count; i += 1)
-				{
-					archetype.World.CreateEntityOnArchetype(archetype);
-				}
-			}
 		}
 	}
 }

src/Rev2/World.cs

@@ -15,22 +15,22 @@ namespace MoonTools.ECS.Rev2
 		internal Dictionary<ArchetypeSignature, Archetype> ArchetypeIndex = new Dictionary<ArchetypeSignature, Archetype>();
 
 		// Going from EntityId to Archetype and storage row
-		Dictionary<Id, Record> EntityIndex = new Dictionary<Id, Record>();
+		internal Dictionary<Id, Record> EntityIndex = new Dictionary<Id, Record>();
 
 		// Going from ComponentId to Archetype list
 		Dictionary<Id, List<Archetype>> ComponentIndex = new Dictionary<Id, List<Archetype>>();
 
 		// ID Management
-		IdAssigner IdAssigner = new IdAssigner();
+		internal IdAssigner IdAssigner = new IdAssigner();
 
 		internal readonly Archetype EmptyArchetype;
 
 		public FilterBuilder FilterBuilder => new FilterBuilder(this);
 
-		private bool IsDisposed;
-
 		public delegate void RefAction<T1, T2>(ref T1 arg1, ref T2 arg2);
 
+		private bool IsDisposed;
+
 		public World()
 		{
 			// Create the Empty Archetype
@@ -39,10 +39,7 @@ namespace MoonTools.ECS.Rev2
 
 		internal Archetype CreateArchetype(ArchetypeSignature signature)
 		{
-			var archetype = new Archetype(this, signature)
+			var archetype = new Archetype(this, signature);
-			{
-				ComponentColumns = new List<Column>(signature.Count)
-			};
 
 			ArchetypeIndex.Add(signature, archetype);
 
@@ -50,8 +47,8 @@ namespace MoonTools.ECS.Rev2
 			{
 				var componentId = signature[i];
 				ComponentIndex[componentId].Add(archetype);
-				archetype.ComponentToColumnIndex.Add(componentId, archetype.ComponentColumns.Count);
+				archetype.ComponentToColumnIndex.Add(componentId, i);
-				archetype.ComponentColumns.Add(new Column(ElementSizes[componentId]));
+				archetype.ComponentColumns[i] = new Column(ElementSizes[componentId]);
 			}
 
 			return archetype;
@@ -69,7 +66,6 @@ namespace MoonTools.ECS.Rev2
 		internal void CreateEntityOnArchetype(Archetype archetype)
 		{
 			var entityId = IdAssigner.Assign();
-			EntityIndex.Add(entityId, new Record(archetype, archetype.Count));
 			archetype.RowToEntity.Add(entityId);
 		}
 
@@ -330,7 +326,7 @@ namespace MoonTools.ECS.Rev2
 			var archetype = record.Archetype;
 			var row = record.Row;
 
-			for (int i = 0; i < archetype.ComponentColumns.Count; i += 1)
+			for (int i = 0; i < archetype.Signature.Count; i += 1)
 			{
 				archetype.ComponentColumns[i].Delete(row);
 			}
@@ -343,14 +339,14 @@ namespace MoonTools.ECS.Rev2
 				EntityIndex[lastRowEntity] = new Record(archetype, row);
 			}
 
-			archetype.RowToEntity.RemoveAt(archetype.Count - 1);
+			archetype.RowToEntity.RemoveLastElement();
 			EntityIndex.Remove(entityId);
 			IdAssigner.Unassign(entityId);
 		}
 
 		private void MoveEntityToHigherArchetype(Id entityId, int row, Archetype from, Archetype to)
 		{
-			for (int i = 0; i < from.ComponentColumns.Count; i += 1)
+			for (int i = 0; i < from.Signature.Count; i += 1)
 			{
 				var componentId = from.Signature[i];
 				var destinationColumnIndex = to.ComponentToColumnIndex[componentId];
@@ -370,7 +366,7 @@ namespace MoonTools.ECS.Rev2
 				EntityIndex[lastRowEntity] = new Record(from, row);
 			}
 
-			from.RowToEntity.RemoveAt(from.Count - 1);
+			from.RowToEntity.RemoveLastElement();
 
 			// update row to entity lookup on to archetype
 			EntityIndex[entityId] = new Record(to, to.Count);
@@ -379,7 +375,7 @@ namespace MoonTools.ECS.Rev2
 
 		private void MoveEntityToLowerArchetype(Id entityId, int row, Archetype from, Archetype to, Id removed)
 		{
-			for (int i = 0; i < from.ComponentColumns.Count; i += 1)
+			for (int i = 0; i < from.Signature.Count; i += 1)
 			{
 				var componentId = from.Signature[i];
 
@@ -402,7 +398,7 @@ namespace MoonTools.ECS.Rev2
 				EntityIndex[lastRowEntity] = new Record(from, row);
 			}
 
-			from.RowToEntity.RemoveAt(from.Count - 1);
+			from.RowToEntity.RemoveLastElement();
 
 			// update row to entity lookup on to archetype
 			EntityIndex[entityId] = new Record(to, to.Count);