implement world copy leveraging archetype copy

Author: josefalanga

src/Arch.Tests/ArchetypeTest.cs

@@ -245,7 +245,6 @@ public void CopyTo([Values(1111,2222,3333)] int sourceAmount, [Values(1111,2222,
 
         // Copy from one chunk into other.
         Archetype.Copy(source, destination);
-        source.Clear();
 
         var sourceCounter = sourceAmount;
         var destinationCounter = destinationAmount;
@@ -358,7 +357,6 @@ public void CopyToShift([Values(1111,2222,3333)] int sourceAmount, [Values(1111,
 
         // Copy from one chunk into other.
         Archetype.Copy(source, destination);
-        source.Clear();
 
         var requiredChunksForSource = Archetype.GetChunkCapacityFor(source.EntitiesPerChunk, sourceAmount);
         var requiredChunksForDestination = Archetype.GetChunkCapacityFor(destination.EntitiesPerChunk, sourceAmount + destinationAmount);

src/Arch.Tests/WorldTest.cs

@@ -866,4 +866,69 @@ public void GeneratedAdd()
         That(_world.GetArchetype(entity2), Is.EqualTo(_world.GetArchetype(entity)));
         That(arch, Is.EqualTo(_world.GetArchetype(entity)));
     }
+    
+    /// <summary>
+    ///     Checks if the <see cref="World"/> is copied correctly over a new instance.
+    /// </summary>
+    [Test]
+    public void Copy()
+    {
+        using var source = World.Create();
+        source.Create(new Transform(), new Rotation());
+        source.Create(new Transform(), new Rotation());
+        source.Create(new Transform());
+        source.Create(new Transform(), new Rotation());
+        source.Create(new Rotation());
+        source.Create(new Transform());
+
+        var copy = source.Copy();
+
+        That(copy.Size, Is.EqualTo(source.Size));
+        That(copy.Capacity, Is.EqualTo(source.Capacity));
+        That(copy.Archetypes.Count, Is.EqualTo(source.Archetypes.Count));
+        //same amount of entities per archetype
+        for (var index = 0; index < copy.Archetypes.Items.Count; index++)
+        {
+            var copyArchetype = copy.Archetypes.Items[index];
+            var sourceArchetypeExists = source.TryGetArchetype(copyArchetype.Signature, out var sourceArchetype);
+            That(sourceArchetypeExists, Is.True);
+            That(sourceArchetype, Is.Not.Null);
+            That(copyArchetype.Count, Is.EqualTo(sourceArchetype.Count));
+            That(copyArchetype.EntityCount, Is.EqualTo(sourceArchetype.EntityCount));
+        }
+    }
+    
+    /// <summary>
+    ///     Checks if the <see cref="World"/> is copied correctly over a new instance, skipping empty archetypes.
+    /// </summary>
+    [Test]
+    public void CopySkipping()
+    {
+        using var source = World.Create();
+        source.Create(new Transform(), new Rotation());
+        source.Create(new Transform(), new Rotation());
+        source.Create(new Transform());
+        source.Create(new Transform(), new Rotation());
+        source.Create(new Transform());
+        source.Create(new Transform());
+        //this will cause one archetype to be empty and ignored in the copy
+        source.Destroy(source.Create(new Rotation()));
+
+        var copy = source.Copy();
+
+        That(copy.Size, Is.EqualTo(source.Size));
+        //LessThan because one archetype got skipped
+        That(copy.Capacity, Is.LessThan(source.Capacity));
+        That(copy.Archetypes.Count, Is.LessThan(source.Archetypes.Count));
+        //same amount of entities per archetype
+        for (var index = 0; index < copy.Archetypes.Items.Count; index++)
+        {
+            var copyArchetype = copy.Archetypes.Items[index];
+            var sourceArchetypeExists = source.TryGetArchetype(copyArchetype.Signature, out var sourceArchetype);
+            That(sourceArchetypeExists, Is.True);
+            That(sourceArchetype, Is.Not.Null);
+            That(copyArchetype.Count, Is.EqualTo(sourceArchetype.Count));
+            That(copyArchetype.EntityCount, Is.EqualTo(sourceArchetype.EntityCount));
+        }
+    }
 }

src/Arch/Core/Archetype.cs

@@ -867,7 +867,8 @@ internal static int GetNextSlots(Archetype archetype, Span<Slot> slots, int amou
     /// </summary>
     /// <param name="source">The source <see cref="Archetype"/>.</param>
     /// <param name="destination">The destination <see cref="Archetype"/>.</param>
-    internal static void Copy(Archetype source, Archetype destination)
+    /// <param name="clearSource">Whether to clear the source Archetype and Chunk counts.</param>
+    internal static void Copy(Archetype source, Archetype destination, bool clearSource = true)
     {
         // Make sure other archetype can fit additional entities from this archetype.
         destination.EnsureEntityCapacity(destination.EntityCount + source.EntityCount);
@@ -880,19 +881,25 @@ internal static void Copy(Archetype source, Archetype destination)
 
             var amountCopied = 0;
             var chunkIndex = 0;
+            var amountLeft = sourceChunk.Count;
 
             // Loop over destination chunk and fill them with the source chunk till either the source chunk is empty or theres no more capacity
-            for (int destinationChunkIndex = destination.Count; destinationChunkIndex < destination.ChunkCapacity && sourceChunk.Count > 0; destinationChunkIndex++)
+            for (int destinationChunkIndex = destination.Count; destinationChunkIndex < destination.ChunkCapacity && amountLeft > 0; destinationChunkIndex++)
             {
                 // Determine amount that can be copied into destination
                 ref var destinationChunk = ref destination.GetChunk(destinationChunkIndex);
                 var remainingCapacity = destinationChunk.Buffer;
-                var amountToCopy = Math.Min(sourceChunk.Count, remainingCapacity);
+                var amountToCopy = Math.Min(amountLeft, remainingCapacity);
 
                 Chunk.Copy(ref sourceChunk, amountCopied, ref sourceSignature, ref destinationChunk, destinationChunk.Count, amountToCopy);
 
                 // Apply copied amount to track the progress
-                sourceChunk.Count -= amountToCopy;
+                amountLeft -= amountToCopy;
+                if (clearSource)
+                {
+                    sourceChunk.Count -= amountToCopy;
+                }
+                
                 destinationChunk.Count += amountToCopy;
                 amountCopied += amountToCopy;
                 chunkIndex = destinationChunkIndex;  // Track the last destination chunk we filled, important
@@ -903,8 +910,11 @@ internal static void Copy(Archetype source, Archetype destination)
 
         // Update entity counts
         destination.EntityCount += source.EntityCount;
-        source.EntityCount = 0;
-        source.Count = 0;
+        if (clearSource)
+        {
+            source.EntityCount = 0;
+            source.Count = 0;
+        }
     }
 
     /// <summary>

src/Arch/Core/World.cs

@@ -1744,4 +1744,31 @@ public Signature GetSignature(Entity entity)
     }
 }
 
+public partial class World
+{
+    /// <summary>
+    /// Creates a deep copy of the world, copying archetypes and chunks.
+    /// </summary>
+    /// <returns>A newly created World with all data copied from the original.</returns>
+    public World Copy()
+    {
+        var copy = Create();
+        var archetypes = Archetypes.Items;
+        for (var index= 0; index < archetypes.Count; index++)
+        {
+            var archetype = archetypes[index];
+            if (archetype.EntityCount <= 0)
+            {
+                continue;
+            }
+            
+            var newArchetype = copy.GetOrCreate(archetype.Signature);
+            Archetype.Copy(archetype, newArchetype, false);
+            copy.Size += newArchetype.EntityCount;
+        }
+        
+        return copy;
+    }
+}
+
 #endregion