garbage optimization changes

src/Collision/Fixed/Collider.cs

@@ -0,0 +1,33 @@
+using System;
+
+namespace MoonWorks.Collision.Fixed
+{
+	public class Collider<T> : IHasAABB2D where T : struct, IShape2D
+	{
+		private readonly T[] Shapes;
+
+		public ReadOnlySpan<T>.Enumerator GetEnumerator() => new ReadOnlySpan<T>(Shapes).GetEnumerator();
+
+		public AABB2D AABB { get; }
+
+		public Collider(T shape)
+		{
+			Shapes = new T[1] { shape };
+			AABB = shape.AABB;
+		}
+
+		public Collider(T[] shapes)
+		{
+			Shapes = new T[shapes.Length];
+			Array.Copy(shapes, Shapes, shapes.Length);
+
+			var aabb = new AABB2D();
+			foreach (var shape in Shapes)
+			{
+				aabb = aabb.Compose(shape.AABB);
+			}
+
+			AABB = aabb;
+		}
+	}
+}

src/Collision/Fixed/ICollidable.cs

@@ -1,12 +0,0 @@
-using System.Collections.Generic;
-using MoonWorks.Math.Fixed;
-
-namespace MoonWorks.Collision.Fixed
-{
-	public interface ICollidable
-	{
-		IEnumerable<IShape2D> Shapes { get; }
-		AABB2D AABB { get; }
-		AABB2D TransformedAABB(Transform2D transform);
-	}
-}

src/Collision/Fixed/IHasAABB2D.cs

@@ -0,0 +1,7 @@
+namespace MoonWorks.Collision.Fixed
+{
+	public interface IHasAABB2D
+	{
+		AABB2D AABB { get; }
+	}
+}

src/Collision/Fixed/IShape2D.cs

@@ -1,8 +1,8 @@
-using MoonWorks.Math.Fixed;
+using MoonWorks.Math.Fixed;
 
 namespace MoonWorks.Collision.Fixed
 {
-	public interface IShape2D : ICollidable, System.IEquatable<IShape2D>
+	public interface IShape2D : IHasAABB2D, System.IEquatable<IShape2D>
 	{
 		/// <summary>
 		/// A Minkowski support function. Gives the farthest point on the edge of a shape along the given direction.

src/Collision/Fixed/MinkowskiDifference.cs

@@ -1,18 +1,18 @@
-using MoonWorks.Math.Fixed;
+using MoonWorks.Math.Fixed;
 
 namespace MoonWorks.Collision.Fixed
 {
 	/// <summary>
 	/// A Minkowski difference between two shapes.
 	/// </summary>
-	public struct MinkowskiDifference : System.IEquatable<MinkowskiDifference>
+	public struct MinkowskiDifference<T, U> where T : IShape2D where U : IShape2D
 	{
-		private IShape2D ShapeA { get; }
+		private T ShapeA { get; }
 		private Transform2D TransformA { get; }
-		private IShape2D ShapeB { get; }
+		private U ShapeB { get; }
 		private Transform2D TransformB { get; }
 
-		public MinkowskiDifference(IShape2D shapeA, Transform2D transformA, IShape2D shapeB, Transform2D transformB)
+		public MinkowskiDifference(T shapeA, Transform2D transformA, U shapeB, Transform2D transformB)
 		{
 			ShapeA = shapeA;
 			TransformA = transformA;
@@ -24,34 +24,5 @@ namespace MoonWorks.Collision.Fixed
 		{
 			return ShapeA.Support(direction, TransformA) - ShapeB.Support(-direction, TransformB);
 		}
-
-		public override bool Equals(object other)
-		{
-			return other is MinkowskiDifference minkowskiDifference && Equals(minkowskiDifference);
-		}
-
-		public bool Equals(MinkowskiDifference other)
-		{
-			return
-				ShapeA == other.ShapeA &&
-				TransformA == other.TransformA &&
-				ShapeB == other.ShapeB &&
-				TransformB == other.TransformB;
-		}
-
-		public override int GetHashCode()
-		{
-			return System.HashCode.Combine(ShapeA, TransformA, ShapeB, TransformB);
-		}
-
-		public static bool operator ==(MinkowskiDifference a, MinkowskiDifference b)
-		{
-			return a.Equals(b);
-		}
-
-		public static bool operator !=(MinkowskiDifference a, MinkowskiDifference b)
-		{
-			return !(a == b);
-		}
 	}
 }

src/Collision/Fixed/NarrowPhase.cs

@@ -1,4 +1,5 @@
-using MoonWorks.Math.Fixed;
+using MoonWorks.Math.Fixed;
+using System.Runtime.CompilerServices;
 
 namespace MoonWorks.Collision.Fixed
 {
@@ -11,11 +12,11 @@ namespace MoonWorks.Collision.Fixed
 			public int Index;
 		}
 
-        public static bool TestCollision(ICollidable collidableA, Transform2D transformA, ICollidable collidableB, Transform2D transformB)
+		public static bool TestCollision<T, U>(Collider<T> colliderA, in Transform2D transformA, Collider<U> colliderB, in Transform2D transformB) where T : struct, IShape2D where U : struct, IShape2D
 		{
-            foreach (var shapeA in collidableA.Shapes)
+			foreach (var shapeA in colliderA)
 			{
-                foreach (var shapeB in collidableB.Shapes)
+				foreach (var shapeB in colliderB)
 				{
 					if (TestCollision(shapeA, transformA, shapeB, transformB))
 					{
@@ -27,47 +28,152 @@ namespace MoonWorks.Collision.Fixed
 			return false;
 		}
 
-		public static bool TestCollision(IShape2D shapeA, Transform2D transformA, IShape2D shapeB, Transform2D transformB)
+		public static bool TestCollision<T, U>(Collider<T> collider, in Transform2D transformA, U shape, in Transform2D transformB) where T : struct, IShape2D where U : struct, IShape2D
 		{
-			// If we can use a fast path check, let's do that!
+			foreach (var colliderShape in collider)
-			if (shapeA is Rectangle rectangleA && shapeB is Rectangle rectangleB && transformA.IsAxisAligned && transformB.IsAxisAligned)
 			{
-				return TestRectangleOverlap(rectangleA, transformA, rectangleB, transformB);
+				if (TestCollision(colliderShape, transformA, shape, transformB))
+				{
+					return true;
+				}
+			}
+
+			return false;
+		}
+
+		public static bool TestCollision<T, U>(U shape, in Transform2D transformA, Collider<T> collider, in Transform2D transformB) where T : struct, IShape2D where U : struct, IShape2D
+		{
+			return TestCollision(collider, transformB, shape, transformA);
+		}
+
+		public static bool TestCollision<T, U>(in T shapeA, in Transform2D transformA, in U shapeB, in Transform2D transformB) where T : struct, IShape2D where U : struct, IShape2D
+		{
+			if (shapeA is Circle circle)
+			{
+				if (shapeB is Circle circleB)
+				{
+					return TestCollision(circle, transformA, circleB, transformB);
+				}
+				else if (shapeB is Point pointB)
+				{
+					return TestCollision(circle, transformA, pointB, transformB);
+				}
+				else if (shapeB is Rectangle rectangleB)
+				{
+					return TestCollision(circle, transformA, rectangleB, transformB);
+				}
+			}
+			else if (shapeA is Point point)
+			{
+				if (shapeB is Circle circleB)
+				{
+					return TestCollision(point, transformA, circleB, transformB);
+				}
+				else if (shapeB is Point pointB)
+				{
+					return TestCollision(point, transformA, pointB, transformB);
+				}
+				else if (shapeB is Rectangle rectangleB)
+				{
+					return TestCollision(point, transformA, rectangleB, transformB);
+				}
+			}
+			else if (shapeA is Rectangle rectangle)
+			{
+				if (shapeB is Circle circleB)
+				{
+					return TestCollision(rectangle, transformA, circleB, transformB);
+				}
+				else if (shapeB is Point pointB)
+				{
+					return TestCollision(rectangle, transformA, pointB, transformB);
+				}
+				else if (shapeB is Rectangle rectangleB)
+				{
+					return TestCollision(rectangle, transformA, rectangleB, transformB);
 				}
-			else if (shapeA is Point && shapeB is Rectangle && transformB.IsAxisAligned)
-			{
-				return TestPointRectangleOverlap((Point) shapeA, transformA, (Rectangle) shapeB, transformB);
-			}
-			else if (shapeA is Rectangle && shapeB is Point && transformA.IsAxisAligned)
-			{
-				return TestPointRectangleOverlap((Point) shapeB, transformB, (Rectangle) shapeA, transformA);
-			}
-			else if (shapeA is Rectangle && shapeB is Circle && transformA.IsAxisAligned && transformB.IsUniformScale)
-			{
-				return TestCircleRectangleOverlap((Circle) shapeB, transformB, (Rectangle) shapeA, transformA);
-			}
-			else if (shapeA is Circle && shapeB is Rectangle && transformA.IsUniformScale && transformB.IsAxisAligned)
-			{
-				return TestCircleRectangleOverlap((Circle) shapeA, transformA, (Rectangle) shapeB, transformB);
-			}
-			else if (shapeA is Circle && shapeB is Point && transformA.IsUniformScale)
-			{
-				return TestCirclePointOverlap((Circle) shapeA, transformA, (Point) shapeB, transformB);
-			}
-			else if (shapeA is Point && shapeB is Circle && transformB.IsUniformScale)
-			{
-				return TestCirclePointOverlap((Circle) shapeB, transformB, (Point) shapeA, transformA);
-			}
-			else if (shapeA is Circle circleA && shapeB is Circle circleB && transformA.IsUniformScale && transformB.IsUniformScale)
-			{
-				return TestCircleOverlap(circleA, transformA, circleB, transformB);
 			}
 
-			// Sad, we can't do a fast path optimization. Time for a simplex reduction.
 			return FindCollisionSimplex(shapeA, transformA, shapeB, transformB).Item1;
 		}
 
-		public static bool TestRectangleOverlap(Rectangle rectangleA, Transform2D transformA, Rectangle rectangleB, Transform2D transformB)
+		public static bool TestCollision(in Rectangle rectangleA, in Transform2D transformA, in Rectangle rectangleB, in Transform2D transformB)
+		{
+			if (transformA.IsAxisAligned && transformB.IsAxisAligned)
+			{
+				return TestRectangleOverlap(rectangleA, transformA, rectangleB, transformB);
+			}
+			else
+			{
+				return FindCollisionSimplex(rectangleA, transformA, rectangleB, transformB).Item1;
+			}
+		}
+
+		public static bool TestCollision(in Point point, in Transform2D transformA, in Rectangle rectangle, in Transform2D transformB)
+		{
+			if (transformB.IsAxisAligned)
+			{
+				return TestPointRectangleOverlap(point, transformA, rectangle, transformB);
+			}
+			else
+			{
+				return FindCollisionSimplex(point, transformA, rectangle, transformB).Item1;
+			}
+		}
+
+		public static bool TestCollision(in Rectangle rectangle, in Transform2D transformA, in Point point, in Transform2D transformB)
+		{
+			return TestCollision(point, transformB, rectangle, transformA);
+		}
+
+		public static bool TestCollision(in Rectangle rectangle, in Transform2D transformA, in Circle circle, in Transform2D transformB)
+		{
+			if (transformA.IsAxisAligned && transformB.IsUniformScale)
+			{
+				return TestCircleRectangleOverlap(circle, transformB, rectangle, transformA);
+			}
+			else
+			{
+				return FindCollisionSimplex(rectangle, transformA, circle, transformB).Item1;
+			}
+		}
+
+		public static bool TestCollision(in Circle circle, in Transform2D transformA, in Rectangle rectangle, in Transform2D transformB)
+		{
+			return TestCollision(rectangle, transformB, circle, transformA);
+		}
+
+		public static bool TestCollision(in Circle circle, in Transform2D transformA, in Point point, in Transform2D transformB)
+		{
+			if (transformA.IsUniformScale)
+			{
+				return TestCirclePointOverlap(circle, transformA, point, transformB);
+			}
+			else
+			{
+				return FindCollisionSimplex(circle, transformA, point, transformB).Item1;
+			}
+		}
+
+		public static bool TestCollision(in Point point, in Transform2D transformA, in Circle circle, in Transform2D transformB)
+		{
+			return TestCollision(circle, transformB, point, transformA);
+		}
+
+		public static bool TestCollision(in Circle circleA, in Transform2D transformA, in Circle circleB, in Transform2D transformB)
+		{
+			if (transformA.IsUniformScale && transformB.IsUniformScale)
+			{
+				return TestCircleOverlap(circleA, transformA, circleB, transformB);
+			}
+			else
+			{
+				return FindCollisionSimplex(circleA, transformA, circleB, transformB).Item1;
+			}
+		}
+
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public static bool TestRectangleOverlap(in Rectangle rectangleA, in Transform2D transformA, in Rectangle rectangleB, in Transform2D transformB)
 		{
 			var firstAABB = rectangleA.TransformedAABB(transformA);
 			var secondAABB = rectangleB.TransformedAABB(transformB);
@@ -75,7 +181,8 @@ namespace MoonWorks.Collision.Fixed
 			return firstAABB.Left < secondAABB.Right && firstAABB.Right > secondAABB.Left && firstAABB.Top < secondAABB.Bottom && firstAABB.Bottom > secondAABB.Top;
 		}
 
-		public static bool TestPointRectangleOverlap(Point point, Transform2D pointTransform, Rectangle rectangle, Transform2D rectangleTransform)
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public static bool TestPointRectangleOverlap(in Point point, in Transform2D pointTransform, in Rectangle rectangle, in Transform2D rectangleTransform)
 		{
 			var transformedPoint = pointTransform.Position;
 			var AABB = rectangle.TransformedAABB(rectangleTransform);
@@ -83,7 +190,8 @@ namespace MoonWorks.Collision.Fixed
 			return transformedPoint.X > AABB.Left && transformedPoint.X < AABB.Right && transformedPoint.Y < AABB.Bottom && transformedPoint.Y > AABB.Top;
 		}
 
-		public static bool TestCirclePointOverlap(Circle circle, Transform2D circleTransform, Point point, Transform2D pointTransform)
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public static bool TestCirclePointOverlap(in Circle circle, in Transform2D circleTransform, in Point point, in Transform2D pointTransform)
 		{
 			var circleCenter = circleTransform.Position;
 			var circleRadius = circle.Radius * circleTransform.Scale.X;
@@ -97,7 +205,8 @@ namespace MoonWorks.Collision.Fixed
 		/// <summary>
 		/// NOTE: The rectangle must be axis aligned, and the scaling of the circle must be uniform.
 		/// </summary>
-		public static bool TestCircleRectangleOverlap(Circle circle, Transform2D circleTransform, Rectangle rectangle, Transform2D rectangleTransform)
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public static bool TestCircleRectangleOverlap(in Circle circle, in Transform2D circleTransform, in Rectangle rectangle, in Transform2D rectangleTransform)
 		{
 			var circleCenter = circleTransform.Position;
 			var circleRadius = circle.Radius * circleTransform.Scale.X;
@@ -113,7 +222,8 @@ namespace MoonWorks.Collision.Fixed
 			return distanceSquared < (circleRadius * circleRadius);
 		}
 
-		public static bool TestCircleOverlap(Circle circleA, Transform2D transformA, Circle circleB, Transform2D transformB)
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		public static bool TestCircleOverlap(in Circle circleA, in Transform2D transformA, in Circle circleB, in Transform2D transformB)
 		{
 			var radiusA = circleA.Radius * transformA.Scale.X;
 			var radiusB = circleB.Radius * transformB.Scale.Y;
@@ -127,9 +237,14 @@ namespace MoonWorks.Collision.Fixed
 			return distanceSquared < radiusSumSquared;
 		}
 
-		public static (bool, Simplex2D) FindCollisionSimplex(IShape2D shapeA, Transform2D transformA, IShape2D shapeB, Transform2D transformB)
+		public static bool TestPointOverlap(in Point pointA, in Transform2D transformA, in Point pointB, in Transform2D transformB)
 		{
-			var minkowskiDifference = new MinkowskiDifference(shapeA, transformA, shapeB, transformB);
+			return transformA.Position == transformB.Position;
+		}
+
+		public static (bool, Simplex2D) FindCollisionSimplex<T, U>(T shapeA, Transform2D transformA, U shapeB, Transform2D transformB) where T : IShape2D where U : IShape2D
+		{
+			var minkowskiDifference = new MinkowskiDifference<T, U>(shapeA, transformA, shapeB, transformB);
 			var c = minkowskiDifference.Support(Vector2.UnitX);
 			var b = minkowskiDifference.Support(-Vector2.UnitX);
 			return Check(minkowskiDifference, c, b);
@@ -208,12 +323,13 @@ namespace MoonWorks.Collision.Fixed
 			};
         }
 
-        private static Vector2 CalculateSupport(IShape2D shapeA, Transform2D Transform2DA, IShape2D shapeB, Transform2D Transform2DB, Vector2 direction)
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
+		private static Vector2 CalculateSupport<T, U>(T shapeA, Transform2D Transform2DA, U shapeB, Transform2D Transform2DB, Vector2 direction) where T : IShape2D where U : IShape2D
         {
             return shapeA.Support(direction, Transform2DA) - shapeB.Support(-direction, Transform2DB);
         }
 
-		private static (bool, Simplex2D) Check(MinkowskiDifference minkowskiDifference, Vector2 c, Vector2 b)
+		private static (bool, Simplex2D) Check<T, U>(MinkowskiDifference<T, U> minkowskiDifference, Vector2 c, Vector2 b) where T : IShape2D where U : IShape2D
         {
             var cb = c - b;
             var c0 = -c;
@@ -221,7 +337,7 @@ namespace MoonWorks.Collision.Fixed
             return DoSimplex(minkowskiDifference, new Simplex2D(b, c), d);
         }
 
-        private static (bool, Simplex2D) DoSimplex(MinkowskiDifference minkowskiDifference, Simplex2D simplex, Vector2 direction)
+        private static (bool, Simplex2D) DoSimplex<T, U>(MinkowskiDifference<T, U> minkowskiDifference, Simplex2D simplex, Vector2 direction) where T : IShape2D where U : IShape2D
         {
             var a = minkowskiDifference.Support(direction);
             var notPastOrigin = Vector2.Dot(a, direction) < Fix64.Zero;
@@ -320,6 +436,7 @@ namespace MoonWorks.Collision.Fixed
             return collinear ? new Vector2(a.Y, -a.X) : d;
         }
 
+		[MethodImpl(MethodImplOptions.AggressiveInlining)]
 		private static bool SameDirection(Vector2 a, Vector2 b)
         {
             return Vector2.Dot(a, b) > Fix64.Zero;

src/Collision/Fixed/Shapes/Circle.cs

@@ -1,4 +1,3 @@
-using System.Collections.Generic;
 using MoonWorks.Math.Fixed;
 
 namespace MoonWorks.Collision.Fixed
@@ -10,13 +9,6 @@ namespace MoonWorks.Collision.Fixed
 	{
 		public Fix64 Radius { get; }
 		public AABB2D AABB { get; }
-		public IEnumerable<IShape2D> Shapes
-		{
-			get
-			{
-				yield return this;
-			}
-		}
 
 		public Circle(Fix64 radius)
 		{

src/Collision/Fixed/Shapes/Line.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using MoonWorks.Math.Fixed;
 
 namespace MoonWorks.Collision.Fixed
@@ -13,14 +13,6 @@ namespace MoonWorks.Collision.Fixed
 
         public AABB2D AABB { get; }
 
-		public IEnumerable<IShape2D> Shapes
-        {
-            get
-            {
-				yield return this;
-			}
-        }
-
 		public Line(Vector2 start, Vector2 end)
         {
             Start = start;

src/Collision/Fixed/Simplex2D.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using MoonWorks.Math.Fixed;
 
 namespace MoonWorks.Collision.Fixed
@@ -54,33 +54,6 @@ namespace MoonWorks.Collision.Fixed
 			}
 		}
 
-		public IEnumerable<Vector2> Vertices
-		{
-			get
-			{
-				yield return (Vector2) a;
-				if (b.HasValue) { yield return (Vector2) b; }
-				if (c.HasValue) { yield return (Vector2) c; }
-			}
-		}
-
-		public Vector2 Support(Vector2 direction, Transform2D transform)
-		{
-			var maxDotProduct = Fix64.MinValue;
-			var maxVertex = a;
-			foreach (var vertex in Vertices)
-			{
-				var transformed = Vector2.Transform(vertex, transform.TransformMatrix);
-				var dot = Vector2.Dot(transformed, direction);
-				if (dot > maxDotProduct)
-				{
-					maxVertex = transformed;
-					maxDotProduct = dot;
-				}
-			}
-			return maxVertex;
-		}
-
 		public void Insert(Vector2 point, int index)
 		{
 			if (index == 0)
@@ -120,7 +93,7 @@ namespace MoonWorks.Collision.Fixed
 
 		public override int GetHashCode()
 		{
-			return System.HashCode.Combine(Vertices);
+			return System.HashCode.Combine(a, b, c);
 		}
 
 		public static bool operator ==(Simplex2D a, Simplex2D b)

src/Collision/Fixed/SpatialHash2D.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 using MoonWorks.Math.Fixed;
 
 namespace MoonWorks.Collision.Fixed
@@ -7,13 +7,12 @@ namespace MoonWorks.Collision.Fixed
 	/// Used to quickly check if two shapes are potentially overlapping.
 	/// </summary>
 	/// <typeparam name="T">The type that will be used to uniquely identify shape-transform pairs.</typeparam>
-	public class SpatialHash2D<T> where T : System.IEquatable<T>
+	public class SpatialHash2D<T, U> where T : struct, System.IEquatable<T> where U : IHasAABB2D
 	{
 		private readonly Fix64 cellSize;
 
 		private readonly Dictionary<long, HashSet<T>> hashDictionary = new Dictionary<long, HashSet<T>>();
-		// FIXME: this ICollidable causes boxing which triggers garbage collection
+		private readonly Dictionary<T, (U, Transform2D, uint)> IDLookup = new Dictionary<T, (U, Transform2D, uint)>();
-		private readonly Dictionary<T, (ICollidable, Transform2D, uint)> IDLookup = new Dictionary<T, (ICollidable, Transform2D, uint)>();
 
 		public int MinX { get; private set; } = 0;
 		public int MaxX { get; private set; } = 0;
@@ -39,9 +38,9 @@ namespace MoonWorks.Collision.Fixed
 		/// <param name="shape"></param>
 		/// <param name="transform2D"></param>
 		/// <param name="collisionGroups">A bitmask value specifying the groups this object belongs to.</param>
-		public void Insert(T id, ICollidable shape, Transform2D transform2D, uint collisionGroups = uint.MaxValue)
+		public void Insert(T id, U shape, Transform2D transform2D, uint collisionGroups = uint.MaxValue)
 		{
-			var box = shape.TransformedAABB(transform2D);
+			var box = AABB2D.Transformed(shape.AABB, transform2D);
 			var minHash = Hash(box.Min);
 			var maxHash = Hash(box.Max);
 
@@ -65,11 +64,9 @@ namespace MoonWorks.Collision.Fixed
 		/// <summary>
 		/// Retrieves all the potential collisions of a shape-transform pair. Excludes any shape-transforms with the given ID.
 		/// </summary>
-		public IEnumerable<(T, ICollidable, Transform2D, uint)> Retrieve(T id, ICollidable shape, Transform2D transform2D, uint collisionMask = uint.MaxValue)
+		public RetrieveEnumerator Retrieve<V>(T id, V hasAABB, Transform2D transform2D, uint collisionMask = uint.MaxValue) where V : IHasAABB2D
 		{
-			var returned = AcquireHashSet();
+			var box = AABB2D.Transformed(hasAABB.AABB, transform2D);
-
-			var box = shape.TransformedAABB(transform2D);
 			var (minX, minY) = Hash(box.Min);
 			var (maxX, maxY) = Hash(box.Max);
 
@@ -78,64 +75,21 @@ namespace MoonWorks.Collision.Fixed
 			if (minY < MinY) { minY = MinY; }
 			if (maxY > MaxY) { maxY = MaxY; }
 
-			foreach (var key in Keys(minX, minY, maxX, maxY))
+			return new RetrieveEnumerator(
-			{
+				this,
-				if (hashDictionary.ContainsKey(key))
+				Keys(minX, minY, maxX, maxY),
-				{
+				id,
-					foreach (var t in hashDictionary[key])
+				collisionMask
-					{
+			);
-						if (!returned.Contains(t))
-						{
-							var (otherShape, otherTransform, collisionGroups) = IDLookup[t];
-							if (!id.Equals(t) && ((collisionGroups & collisionMask) > 0) && AABB2D.TestOverlap(box, otherShape.TransformedAABB(otherTransform)))
-							{
-								returned.Add(t);
-								yield return (t, otherShape, otherTransform, collisionGroups);
-							}
-						}
-					}
-				}
-			}
-
-			FreeHashSet(returned);
 		}
 
 		/// <summary>
 		/// Retrieves all the potential collisions of a shape-transform pair.
 		/// </summary>
-		public IEnumerable<(T, ICollidable, Transform2D, uint)> Retrieve(ICollidable shape, Transform2D transform2D, uint collisionMask = uint.MaxValue)
+		public RetrieveEnumerator Retrieve<V>(V hasAABB, Transform2D transform2D, uint collisionMask = uint.MaxValue) where V : IHasAABB2D
 		{
-			var returned = AcquireHashSet();
+			var box = AABB2D.Transformed(hasAABB.AABB, transform2D);
-
+			return Retrieve(box, collisionMask);
-			var box = shape.TransformedAABB(transform2D);
-			var (minX, minY) = Hash(box.Min);
-			var (maxX, maxY) = Hash(box.Max);
-
-			if (minX < MinX) { minX = MinX; }
-			if (maxX > MaxX) { maxX = MaxX; }
-			if (minY < MinY) { minY = MinY; }
-			if (maxY > MaxY) { maxY = MaxY; }
-
-			foreach (var key in Keys(minX, minY, maxX, maxY))
-			{
-				if (hashDictionary.ContainsKey(key))
-				{
-					foreach (var t in hashDictionary[key])
-					{
-						if (!returned.Contains(t))
-						{
-							var (otherShape, otherTransform, collisionGroups) = IDLookup[t];
-							if (((collisionGroups & collisionMask) > 0) && AABB2D.TestOverlap(box, otherShape.TransformedAABB(otherTransform)))
-							{
-								returned.Add(t);
-								yield return (t, otherShape, otherTransform, collisionGroups);
-							}
-						}
-					}
-				}
-			}
-
-			FreeHashSet(returned);
 		}
 
 		/// <summary>
@@ -143,10 +97,8 @@ namespace MoonWorks.Collision.Fixed
 		/// </summary>
 		/// <param name="aabb">A transformed AABB.</param>
 		/// <returns></returns>
-		public IEnumerable<(T, ICollidable, Transform2D, uint)> Retrieve(AABB2D aabb, uint collisionMask = uint.MaxValue)
+		public RetrieveEnumerator Retrieve(AABB2D aabb, uint collisionMask = uint.MaxValue)
 		{
-			var returned = AcquireHashSet();
-
 			var (minX, minY) = Hash(aabb.Min);
 			var (maxX, maxY) = Hash(aabb.Max);
 
@@ -155,28 +107,14 @@ namespace MoonWorks.Collision.Fixed
 			if (minY < MinY) { minY = MinY; }
 			if (maxY > MaxY) { maxY = MaxY; }
 
-			foreach (var key in Keys(minX, minY, maxX, maxY))
+			return new RetrieveEnumerator(
-			{
+				this,
-				if (hashDictionary.ContainsKey(key))
+				Keys(minX, minY, maxX, maxY),
-				{
+				collisionMask
-					foreach (var t in hashDictionary[key])
+			);
-					{
-						if (!returned.Contains(t))
-						{
-							var (otherShape, otherTransform, collisionGroups) = IDLookup[t];
-							if (((collisionGroups & collisionMask) > 0) && AABB2D.TestOverlap(aabb, otherShape.TransformedAABB(otherTransform)))
-							{
-								yield return (t, otherShape, otherTransform, collisionGroups);
-							}
-						}
-					}
-				}
 		}
 
-			FreeHashSet(returned);
+		public void Update(T id, U shape, Transform2D transform2D, uint collisionGroups = uint.MaxValue)
-		}
-
-		public void Update(T id, ICollidable shape, Transform2D transform2D, uint collisionGroups = uint.MaxValue)
 		{
 			Remove(id);
 			Insert(id, shape, transform2D, collisionGroups);
@@ -189,7 +127,7 @@ namespace MoonWorks.Collision.Fixed
 		{
 			var (shape, transform, collisionGroups) = IDLookup[id];
 
-			var box = shape.TransformedAABB(transform);
+			var box = AABB2D.Transformed(shape.AABB, transform);
 			var minHash = Hash(box.Min);
 			var maxHash = Hash(box.Max);
 
@@ -222,18 +160,12 @@ namespace MoonWorks.Collision.Fixed
 			return ((long) left << 32) | ((uint) right);
 		}
 
-		private IEnumerable<long> Keys(int minX, int minY, int maxX, int maxY)
+		internal static KeysEnumerator Keys(int minX, int minY, int maxX, int maxY)
 		{
-			for (var i = minX; i <= maxX; i++)
+			return new KeysEnumerator(minX, minY, maxX, maxY);
-			{
-				for (var j = minY; j <= maxY; j++)
-				{
-					yield return MakeLong(i, j);
-				}
-			}
 		}
 
-		private HashSet<T> AcquireHashSet()
+		internal HashSet<T> AcquireHashSet()
 		{
 			if (hashSetPool.Count == 0)
 			{
@@ -245,9 +177,147 @@ namespace MoonWorks.Collision.Fixed
 			return hashSet;
 		}
 
-		private void FreeHashSet(HashSet<T> hashSet)
+		internal void FreeHashSet(HashSet<T> hashSet)
 		{
 			hashSetPool.Enqueue(hashSet);
 		}
+
+		internal ref struct KeysEnumerator
+		{
+			private int MinX;
+			private int MinY;
+			private int MaxX;
+			private int MaxY;
+			private int i, j;
+
+			public KeysEnumerator GetEnumerator() => this;
+
+			public KeysEnumerator(int minX, int minY, int maxX, int maxY)
+			{
+				MinX = minX;
+				MinY = minY;
+				MaxX = maxX;
+				MaxY = maxY;
+				i = minX;
+				j = minY - 1;
+			}
+
+			public bool MoveNext()
+			{
+				if (j < MaxY)
+				{
+					j += 1;
+					return true;
+				}
+				else if (i < MaxX)
+				{
+					i += 1;
+					j = MinY;
+					return true;
+				}
+
+				return false;
+			}
+
+			public long Current
+			{
+				get
+				{
+					return MakeLong(i, j);
+				}
+			}
+		}
+
+		public ref struct RetrieveEnumerator
+		{
+			public SpatialHash2D<T, U> SpatialHash;
+			private KeysEnumerator KeysEnumerator;
+			private HashSet<T>.Enumerator HashSetEnumerator;
+			private bool HashSetEnumeratorActive;
+			private HashSet<T> Duplicates;
+			private T? ID;
+			private uint CollisionMask;
+
+			public RetrieveEnumerator GetEnumerator() => this;
+
+			internal RetrieveEnumerator(
+				SpatialHash2D<T, U> spatialHash,
+				KeysEnumerator keysEnumerator,
+				T id,
+				uint collisionMask
+			) {
+				SpatialHash = spatialHash;
+				KeysEnumerator = keysEnumerator;
+				HashSetEnumerator = default;
+				HashSetEnumeratorActive = false;
+				Duplicates = SpatialHash.AcquireHashSet();
+				ID = id;
+				CollisionMask = collisionMask;
+			}
+
+			internal RetrieveEnumerator(
+				SpatialHash2D<T, U> spatialHash,
+				KeysEnumerator keysEnumerator,
+				uint collisionMask
+			) {
+				SpatialHash = spatialHash;
+				KeysEnumerator = keysEnumerator;
+				HashSetEnumerator = default;
+				HashSetEnumeratorActive = false;
+				Duplicates = SpatialHash.AcquireHashSet();
+				ID = null;
+				CollisionMask = collisionMask;
+			}
+
+			public bool MoveNext()
+			{
+				if (!HashSetEnumeratorActive || !HashSetEnumerator.MoveNext())
+				{
+					if (!KeysEnumerator.MoveNext())
+					{
+						SpatialHash.FreeHashSet(Duplicates);
+						return false;
+					}
+
+					if (SpatialHash.hashDictionary.TryGetValue(KeysEnumerator.Current, out var hashset))
+					{
+						HashSetEnumerator = hashset.GetEnumerator();
+						HashSetEnumeratorActive = true;
+					}
+
+					return MoveNext();
+				}
+
+				// conditions
+				var t = HashSetEnumerator.Current;
+				var collisionGroups = SpatialHash.IDLookup[t].Item3;
+
+				if (Duplicates.Contains(t))
+				{
+					return MoveNext();
+				}
+
+				if (ID.HasValue)
+				{
+					if (ID.Value.Equals(t) || (CollisionMask & collisionGroups) == 0)
+					{
+						return MoveNext();
+					}
+				}
+
+				Duplicates.Add(t);
+				return true;
+			}
+
+			public (T, U, Transform2D, uint) Current
+			{
+				get
+				{
+					var t = HashSetEnumerator.Current;
+					var (u, transform, groups) = SpatialHash.IDLookup[t];
+					return (t, u, transform, groups);
+				}
+			}
+		}
 	}
 }