Make PhysicsHull a ref struct (#5297)

* Make PhysicsHull a ref struct

First time I've used it but seemed like a good candidate considering it's temporary.

* weh

---------

Co-authored-by: Pieter-Jan Briers <pieterjan.briers+git@gmail.com>

RELEASE-NOTES.md

@@ -36,6 +36,7 @@ END TEMPLATE-->
 ### Breaking changes
 
 * Fixes large entities causing entity spawn menu to break.
+* Made PhysicsHull an internal ref struct for some PolygonShape speedup.
 
 ### New features
 

Robust.Shared/Physics/Collision/PhysicsHull.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Numerics;
+using System.Runtime.CompilerServices;
 using Robust.Shared.Maths;
 using Robust.Shared.Utility;
 
@@ -8,11 +9,17 @@ namespace Robust.Shared.Physics;
 /// <summary>
 /// Convex hull used for poly collision.
 /// </summary>
-public record struct PhysicsHull()
+internal ref struct PhysicsHull()
 {
-    public readonly Vector2[] Points = new Vector2[PhysicsConstants.MaxPolygonVertices];
+    public Span<Vector2> Points;
     public int Count;
 
+    public PhysicsHull(Span<Vector2> vertices, int count) : this()
+    {
+        Count = count;
+        Points = vertices[..count];
+    }
+
     private static PhysicsHull RecurseHull(Vector2 p1, Vector2 p2, Span<Vector2> ps, int count)
     {
         PhysicsHull hull = new();
@@ -58,6 +65,7 @@ public record struct PhysicsHull()
             return hull;
         }
 
+        hull.Points = new Vector2[PhysicsConstants.MaxPolygonVertices];
         var bestPoint = ps[bestIndex];
 
         // compute hull to the right of p1-bestPoint
@@ -93,7 +101,8 @@ public record struct PhysicsHull()
         PhysicsHull hull = new();
 
         if (count is < 3 or > PhysicsConstants.MaxPolygonVertices)
-	    {
+        {
+            hull.Count = 0;
             DebugTools.Assert(false);
 		    // check your data
 		    return hull;
@@ -206,11 +215,14 @@ public record struct PhysicsHull()
 	    var hull2 = RecurseHull(p2, p1, leftPoints, leftCount);
 
 	    if (hull1.Count == 0 && hull2.Count == 0)
-	    {
+        {
+            hull.Count = 0;
 		    // all points collinear
 		    return hull;
 	    }
 
+        hull.Points = new Vector2[PhysicsConstants.MaxPolygonVertices];
+
 	    // stitch hulls together, preserving CCW winding order
 	    hull.Points[hull.Count++] = p1;
 

Robust.Shared/Physics/Collision/Shapes/PolygonShape.cs

@@ -23,6 +23,7 @@
 using System;
 using System.Collections.Generic;
 using System.Numerics;
+using System.Runtime.InteropServices;
 using Robust.Shared.Configuration;
 using Robust.Shared.IoC;
 using Robust.Shared.Maths;
@@ -61,18 +62,12 @@ namespace Robust.Shared.Physics.Collision.Shapes
         /// <summary>
         /// The radius of this polygon.
         /// </summary>
-        [DataField("radius"), Access(typeof(SharedPhysicsSystem), Friend = AccessPermissions.ReadWriteExecute, Other = AccessPermissions.Read)]
+        [DataField, Access(typeof(SharedPhysicsSystem), Friend = AccessPermissions.ReadWriteExecute, Other = AccessPermissions.Read)]
         public float Radius { get; set; } = PhysicsConstants.PolygonRadius;
 
         public bool Set(List<Vector2> vertices)
         {
-            Span<Vector2> verts = stackalloc Vector2[vertices.Count];
-
-            for (var i = 0; i < vertices.Count; i++)
-            {
-                verts[i] = vertices[i];
-            }
-
+            var verts = CollectionsMarshal.AsSpan(vertices);
             return Set(verts, vertices.Count);
         }
 
@@ -91,7 +86,7 @@ namespace Robust.Shared.Physics.Collision.Shapes
             return true;
         }
 
-        public void Set(PhysicsHull hull)
+        internal void Set(PhysicsHull hull)
         {
             DebugTools.Assert(hull.Count >= 3);
             var vertexCount = hull.Count;
@@ -188,17 +183,16 @@ namespace Robust.Shared.Physics.Collision.Shapes
             Set(Vertices.AsSpan(), VertexCount);
         }
 
-        public bool Set(Box2Rotated bounds)
+        public void Set(Box2Rotated bounds)
         {
-            Span<Vector2> vertices = stackalloc Vector2[]
-            {
-                bounds.BottomLeft,
-                bounds.BottomRight,
-                bounds.TopRight,
-                bounds.TopLeft,
-            };
+            Span<Vector2> verts = stackalloc Vector2[4];
+            verts[0] = bounds.BottomLeft;
+            verts[1] = bounds.BottomRight;
+            verts[2] = bounds.TopRight;
+            verts[3] = bounds.TopLeft;
 
-            return Set(vertices, 4);
+            var hull = new PhysicsHull(verts, 4);
+            Set(hull);
         }
 
         public void SetAsBox(Box2 box)