From e4893a9f2efc6d7d1b9a14f3420e0b311960a057 Mon Sep 17 00:00:00 2001
From: Erik Onarheim <erik.onarheim@gmail.com>
Date: Fri, 4 Oct 2024 21:43:58 -0500
Subject: [PATCH] fix: [#3205] Prevent degenerate PolygonColliders

closes: #3205
---
 CHANGELOG.md                                      |  1 +
 src/engine/Collision/Colliders/PolygonCollider.ts |  6 +++++-
 src/spec/CollisionShapeSpec.ts                    | 15 ++++++++++++---
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c836953dc..cf837245a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -104,6 +104,7 @@ are doing mtv adjustments during precollision.
 
 ### Fixed
 
+- Fixed issue where `ex.PolygonColliders` would get trapped in infinite loop for degenerate polygons (< 3 vertices)
 - Fixed issue where certain devices that support large numbers of texture slots exhaust the maximum number of if statements (complexity) in the shader.
 - Fixed issue where `ex.Label` where setting the opacity of caused a multiplicative opacity effect when actor opacity set
 - Fixed issue where the `ex.Loader` would have a low res logo on small configured resolution sizes
diff --git a/src/engine/Collision/Colliders/PolygonCollider.ts b/src/engine/Collision/Colliders/PolygonCollider.ts
index 41429fb51..91167064b 100644
--- a/src/engine/Collision/Colliders/PolygonCollider.ts
+++ b/src/engine/Collision/Colliders/PolygonCollider.ts
@@ -23,6 +23,7 @@ export interface PolygonColliderOptions {
   offset?: Vector;
   /**
    * Points in the polygon in order around the perimeter in local coordinates. These are relative from the body transform position.
+   * **Must be at least 3 points**
    */
   points: Vector[];
 
@@ -61,6 +62,9 @@ export class PolygonCollider extends Collider {
    * Excalibur stores these in counter-clockwise order
    */
   public set points(points: Vector[]) {
+    if (points.length < 3) {
+      throw new Error('PolygonCollider cannot be created with less that 3 points');
+    }
     this._points = points;
     this._checkAndUpdateWinding(this._points);
     this._calculateNormals();
@@ -97,7 +101,7 @@ export class PolygonCollider extends Collider {
     this.offset = options.offset ?? Vector.Zero;
     this._transform.pos.x += this.offset.x;
     this._transform.pos.y += this.offset.y;
-    this.points = options.points ?? [];
+    this.points = options.points;
 
     if (!this.isConvex()) {
       if (!options.suppressConvexWarning) {
diff --git a/src/spec/CollisionShapeSpec.ts b/src/spec/CollisionShapeSpec.ts
index 9f69a1568..8183235a7 100644
--- a/src/spec/CollisionShapeSpec.ts
+++ b/src/spec/CollisionShapeSpec.ts
@@ -474,21 +474,30 @@ describe('Collision Shape', () => {
 
     it('can be constructed with empty args', () => {
       const poly = new ex.PolygonCollider({
-        points: [ex.Vector.One]
+        points: [ex.Vector.One, ex.Vector.Zero, ex.Vector.Right]
       });
       expect(poly).not.toBe(null);
     });
 
+    it('does not allow degenerate polygons', () => {
+      const action = () => {
+        const poly = new ex.PolygonCollider({
+          points: [ex.Vector.One]
+        });
+      };
+      expect(action).toThrowError('PolygonCollider cannot be created with less that 3 points');
+    });
+
     it('can be cloned', () => {
       const actor1 = new ex.Actor({ x: 0, y: 0, width: 20, height: 20 });
-      const poly = actor1.collider.usePolygonCollider([ex.Vector.One, ex.Vector.Half], new ex.Vector(20, 25));
+      const poly = actor1.collider.usePolygonCollider([ex.Vector.One, ex.Vector.Half, ex.Vector.Right], new ex.Vector(20, 25));
 
       const sut = poly.clone();
 
       expect(sut).not.toBe(poly);
       expect(sut.offset).toBeVector(poly.offset);
       expect(sut.offset).not.toBe(poly.offset);
-      expect(sut.points.length).toBe(2);
+      expect(sut.points.length).toBe(3);
     });
 
     it('can be constructed with points', () => {