feat: add new function to compute face normals

Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>

src/js/shape.js

@@ -1,5 +1,5 @@
 import { gravity } from './constants.js'
-import { Vec2 } from './vector.js'
+import { normalize, subtract, Vec2 } from './vector.js'
 
 /** @typedef {import('./vector.js').Vector2D} Vector2D
 
@@ -67,5 +67,19 @@ export function RigidShape ({
     ]
   }
 
+  computeNormals(shape)
   return shape
 }
+
+/**
+ * Compute face normals
+ *
+ * @param {Shape} shape
+ */
+export function computeNormals (shape) {
+  shape.N.forEach(function (normal, index) {
+    shape.N[index] = normalize(
+      subtract(shape.X[(index + 1) % 4], shape.X[(index + 2) % 4])
+    )
+  })
+}

test/js/shape.test.js

@@ -1,6 +1,6 @@
 import { expect } from 'chai'
 
-import { RigidShape } from '../../src/js/shape.js'
+import { computeNormals, RigidShape } from '../../src/js/shape.js'
 import { Vec2 } from '../../src/js/vector.js'
 
 describe('RigidShape', function () {
@@ -28,4 +28,39 @@ describe('RigidShape', function () {
     // Assert
     expect(shape).to.be.an('object')
   })
+
+  describe('computeNormals', function () {
+    it('should compute the face normals of a shape', function () {
+      // Arrange
+      const center = Vec2(500, 200)
+      const friction = 20
+      const restitution = 0
+      const mass = 400
+      const bounds = 1
+      const width = 5
+      const height = 5
+
+      const shape = RigidShape({
+        center,
+        mass,
+        friction,
+        restitution,
+        bounds,
+        width,
+        height
+      })
+      const originalFaces = shape.N.map(function (face) {
+        return {...face}
+      })
+
+      // Act
+      computeNormals(shape)
+
+      // Assert
+      shape.N.forEach(function (face, index) {
+	const originalFace = originalFaces[index]
+        expect(face).to.not.deep.equal(originalFace)
+      })
+    })
+  })
 })

types/shape.d.ts

@@ -39,6 +39,12 @@ export function RigidShape({ center, mass, friction, restitution, bounds, width,
     width: number;
     height: number;
 }): Shape;
+/**
+ * Compute face normals
+ *
+ * @param {Shape} shape
+ */
+export function computeNormals(shape: Shape): void;
 /**
  * /**
  */