From a5dd1755d95b2053f87bb87fc76383e5c90e0869 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Andr=C3=A9=20Jaenisch?= <andre.jaenisch@posteo.de>
Date: Sun, 15 Aug 2021 14:01:44 +0200
Subject: [PATCH] feat: add new function to rotate a vector
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Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>
---
 src/js/vector.js       | 18 ++++++++++++++++++
 test/js/vector.test.js | 17 +++++++++++++++++
 types/vector.d.ts      |  8 ++++++++
 3 files changed, 43 insertions(+)

diff --git a/src/js/vector.js b/src/js/vector.js
index 5636623..1bc573d 100644
--- a/src/js/vector.js
+++ b/src/js/vector.js
@@ -100,3 +100,21 @@ export function distance (v, w) {
 export function normalize (v) {
   return scale(v, 1 / (length(v) || 1))
 }
+
+/**
+ * Rotate a vector around a given centre for a given angle in radians.
+ *
+ * @param {Vector2D} v
+ * @param {Vector2D} centre
+ * @param {number} angle
+ */
+export function rotate (v, centre, angle) {
+  const { cos, sin } = Math
+  const x = v.x - centre.x
+  const y = v.y - centre.y
+
+  return Vec2(
+    x * cos(angle) - y * sin(angle) + centre.x,
+    x * sin(angle) + y * cos(angle) + centre.y
+  )
+}
diff --git a/test/js/vector.test.js b/test/js/vector.test.js
index d88c52d..61fab92 100644
--- a/test/js/vector.test.js
+++ b/test/js/vector.test.js
@@ -7,6 +7,7 @@ import {
   dot,
   length,
   normalize,
+  rotate,
   scale,
   subtract,
   Vec2,
@@ -140,4 +141,20 @@ describe('Vector library', function () {
       expect(normalized.y).to.be.approximately(0.8, 0.001)
     })
   })
+
+  describe('rotate', function () {
+    it('should rotate a shape around its centre on given angle', function () {
+      // Arrange
+      const v = Vec2(1, 0)
+      const c = Vec2(0, 0)
+      const angle = Math.PI
+
+      // Act
+      const rotated = rotate(v, c, angle)
+
+      // Assert
+      expect(rotated.x).to.be.approximately(-1, 0.001)
+      expect(rotated.y).to.be.approximately(0, 0.001)
+    })
+  })
 })
diff --git a/types/vector.d.ts b/types/vector.d.ts
index 1a2b584..b148fe7 100644
--- a/types/vector.d.ts
+++ b/types/vector.d.ts
@@ -73,6 +73,14 @@ export function distance(v: Vector2D, w: Vector2D): number;
  * @returns {Vector2D}
  */
 export function normalize(v: Vector2D): Vector2D;
+/**
+ * Rotate a vector around a given centre for a given angle in radians.
+ *
+ * @param {Vector2D} v
+ * @param {Vector2D} centre
+ * @param {number} angle
+ */
+export function rotate(v: Vector2D, centre: Vector2D, angle: number): Vector2D;
 export type Vector2D = {
     x: number;
     y: number;