feat(math): Add Vec2.rotate()

src/math/Vec2.js

@@ -474,7 +474,7 @@ export class Vec2 {
 	}
 
 	/**
-	 * Computes the cross product between this and another vector and changes the value of this vector.
+	 * Computes the cross product between this and another vector.
 	 *
 	 * [Cross product visualisation](https://www.geogebra.org/m/psMTGDgc) (in 3d)
 	 *
@@ -525,6 +525,36 @@ export class Vec2 {
 		return this.set(other);
 	}
 
+	/**
+	 * Rotates the vector (around 0, 0) and modifies it.
+	 * Positive angles rotate the vector clockwise. (Positive x is right, positive y is down)
+	 *
+	 * For instance, say you have a `Vec2<1, 1>`:
+	 * ```none
+	 * o
+	 *  \
+	 *   \
+	 *    V
+	 * ```
+	 * then calling `rotate(Math.PI * 0.5)` results in a `Vec2<-1, 1>`:
+	 * ```none
+	 *    o
+	 *   /
+	 *  /
+	 * V
+	 * ```
+	 * @param {number} angle Angle in radians.
+	 */
+	rotate(angle) {
+		const cos = Math.cos(angle);
+		const sin = Math.sin(angle);
+
+		return this.set(
+			this._x * cos - this._y * sin,
+			this._x * sin + this._y * cos,
+		);
+	}
+
 	/**
 	 * @returns {[x: number, y: number]}
 	 */

test/unit/src/math/Vec2.test.js

@@ -736,6 +736,48 @@ Deno.test({
 	},
 });
 
+Deno.test({
+	name: "rotate()",
+	fn() {
+		const tests = [
+			{ vec: [0, 1], angle: 0, result: [0, 1] },
+			{ vec: [0, 1], angle: Math.PI, result: [0, -1] },
+			{ vec: [1, 0], angle: Math.PI * 0.5, result: [0, 1] },
+			{ vec: [1, 1], angle: Math.PI * 0.5, result: [-1, 1] },
+			{ vec: [0, 0], angle: 100, result: [0, 0] },
+			{ vec: [12, 34], angle: Math.PI * 2, result: [12, 34] },
+			{ vec: [12, 34], angle: 0.1, result: [8.54571, 35.02814] },
+		];
+
+		for (const { vec, angle, result } of tests) {
+			const actual = new Vec2(vec).rotate(angle);
+
+			assertVecAlmostEquals(actual, result);
+		}
+	},
+});
+
+Deno.test({
+	name: "rotate() angle matches the direction of clockwiseAngleTo",
+	fn() {
+		const tests = [
+			[0.123, -0.456],
+			[0.5, 2],
+			[-10, 20],
+			[-10, -20],
+		];
+
+		for (const test of tests) {
+			const vec = new Vec2(test);
+			const newVec = new Vec2(0, 1);
+			const angle = newVec.clockwiseAngleTo(vec);
+			newVec.rotate(angle);
+			newVec.magnitude = vec.magnitude;
+			assertVecAlmostEquals(newVec, test);
+		}
+	},
+});
+
 // ======== onChange Callbacks ========
 
 Deno.test({
@@ -894,6 +936,17 @@ Deno.test({
 		fireResults.push(-1);
 		expectedResult.push(-1);
 
+		vec.set(0, 1);
+		expectedResult.push(0x11);
+
+		vec.rotate(0);
+		// rotating 0 radians shouldn't fire the callback
+		fireResults.push(-1);
+		expectedResult.push(-1);
+
+		vec.rotate(0.5);
+		expectedResult.push(0x11);
+
 		assertEquals(fireResults, expectedResult);
 	},
 });