math.vec: cleanup angle_between implementation, add more tests

vlib/math/vec/vec2.v

@@ -316,13 +316,9 @@ pub fn (v Vec2[T]) manhattan_distance(u Vec2[T]) T {
 // angle_between returns the angle in radians to the vector `u`.
 pub fn (v Vec2[T]) angle_between(u Vec2[T]) T {
 	$if T is f64 {
-		return math.atan2(u.y * v.x - u.x * v.y, u.x * v.x - u.y * v.y)
+		return math.atan2(v.cross(u), v.dot(u))
 	} $else {
-		vx := f64(v.x)
-		vy := f64(v.y)
-		ux := f64(u.x)
-		uy := f64(u.y)
-		return T(math.atan2(uy * vx - ux * vy, ux * vx - uy * vy))
+		return T(math.atan2(f64(v.cross(u)), f64(v.dot(u))))
 	}
 }
 

vlib/math/vec/vec2_test.v

@@ -92,4 +92,13 @@ fn test_vec2_angle_between() {
 	assert vec.vec2(1.0, 0.0).angle_between(vec.vec2(-1.0, -1.0)) == math.pi * 5.0 / 4.0 - 2 * math.pi
 	assert vec.vec2(1.0, 0.0).angle_between(vec.vec2(0.0, -1.0)) == math.pi * 6.0 / 4.0 - 2 * math.pi
 	assert vec.vec2(1.0, 0.0).angle_between(vec.vec2(1.0, -1.0)) == math.pi * 7.0 / 4.0 - 2 * math.pi
+
+	// check the angle between non-normalised vectors too:
+	assert vec.vec2(5.0, 0.0).angle_between(vec.vec2(2.0, 2.0)) == math.pi * 1.0 / 4.0
+	assert vec.vec2(5.0, 0.0).angle_between(vec.vec2(0.0, 2.0)) == math.pi * 2.0 / 4.0
+	assert vec.vec2(5.0, 0.0).angle_between(vec.vec2(-2.0, 2.0)) == math.pi * 3.0 / 4.0
+	assert vec.vec2(5.0, 0.0).angle_between(vec.vec2(-2.0, 0.0)) == math.pi
+	assert vec.vec2(5.0, 0.0).angle_between(vec.vec2(-2.0, -2.0)) == math.pi * 5.0 / 4.0 - 2 * math.pi
+	assert vec.vec2(5.0, 0.0).angle_between(vec.vec2(0.0, -2.0)) == math.pi * 6.0 / 4.0 - 2 * math.pi
+	assert vec.vec2(5.0, 0.0).angle_between(vec.vec2(2.0, -2.0)) == math.pi * 7.0 / 4.0 - 2 * math.pi
 }