tuples: implement reflect.

rtchallenge/src/tuples.rs

@@ -46,6 +46,28 @@ impl Tuple {
     }
 }
 
+/// Reflects vector v across normal n.
+///
+/// # Examples
+/// ```
+/// use rtchallenge::tuples::{reflect, Tuple};
+///
+/// // Reflecting a vector approaching at 45°
+/// let v = Tuple::vector(1., -1., 0.);
+/// let n = Tuple::vector(0., 1., 0.);
+/// let r = reflect(v, n);
+/// assert_eq!(r, Tuple::vector(1., 1., 0.));
+///
+/// // Reflecting off a slanted surface.
+/// let v = Tuple::vector(0., -1., 0.);
+/// let n = Tuple::vector(2_f32.sqrt() / 2., 2_f32.sqrt() / 2., 0.);
+/// let r = reflect(v, n);
+/// assert_eq!(r, Tuple::vector(1., 0., 0.));
+/// ```
+pub fn reflect(v: Tuple, n: Tuple) -> Tuple {
+    v - n * 2. * dot(v, n)
+}
+
 impl Add for Tuple {
     type Output = Self;
     fn add(self, other: Self) -> Self {