intersections: implement basic prepare_computations.

rtchallenge/src/intersections.rs

@@ -1,6 +1,6 @@
 use std::ops::Index;
 
-use crate::spheres::Sphere;
+use crate::{rays::Ray, spheres::Sphere, tuples::Tuple};
 
 #[derive(Debug, Clone, PartialEq)]
 pub struct Intersection<'i> {
@@ -129,3 +129,43 @@ impl<'i> Index<usize> for Intersections<'i> {
         &self.0[idx]
     }
 }
+
+pub struct PrecomputedData<'i> {
+    pub t: f32,
+    pub object: &'i Sphere,
+    pub point: Tuple,
+    pub eyev: Tuple,
+    pub normalv: Tuple,
+}
+
+/// Precomputes data common to all intersections.
+///
+/// # Examples
+/// ```
+/// use rtchallenge::{
+///     intersections::{prepare_computations, Intersection, Intersections},
+///     rays::Ray,
+///     spheres::{intersect, Sphere},
+///     tuples::Tuple,
+/// };
+///
+/// let r = Ray::new(Tuple::point(0., 0., -5.), Tuple::vector(0., 0., 1.));
+/// let shape = Sphere::default();
+/// let i = Intersection::new(4., &shape);
+/// let comps = prepare_computations(&i, &r);
+/// assert_eq!(comps.t, i.t);
+/// assert_eq!(comps.object, i.object);
+/// assert_eq!(comps.point, Tuple::point(0., 0., -1.));
+/// assert_eq!(comps.eyev, Tuple::vector(0., 0., -1.));
+/// assert_eq!(comps.normalv, Tuple::vector(0., 0., -1.));
+/// ```
+pub fn prepare_computations<'i>(i: &'i Intersection, r: &Ray) -> PrecomputedData<'i> {
+    let point = r.position(i.t);
+    PrecomputedData {
+        t: i.t,
+        object: i.object,
+        point,
+        eyev: -r.direction,
+        normalv: i.object.normal_at(point),
+    }
+}