spheres: add transform member to Sphere

rtchallenge/src/spheres.rs

@@ -1,12 +1,37 @@
 use crate::{
     intersections::Intersection,
+    matrices::Matrix4x4,
     rays::Ray,
     tuples::{dot, Tuple},
 };
 
-#[derive(Default, Debug, PartialEq)]
+#[derive(Debug, PartialEq)]
 /// Sphere represents the unit-sphere (radius of unit 1.) at the origin 0., 0., 0.
-pub struct Sphere {}
+pub struct Sphere {
+    pub transform: Matrix4x4,
+}
+
+impl Default for Sphere {
+    /// # Examples
+    /// ```
+    /// use rtchallenge::{matrices::Matrix4x4, spheres::Sphere};
+    ///
+    /// // A sphere's default transform is the identity matrix.
+    /// let s = Sphere::default();
+    /// assert_eq!(s.transform, Matrix4x4::identity());
+    ///
+    /// // It can be changed by directly setting the transform member.
+    /// let mut s = Sphere::default();
+    /// let t = Matrix4x4::translate(2., 3., 4.);
+    /// s.transform = t.clone();
+    /// assert_eq!(s.transform, t);
+    /// ```
+    fn default() -> Sphere {
+        Sphere {
+            transform: Matrix4x4::identity(),
+        }
+    }
+}
 
 /// Intersect a ray with a sphere.
 ///