Fix hit determination inside a sphere

src/hittable.rs

@@ -42,16 +42,16 @@ impl Hittable for Sphere {
         let discriminant = half_b * half_b - a * c;
         if discriminant < 0.0 { return None; }
 
-        let sqrtd = f64::sqrt(discriminant);
-        let root = (-half_b - sqrtd) / a;
+        let sqrtd = discriminant.sqrt();
+        let mut root = (-half_b - sqrtd) / a;
         if root < t_min || t_max < root {
-            let root = (-half_b + sqrtd) / a;
+            root = (-half_b + sqrtd) / a;
             if root < t_min || t_max < root { return None; }
         }
-
         let point = ray.at(root);
         let normal = (point - self.center) / self.radius;
-        let front_face = ray.direction().dot(&normal) < 0.0;
+        let dot = ray.direction().dot(&normal);
+        let front_face = dot < 0.0;
         let normal = if front_face { normal } else { -normal };
         Some(HitRecord {
             point,