add hittable spheres

in_one_weekend/src/hittable.zig

@@ -0,0 +1,21 @@
+const Point3 = @import("vec3.zig").Point3;
+const Vec3 = @import("vec3.zig").Vec3;
+const Ray = @import("ray.zig").Ray;
+
+pub const HitRecord = struct {
+    p: Point3,
+    normal: Vec3,
+    t: f32,
+    front_face: bool,
+
+    pub fn setFaceNormal(hit: *HitRecord, ray: Ray, outward_normal: Vec3) void {
+        const front_face = Vec3.dot(ray.direction, outward_normal) < 0;
+        hit.*.front_face = front_face;
+        if (front_face) {
+            hit.*.normal = outward_normal;
+        } else {
+            const origin = Vec3{};
+            hit.*.normal = origin.sub(outward_normal);
+        }
+    }
+};