sphere.h

#ifndef SPHERE_H
#define SPHERE_H

#include "hittable.h"
#include "vec3.h"

class sphere : public hittable {
public:
    sphere(const point3& static_center, double radius, shared_ptr<material> mat)
        : center(static_center, vec3(0, 0, 0)), radius(std::fmax(0, radius)), mat(mat)
    {
        auto rvec = vec3(radius, radius, radius);
        bbox = aabb(static_center - rvec, static_center + rvec);
    }

    sphere(const point3& center1, const point3& center2, double radius,
        shared_ptr<material> mat)
        : center(center1, center2 - center1), radius(std::fmax(0, radius)), mat(mat)
    {
        auto rvec = vec3(radius, radius, radius);
        aabb box1(center.at(0) - rvec, center.at(0) + rvec);
        aabb box2(center.at(1) - rvec, center.at(1) + rvec);
        bbox = aabb(box1, box2);
    }

    bool hit(const ray& r, interval ray_t, hit_record& rec) const override {
        point3 current_center = center.at(r.time());
        vec3 oc = current_center - r.origin();

        auto a = r.direction().length_squared();
        auto h = dot(r.direction(), oc);
        auto c = oc.length_squared() - radius * radius;

        auto discriminant = h * h - a * c;
        if (discriminant < 0)
            return false;

        auto sqrtd = sqrt(discriminant);
        auto root = (h - sqrtd) / a;
        if (!ray_t.contains(root)) {
            root = (h + sqrtd) / a;
            if (!ray_t.contains(root))
                return false;
        }

        rec.t = root;
        rec.p = r.at(rec.t);
        vec3 outward_normal = (rec.p - current_center) / radius;
        rec.set_face_normal(r, outward_normal);
        get_sphere_uv(outward_normal, rec.u, rec.v);
        rec.mat = mat;

        return true;
    }


    aabb bounding_box() const override { return bbox; }
private:
    static void get_sphere_uv(const point3& p, double& u, double& v) {
        auto theta = std::acos(-p.y());
        auto phi = std::atan2(-p.z(), p.x()) + pi;
        u = phi / (2 * pi);
        v = theta / pi;
    }
    ray center; 
    double radius;
    shared_ptr<material> mat;
    aabb bbox;
};

#endif