#include "obj.h"

#include <math.h>

bool intersect_sphere(
    obj_params_t params, ray_t ray, hit_t *hit_out, double t_min,
    double t_max)
{
	const vec3_t centre = params.sphere.centre;
	const double r = params.sphere.radius;
	const vec3_t disp = vec3_sub(centre, ray.orig);

	const double a = vec3_dot(ray.dir, ray.dir);
	const double h = vec3_dot(ray.dir, disp);
	const double c = vec3_dot(disp, disp) - r * r;

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

	const double sqrtd = sqrt(discriminant);
	const double low = (h - sqrtd) / a;
	const double t = low > t_min ? low : (h + sqrtd) / a;
	if (t < t_min || t_max < t)
		return false;

	const vec3_t point = vec3_add(ray.orig, vec3_scale(ray.dir, t));
	const vec3_t normal = vec3_unit(vec3_sub(point, centre));

	hit_out->point = point;
	hit_out->normal = normal;
	hit_out->t = t;
	hit_out->front = vec3_dot(ray.dir, normal) < 0.0;
	return true;
}