#include "material.h"

#include <math.h>
#include <stdio.h>

static vec3_t reflect(vec3_t v, vec3_t n)
{
	const vec3_t dn = vec3_scale(n, -2 * vec3_dot(n, v));
	return vec3_unit(vec3_add(v, dn));
}

static vec3_t refract(vec3_t v, vec3_t n, double eta_ratio)
{
	const double cos_theta = fmin(-vec3_dot(v, n), 1.0);
	const vec3_t perp
	    = vec3_scale(vec3_add(v, vec3_scale(n, cos_theta)), eta_ratio);
	const vec3_t para
	    = vec3_scale(n, -sqrt(fabs(1.0 - vec3_len_squared(perp))));
	return vec3_unit(vec3_add(perp, para));
}

bool scatter_lambertian(
    material_params_t params, hit_t hit, rng_t *rng, ray_t *ray,
    vec3_t *atten_out)
{
	const vec3_t dir = vec3_unit(vec3_add(hit.normal, rng_vec3(rng)));
	ray->orig = hit.point;
	ray->dir = vec3_len(dir) == 0 ? hit.normal : dir;
	*atten_out = params.lambertian.albedo;
	return true;
}

bool scatter_reflective(
    material_params_t params, hit_t hit, rng_t *rng, ray_t *ray,
    vec3_t *atten_out)
{
	vec3_t reflected = reflect(ray->dir, hit.normal);
	if (params.reflective.fuzz != 0.0) {
		const double weight = params.reflective.fuzz * rng_canon(rng);
		const vec3_t scatter = vec3_scale(rng_vec3(rng), weight);
		reflected = vec3_unit(vec3_add(reflected, scatter));
	}

	ray->orig = hit.point;
	ray->dir = reflected;
	*atten_out = params.reflective.tint;
	return true;
}

bool scatter_dielectric(
    material_params_t params, hit_t hit, rng_t *rng, ray_t *ray,
    vec3_t *atten_out)
{
	const double eta_ratio
	    = hit.front ? 1.0 / params.dielectric.eta : params.dielectric.eta;
	const vec3_t n = hit.front ? hit.normal : vec3_scale(hit.normal, -1);
	const double cos_theta = fmin(-vec3_dot(ray->dir, n), 1.0);
	const double sin_theta = sqrt(1.0 - cos_theta * cos_theta);
	const double r = pow((1 - eta_ratio) / (1 + eta_ratio), 2);
	const double reflectance = r + (1 - r) * pow(1 - cos_theta, 5);

	ray->orig = hit.point;
	if (sin_theta * eta_ratio > 1.0 || rng_canon(rng) < reflectance)
		ray->dir = reflect(ray->dir, n);
	else
		ray->dir = refract(ray->dir, n, eta_ratio);
	*atten_out = (vec3_t) { 1.0, 1.0, 1.0 };

	return true;
}