#include "bvh.h"

#include <algorithm>
#include <functional>

bool Bvh_node::bounding_box(double t0, double t1, Aabb& output_box) const {
    output_box = box;
    return true;
}

bool Bvh_node::hit(const Ray& r, double t_min, double t_max, hit_record& rec) const {
    if (!box.hit(r, t_min, t_max))
        return false;

    bool hit_left = left->hit(r, t_min, t_max, rec);
    bool hit_right = right->hit(r, t_min, hit_left ? rec.t : t_max, rec);

    return hit_left || hit_right;
}

bool box_compare(const std::shared_ptr<Hittable> a, const std::shared_ptr<Hittable> b, int axis, double t0, double t1) {
    Aabb box_a;
    Aabb box_b;

    if (!a->bounding_box(t0, t1, box_a) || !b->bounding_box(t0, t1, box_b)) {
        std::cerr << "No bounding box in Bvh_node constructor.\n";
        std::exit(1);
    }

    return box_a.min()[axis] < box_b.min()[axis];
}

Bvh_node::Bvh_node(std::vector<std::shared_ptr<Hittable>>& objects, int start, int end, double time0, double time1) {
    using namespace std::placeholders;

    int axis = random_int(0, 2);
    auto comparator = std::bind(box_compare, _1, _2, axis, time0, time1);

    int object_span = end - start;

    if (object_span == 1) {
        left = right = objects[start];
    } else if (object_span == 2) {
        if (comparator(objects[start], objects[start+1])) {
            left = objects[start];
            right = objects[start+1];
        } else {
            left = objects[start+1];
            right = objects[start];
        }
    } else {
        std::sort(objects.begin() + start, objects.begin() + end, comparator);

        auto mid = start + object_span/2;
        left = std::make_shared<Bvh_node>(objects, start, mid, time0, time1);
        right = std::make_shared<Bvh_node>(objects, mid, end, time0, time1);
    }

    Aabb box_left, box_right;

    if (!left->bounding_box(time0, time1, box_left) || !right->bounding_box(time0, time1, box_right)) {
        std::cerr << "No bounding box in Bvh_node constructor.\n";
        std::exit(1);
    }

    box = surrounding_box(box_left, box_right);
}