std::vector<std::unique_ptr<graph_state>> get_successors() const override {
        std::vector<std::unique_ptr<graph_state>> r;
        std::vector<move_direction> possibilities = {
                move_direction::UP,
                move_direction::DOWN,
                move_direction::LEFT,
                move_direction::RIGHT,
        };

        for (move_direction n : possibilities) {
            auto state_copy = clone();
            if(!dynamic_cast<sliding_puzzle &>(*state_copy).make_move(n))
                continue;
            state_copy->update_score(state_copy->get_g() + 1);
            state_copy->set_parent(this);
            r.push_back(std::move(state_copy));
        }

        return r;
    }