struct task_system
{
  task_system(int start, int end, int step):
    n_tasks(static_cast<int>(
      std::floor((end-start)/step)
    )),
    _barrier(n_tasks)
  {
  }

  template<typename F>
  void call(const F & f)
  {
    std::vector<F> vec(n_tasks, f);
    for(int i = 0; i < n_tasks; ++i) {
      vec[i](*this, i);
    }
  }
private:
  int n_tasks;
};

template<typename F>
void spawn(int start, int end, int step, const F & f)
{
  task_system tasks(start, end, step);
  tasks.call(f);
}