template <typename Iterator, typename Func>
void
parallel_for_each(Iterator first, Iterator last, Func f)
{
  unsigned long const length = std::distance(first, last);

  if (!length)
    return;

  unsigned long const min_per_thread = 25;

  if (length < (2 * min_per_thread)) {
    std::for_each(first, last, f);
  }
  else {
    Iterator const mid_point = first + length / 2;
    std::future<void> first_half =
        std::async(&parallel_for_each<Iterator, Func>, first, mid_point, f);
    parallel_for_each(mid_point, last, f);
    first_half.get();
  }
}