template <typename Iterator, typename T>
struct accumulate_block {
  T operator()(Iterator first, Iterator last)
  {
    return std::accumulate(first, last, T());
  }
};

template <typename Iterator, typename T>
T
parallel_accumulate(Iterator first, Iterator last, T init)
{
  unsigned long const length = std::distance(first, last);

  if (!length)
    return init;

  unsigned long const min_per_thread = 25;
  unsigned long const max_threads =
      (length + min_per_thread - 1) / min_per_thread;

  unsigned long const hardware_threads = std::thread::hardware_concurrency();

  unsigned long const num_threads =
      std::min(hardware_threads != 0 ? hardware_threads : 2, max_threads);

  unsigned long const block_size = length / num_threads;

  std::vector<std::future<T>> futures(num_threads - 1);
  std::vector<std::thread> threads(num_threads - 1);

  Iterator block_start = first;
  for (unsigned long i = 0; i < (num_threads - 1); ++i) {
    Iterator block_end = block_start;
    std::advance(block_end, block_size);
    std::packaged_task<T(Iterator, Iterator)> task(
        accumulate_block<Iterator, T>());
    futures[i]  = task.get_future();
    threads[i]  = std::thread(std::move(task), block_start, block_end);
    block_start = block_end;
  }
  T last_result = accumulate_block()(block_start, last);

  std::for_each(
      threads.begin(), threads.end(), std::mem_fn(&std::thread::join));

  T result = init;
  for (unsigned long i = 0; i < (num_threads - 1); ++i) {
    result += futures[i].get();
  }
  result += last_result;
  return result;
}