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 block_size = 25;
  unsigned long const num_blocks = (length + block_size - 1) / block_size;

  std::vector<std::future<T>> futures(num_blocks - 1);
  thread_pool pool;

  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);
    futures[i]  = pool.submit(accumulate_block<Iterator, T>());
    block_start = block_end;
  }
  T last_result = accumulate_block()(block_start, last);
  T result      = init;
  for (unsigned long i = 0; i < (num_blocks - 1); ++i) {
    result += futures[i].get();
  }
  result += last_result;
  return result;
}