class barrier {
  unsigned const count;
  std::atomic<unsigned> spaces;
  std::atomic<unsigned> generation;

public:
  explicit barrier(unsigned count_)
      : count(count_), spaces(count), generation(0)
  {
  }
  void wait()
  {
    unsigned const my_generation = generation;
    if (!--spaces) {
      spaces = count;
      ++generation;
    }
    else {
      while (generation == my_generation) std::this_thread::yield();
    }
  }
};