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#include <condition_variable>
#include <iostream>
#include <mutex>
#include <thread>
 
std::mutex mutex;
std::condition_variable cond;

void run_step(size_t my_step) {
  static size_t current_step = 0;
  std::unique_lock<std::mutex> lk(mutex);

  std::cout << "Thread " << my_step << " has the lock" << std::endl;

  while (current_step != my_step) {
    std::cout << "Thread " << my_step << " is sleeping..." << std::endl;
    cond.wait(lk);
    std::cout << "Thread " << my_step << " woke up" << std::endl;
  }

  // Do processing...
  std::cout << "Thread " << my_step << " is processing..." << std::endl;
  current_step++;

  // Signal awaiting task.
  cond.notify_one();

  std::cout << "Thread " << my_step << " is exiting..." << std::endl;
}

int main() {
  constexpr size_t NumThreads = 5;
  std::thread threads[NumThreads];

  // Create threads.
  for (size_t i = 0; i < NumThreads; ++i) {
    threads[i] = std::thread(run_step, i);
  }

  // Wait for all threads to complete.
  for (size_t i = NumThreads; i != 0; --i) {
    threads[i - 1].join();
  }
}