//---------------------------------------------------------
// monitor.cc
//---------------------------------------------------------

#include <condition_variable>
#include <iostream>
#include <mutex>
#include <string>
#include <syncstream>
#include <thread>

//---------------------------------------------------------

using namespace std::literals;

//---------------------------------------------------------

const size_t N = std::thread::hardware_concurrency();

//---------------------------------------------------------

class barrera_t
{
public:
    barrera_t(size_t m): cicle(0), counter(m), max(m) {}

    void esperar()
    {
        std::unique_lock<std::mutex> lock(mutex);
        std::size_t now = cicle;
        if (--counter)
        {
            cv.wait(lock, [&] { return cicle != now; });
        }
        else
        {
            counter = max;
            ++cicle;
            cv.notify_all();
        }
    }

private:
    std::mutex mutex;
    std::condition_variable cv;
    std::size_t cicle, counter, max;
} barrera(N);

//---------------------------------------------------------

void hebra(size_t yo)
{
    std::string antes = std::to_string(yo) + ": antes\n",
                despues = std::to_string(yo) + ": después\n";
    while (true)
    {
        std::osyncstream(std::cout) << antes;
        barrera.esperar();
        std::osyncstream(std::cout) << despues;
    }
}

//---------------------------------------------------------

int main()
{
    for (size_t i = 0; i < N; ++i)
        std::thread(hebra, i).detach();
    std::this_thread::sleep_for(25ms);
    std::osyncstream(std::cout) << std::flush;
}

//---------------------------------------------------------