//---------------------------------------------------------
// barrera.cc: ¿Qué podría salir mal?
//---------------------------------------------------------

#include <iostream>
#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 l): uso(0), en_espera{0, 0}, limite(l) {}

    void esperar()
    {
        size_t local = uso;

        if (++en_espera[local] != limite)
        {
            while (en_espera[uso]);
        }
        else
        {
            uso ^= 1;
            en_espera[local] = 0;
        }

    }

private:
    size_t uso, en_espera[2], limite;
} 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;
}

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