//-----------------------------------------------
// popov.h: reader/writer fairness
//-----------------------------------------------

#ifndef popov_h
#define popov_h

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

#include <cstddef>   // std::size_t
#include <mutex>     // std::mutex
#include <semaphore> // std::counting_semaphore

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

class popov_lock
{
public:
    popov_lock(std::size_t) {}

    void reader_lock()
    {
        in.acquire();
        ++ctrin;
        in.release();
    }

    void reader_unlock()
    {
        out.acquire();
        ++ctrout;
        if (wait && ctrin == ctrout)
            wrt.release();
        out.release();
    }

    void writer_lock()
    {
        in.acquire();
        out.acquire();
        if (ctrin == ctrout)
        {
            out.release();
        }
        else
        {
            wait = true;
            out.release();
            wrt.acquire();
            wait = false;
        }
    }

    void writer_unlock() { in.release(); }

private:
    std::binary_semaphore in{1}, out{1}, wrt{0};
    std::size_t ctrin = 0, ctrout = 0;
    bool wait = false;
};

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

#endif // popov_h