class RW_mutex{

public:
    RW_mutex() : writers(0), readers(0), writing(false) {}

    void write_lock(){
        std::unique_lock<std::mutex> lock(gate);
        writers++;
        while (writing || readers > 0){
            room_empty.wait(lock);
        }
        writing = true;
    }

    void read_lock(){
        std::unique_lock<std::mutex> lock(gate);
        while (writers > 0){
            room_empty.wait(lock);
        }
        readers++;
    }

    void write_unlock(){
        std::unique_lock<std::mutex> lock(gate);
        writers--;
        writing = false;
        room_empty.notify_all();
    }

    void read_unlock(){
        std::unique_lock<std::mutex> lock(gate);
        readers--;
        if (readers == 0) room_empty.notify_all();
    }

private:
    std::mutex gate;
    std::size_t writers;
    std::size_t readers;
    std::condition_variable room_empty;
    bool writing;
};