ordered_mutex.h

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// Minetest
// SPDX-License-Identifier: LGPL-2.1-or-later
 
#pragma once
 
#include <cstdint>
#include <condition_variable>
 
/*
        Fair mutex based on ticketing approach.
        Satisfies `Mutex` C++11 requirements.
*/
class ordered_mutex {
public:
        ordered_mutex() : next_ticket(0), counter(0) {}
 
        void lock()
        {
                std::unique_lock autolock(cv_lock);
                const auto ticket = next_ticket++;
                cv.wait(autolock, [&] { return counter == ticket; });
        }
 
        bool try_lock()
        {
                std::lock_guard autolock(cv_lock);
                if (counter != next_ticket)
                        return false;
                next_ticket++;
                return true;
        }
 
        void unlock()
        {
                {
                        std::lock_guard autolock(cv_lock);
                        counter++;
                }
                cv.notify_all(); // intentionally outside lock
        }
 
private:
        std::condition_variable cv;
        std::mutex cv_lock;
        uint_fast32_t next_ticket, counter;
};