id_factory.hpp

// Copyright Take Vos 2021-2023.
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
 
#pragma once
 
#include "unfair_mutex.hpp"
#include "../utility/utility.hpp"
#include "../macros.hpp"
#include <mutex>
#include <vector>
 
hi_export_module(hikogui.concurrency.id_factory);
 
hi_export namespace hi { inline namespace v1 {

template<std::unsigned_integral T>
class id_factory {
public:
    using value_type = T;
 
    id_factory() = default;
    id_factory(id_factory const &) = delete;
    id_factory(id_factory &&) = delete;
    id_factory &operator=(id_factory const &) = delete;
    id_factory &operator=(id_factory &&) = delete;
   
    [[nodiscard]] value_type acquire() noexcept
    {
        auto expected = _released_count.load(std::memory_order::relaxed);
        if (expected != 0) {
            // There MAY be released IDs.
 
            if (_released_count.compare_exchange_weak(expected, expected - 1, std::memory_order::acquire, std::memory_order::relaxed)) {
                // There ARE released IDs.
 
                // unfair_mutex::try_lock() is wait-free.
                if (_mutex.try_lock()) {
                    // We got a lock, pop the last ID.
                    hi_axiom(not _released.empty());
 
                    // std::vector::pop_back() should be wait-free since there are no reallocations.
                    auto const tmp = _released.back();
                    _released.pop_back();
 
                    _mutex.unlock();
                    return tmp;
 
                } else {
                    // We couldn't get a lock; access to release-stack may be slow due to reallocation.
                    // Since we didn't pop, increment the release-count again.
                    _released_count.fetch_add(1, std::memory_order::relaxed);
                }
            }
        }
 
        auto const tmp = _v.fetch_add(1, std::memory_order::relaxed) + 1;
        hi_assert(tmp != 0, "id_factory overflow.");
        return tmp;
    }

    void release(value_type v) noexcept
    {
        auto const lock = std::scoped_lock(_mutex);
        _released.push_back(v);
        _released_count.fetch_add(1, std::memory_order::release);
    }

    [[nodiscard]] value_type operator++() noexcept
    {
        return acquire();
    }
 
private:
    mutable unfair_mutex _mutex = {};
    std::atomic<value_type> _v = 0;
 
    std::atomic<size_t> _released_count = 0;
    std::vector<value_type> _released = {};
};
 
}}