Function determine_thread_count

include/BS_thread_pool.hpp:2125–2140

* @brief Determine how many threads the pool should have, based on the parameter passed to the constructor or reset(). * * @param num_threads The parameter passed to the constructor or `reset()`. If the parameter is a positive number, then the pool will be created with this number of threads. If the parameter is zero, or a parameter was not supplied (in which case it will have the defaul

     * @param num_threads The parameter passed to the constructor or `reset()`. If the parameter is a positive number, then the pool will be created with this number of threads. If the parameter is zero, or a parameter was not supplied (in which case it will have the default value of 0), then the pool will be created with the total number of hardware threads available, as obtained from `thread_t::hardware_concurrency()`. If the latter returns zero for some reason, then the pool will be created with just one thread. If the native extensions are enabled, the pool will instead use the number of threads available to the process, as obtained from `BS::get_os_process_affinity()`, which can be less than the number of hardware threads.
     */
    [[nodiscard]] static std::size_t determine_thread_count(const std::size_t num_threads) noexcept(!thread_pool_native_extensions)
    {
        if (num_threads > 0)
            return num_threads;
#ifdef BS_THREAD_POOL_NATIVE_EXTENSIONS
        const std::optional<std::vector<bool>> affinity = BS::get_os_process_affinity();
        if (affinity.has_value())
        {
            const std::size_t affinity_thread_count = static_cast<std::size_t>(std::count(affinity->begin(), affinity->end(), true));
            return (affinity_thread_count > 0) ? affinity_thread_count : 1;
        }
#endif
        if (thread_t::hardware_concurrency() > 0)
            return thread_t::hardware_concurrency();
        return 1;
    }

    /**
     * @brief A helper function for `detach_blocks()` and `submit_blocks()`.