#include <vector>
#include <thread>
#include <future>
#include <numeric>
#include <iostream>

using It = std::vector<int>::const_iterator;

void task(It begin, It end, std::promise<int> promise)
{ promise.set_value(std::reduce(begin, end)); }

int sum(std::size_t thread_count, std::vector<int> const& values)
{
    std::size_t const size { values.size() / thread_count };
    std::vector<std::jthread> pool { };
    std::vector<std::future<int>> futures { };

    for (std::size_t i { 0 }; i < thread_count; ++i)
    {
        auto begin { values.begin() + size * i };
        auto end   { values.begin() + size * (i + 1) };
        std::promise<int> promise { };

        futures.push_back(std::move(promise.get_future()));
        pool.emplace_back(task, begin, end, std::move(promise));
    }

    return std::accumulate(futures.begin(), futures.end(), 0,
                           [](int result, std::future<int>& future)
                           { return result + future.get(); });
}

int main()
{
    std::vector<int> v(1000000, 1);
    int answer { sum(20000, v) };

    std::cout << answer << std::endl;
}