c++17 - Decompose a std::chrono::duration into several component durations.

decompose.cpp

#include <chrono>
#include <iostream>
#include <tuple>

template <typename lhs_t, typename rhs_t, typename... other_ts>
constexpr bool has_decreasing_periods() {
    if constexpr (std::ratio_less_v<typename lhs_t::period,
                                    typename rhs_t::period>) {
        return false;
    } else if constexpr (sizeof...(other_ts)) {
        return has_decreasing_periods<rhs_t, other_ts...>();
    } else {
        return true;
    }
}

template <typename return_duration_t, typename input_duration_t>
return_duration_t chrono_extract(input_duration_t& value) {
    auto extracted = std::chrono::duration_cast<return_duration_t>(value);
    value -= extracted;
    return extracted;
}

template <typename... return_ts, typename duration_t>
std::tuple<return_ts...> chrono_decompose(duration_t value) {
    static_assert(has_decreasing_periods<return_ts...>());
    using smallest_t = std::tuple_element_t<sizeof...(return_ts) - 1,
                                            std::tuple<return_ts...>>;
    auto small_value = std::chrono::duration_cast<smallest_t>(value);
    return {chrono_extract<return_ts>(small_value)...};
}

int main()
{
    std::chrono::seconds before(19874);

    {
        auto [h, m] = chrono_decompose<std::chrono::hours,
                                       std::chrono::minutes>(before);
        std::chrono::seconds after(h + m);
        std::cout << h.count() << " hours "
                  << m.count() << " minutes = "
                  << after.count() << " seconds\n";
    }

    {
        auto [m, s, ms] = chrono_decompose<std::chrono::minutes,
                                           std::chrono::seconds,
                                           std::chrono::milliseconds>(before);
        auto after =
            std::chrono::duration_cast<std::chrono::seconds>(m + s + ms);
        std::cout << m.count() << " minutes "
                  << s.count() << " seconds "
                  << ms.count() << " milliseconds = "
                  << after.count() << " seconds\n";
    }
}