text: // The MIT License (MIT)
  //
  // Copyright (c) 2018 Mateusz Pusz
  //
  // Permission is hereby granted, free of charge, to any person obtaining a copy
  // of this software and associated documentation files (the "Software"), to deal
  // in the Software without restriction, including without limitation the rights
  // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  // copies of the Software, and to permit persons to whom the Software is
  // furnished to do so, subject to the following conditions:
  //
  // The above copyright notice and this permission notice shall be included in all
  // copies or substantial portions of the Software.
  //
  // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  // SOFTWARE.
  
  #pragma once
  
  #include <units/customization_points.h>
  // IWYU pragma: begin_exports
  #include <units/isq/si/time.h>
  #include <units/point_origin.h>
  #include <chrono>
  // IWYU pragma: end_exports
  
  namespace units {
  
  template<typename Rep, typename Period>
  struct quantity_like_traits<std::chrono::duration<Rep, Period>> {
    using dimension = isq::si::dim_time;
    using unit = downcast_unit<dimension, ratio(Period::num, Period::den)>;
    using rep = Rep;
    [[nodiscard]] static constexpr rep number(const std::chrono::duration<Rep, Period>& q) { return q.count(); }
  };
  
  template<typename C>
  struct clock_origin : point_origin<isq::si::dim_time> {};
  
  template<typename C, typename Rep, typename Period>
  struct quantity_point_like_traits<std::chrono::time_point<C, std::chrono::duration<Rep, Period>>> {
    using origin = clock_origin<C>;
    using unit = downcast_unit<typename origin::dimension, ratio(Period::num, Period::den)>;
    using rep = Rep;
    [[nodiscard]] static constexpr auto relative(const std::chrono::time_point<C, std::chrono::duration<Rep, Period>>& qp)
    {
      return qp.time_since_epoch();
    }
  };
  
  namespace detail {
  
  template<typename C, typename Rep, typename Period>
  inline constexpr bool is_quantity_point_like<std::chrono::time_point<C, std::chrono::duration<Rep, Period>>> = true;
  
  constexpr std::intmax_t pow_10(std::intmax_t v)
  {
    gsl_Expects(v > 0);
    std::intmax_t res = 1;
    for (std::intmax_t i = 0; i < v; i++) res *= 10;
    return res;
  }
  
  template<ratio R>
  constexpr auto to_std_ratio_impl()
  {
    if constexpr (R.exp == 0)
      return std::ratio<R.num, R.den>{};
    else if constexpr (R.exp > 0)
      return std::ratio<R.num * pow_10(R.exp), R.den>{};
    else
      return std::ratio<R.num, R.den * pow_10(-R.exp)>{};
  }
  
  }  // namespace detail
  
  // TODO ICE below on gcc-11 when `ratio` is used instead of `auto`
  template<auto R>
  using to_std_ratio = decltype(detail::to_std_ratio_impl<R>());
  
  template<typename U, typename Rep>
  [[nodiscard]] constexpr auto to_std_duration(const quantity<isq::si::dim_time, U, Rep>& q)
  {
    return std::chrono::duration<Rep, to_std_ratio<U::ratio>>(q.number());
  }
  
  template<typename C, typename U, typename Rep>
  [[nodiscard]] constexpr auto to_std_time_point(const quantity_point<clock_origin<C>, U, Rep>& qp)
  {
    using ret_type = std::chrono::time_point<C, std::chrono::duration<Rep, to_std_ratio<U::ratio>>>;
    return ret_type(to_std_duration(qp.relative()));
  }
  
  }  // namespace units
  