text: /*
   Copyright (c) 2003-2020 Andy Little.
  
   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License or
   (at your option) any later version.
  
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
   GNU General Public License for more details.
  
   You should have received a copy of the GNU General Public License
   along with this program. If not, see http://www.gnu.org/licenses./
  */
  
  #include <units/physical/si/length.h>
  #include <units/format.h>
  #include <iostream>
  
  /*
    get conversion factor from one dimensionally equivalent
    quantity type to another
  */
  
  namespace {
  
  template<units::Quantity Target, units::Quantity Source>
    requires units::equivalent_dim<typename Source::dimension, typename Target::dimension>
  inline constexpr std::common_type_t<typename Target::rep, typename Source::rep> conversion_factor(Target, Source)
  {
    // get quantities looking like inputs but with Q::rep that doesn't have narrowing conversion
    typedef std::common_type_t<typename Target::rep, typename Source::rep> rep;
    typedef units::quantity<typename Source::dimension, typename Source::unit, rep> source;
    typedef units::quantity<typename Target::dimension, typename Target::unit, rep> target;
    return target{source{1}}.count();
  }
  
  }  // namespace
  
  int main()
  {
    using namespace units::si;
  
    std::cout << "conversion factor in mp-units...\n\n";
  
    constexpr length<metre> lengthA = 2.0q_m;
    constexpr length<millimetre> lengthB = lengthA;
  
    std::cout << fmt::format("lengthA( {} ) and lengthB( {} )\n", lengthA, lengthB)
              << "represent the same length in different units.\n\n";
  
    std::cout << fmt::format("therefore ratio lengthA / lengthB == {}\n\n", lengthA / lengthB);
  
    std::cout << fmt::format("conversion factor from lengthA::unit of {:%q} to lengthB::unit of {:%q}:\n\n", lengthA, lengthB)
              << fmt::format("lengthB.count( {} ) == lengthA.count( {} ) * conversion_factor( {} )\n",
                             lengthB.count(), lengthA.count(), conversion_factor(lengthB, lengthA));
  }
  