text: /*!
  @file
  Defines `boost::hana::cartesian_product`.
  
  @copyright Louis Dionne 2013-2017
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   */
  
  #ifndef BOOST_HANA_CARTESIAN_PRODUCT_HPP
  #define BOOST_HANA_CARTESIAN_PRODUCT_HPP
  
  #include <boost/hana/fwd/cartesian_product.hpp>
  
  #include <boost/hana/at.hpp>
  #include <boost/hana/concept/sequence.hpp>
  #include <boost/hana/config.hpp>
  #include <boost/hana/core/dispatch.hpp>
  #include <boost/hana/core/make.hpp>
  #include <boost/hana/detail/array.hpp>
  #include <boost/hana/integral_constant.hpp>
  #include <boost/hana/length.hpp>
  #include <boost/hana/unpack.hpp>
  
  #include <cstddef>
  #include <utility>
  
  
  BOOST_HANA_NAMESPACE_BEGIN
      //! @cond
      template <typename Xs>
      constexpr auto cartesian_product_t::operator()(Xs&& xs) const {
          using S = typename hana::tag_of<Xs>::type;
          using CartesianProduct = BOOST_HANA_DISPATCH_IF(
              cartesian_product_impl<S>,
              hana::Sequence<S>::value
          );
  
      #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
          static_assert(hana::Sequence<S>::value,
          "hana::cartesian_product(xs) requires 'xs' to be a Sequence");
      #endif
  
          return CartesianProduct::apply(static_cast<Xs&&>(xs));
      }
      //! @endcond
  
      namespace detail {
          template <std::size_t ...Lengths>
          struct cartesian_product_indices {
              static constexpr std::size_t total_length() {
                  std::size_t lengths[sizeof...(Lengths)] = {Lengths...};
                  std::size_t r = 1;
                  for (std::size_t len: lengths)
                      r *= len;
                  return r;
              }
  
              static constexpr std::size_t length = total_length();
  
              static constexpr auto indices_of(std::size_t i) {
                  constexpr std::size_t lengths[sizeof...(Lengths)] = {Lengths...};
                  constexpr std::size_t n = sizeof...(Lengths);
                  detail::array<std::size_t, n> result{};
                  for (std::size_t j = n; j--;) {
                      result[j] = i % lengths[j];
                      i /= lengths[j];
                  }
                  return result;
              }
  
              template <typename S, std::size_t n, std::size_t ...k, typename ...Xs>
              static constexpr auto
              product_element(std::index_sequence<k...>, Xs&& ...xs) {
                  constexpr auto indices = indices_of(n);
                  return hana::make<S>(hana::at_c<indices[k]>(xs)...);
              }
  
              template <typename S, std::size_t ...n, typename ...Xs>
              static constexpr auto
              create_product(std::index_sequence<n...>, Xs&& ...xs) {
                  return hana::make<S>(product_element<S, n>(
                      std::make_index_sequence<sizeof...(Xs)>{}, xs...
                  )...);
              }
          };
      }
  
      // Credits: implementation adapted from http://github.com/alexk7/hel.
      template <typename S, bool condition>
      struct cartesian_product_impl<S, when<condition>> : default_ {
          template <typename Xs>
          static constexpr auto apply(Xs&& xs) {
              return hana::unpack(static_cast<Xs&&>(xs), cartesian_product_impl{});
          }
  
          template <typename ...Xs>
          constexpr auto operator()(Xs&& ...xs) const {
              using indices = detail::cartesian_product_indices<
                  decltype(hana::length(xs))::value...
              >;
              return indices::template create_product<S>(
                          std::make_index_sequence<indices::length>{},
                          static_cast<Xs&&>(xs)...);
          }
  
          constexpr auto operator()() const {
              return hana::make<S>();
          }
      };
  BOOST_HANA_NAMESPACE_END
  
  #endif // !BOOST_HANA_CARTESIAN_PRODUCT_HPP
  