boost/hana/cartesian_product.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::cartesian_product`.
   4
   5 @copyright Louis Dionne 2013-2017
   6 Distributed under the Boost Software License, Version 1.0.
   7 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   8  */
   9
  10 #ifndef BOOST_HANA_CARTESIAN_PRODUCT_HPP
  11 #define BOOST_HANA_CARTESIAN_PRODUCT_HPP
  12
  13 #include <boost/hana/fwd/cartesian_product.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concept/sequence.hpp>
  17 #include <boost/hana/config.hpp>
  18 #include <boost/hana/core/dispatch.hpp>
  19 #include <boost/hana/core/make.hpp>
  20 #include <boost/hana/detail/array.hpp>
  21 #include <boost/hana/integral_constant.hpp>
  22 #include <boost/hana/length.hpp>
  23 #include <boost/hana/unpack.hpp>
  24
  25 #include <cstddef>
  26 #include <utility>
  27
  28
  29 BOOST_HANA_NAMESPACE_BEGIN
  30     //! @cond
  31     template <typename Xs>
  32     constexpr auto cartesian_product_t::operator()(Xs&& xs) const {
  33         using S = typename hana::tag_of<Xs>::type;
  34         using CartesianProduct = BOOST_HANA_DISPATCH_IF(
  35             cartesian_product_impl<S>,
  36             hana::Sequence<S>::value
  37         );
  38
  39     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  40         static_assert(hana::Sequence<S>::value,
  41         "hana::cartesian_product(xs) requires 'xs' to be a Sequence");
  42     #endif
  43
  44         return CartesianProduct::apply(static_cast<Xs&&>(xs));
  45     }
  46     //! @endcond
  47
  48     namespace detail {
  49         template <std::size_t ...Lengths>
  50         struct cartesian_product_indices {
  51             static constexpr std::size_t total_length() {
  52                 std::size_t lengths[] = {Lengths...};
  53                 std::size_t r = 1;
  54                 for (std::size_t len: lengths)
  55                     r *= len;
  56                 return r;
  57             }
  58
  59             static constexpr std::size_t length = total_length();
  60
  61             static constexpr auto indices_of(std::size_t i) {
  62                 constexpr std::size_t lengths[] = {Lengths...};
  63                 constexpr std::size_t n = sizeof...(Lengths);
  64                 detail::array<std::size_t, n> result{};
  65                 for (std::size_t j = n; j--;) {
  66                     result[j] = i % lengths[j];
  67                     i /= lengths[j];
  68                 }
  69                 return result;
  70             }
  71
  72             template <typename S, std::size_t n, std::size_t ...k, typename ...Xs>
  73             static constexpr auto
  74             product_element(std::index_sequence<k...>, Xs&& ...xs) {
  75                 constexpr auto indices = indices_of(n);
  76                 return hana::make<S>(hana::at_c<indices[k]>(xs)...);
  77             }
  78
  79             template <typename S, std::size_t ...n, typename ...Xs>
  80             static constexpr auto
  81             create_product(std::index_sequence<n...>, Xs&& ...xs) {
  82                 return hana::make<S>(product_element<S, n>(
  83                     std::make_index_sequence<sizeof...(Xs)>{}, xs...
  84                 )...);
  85             }
  86         };
  87     }
  88
  89     // Credits: implementation adapted from http://github.com/alexk7/hel.
  90     template <typename S, bool condition>
  91     struct cartesian_product_impl<S, when<condition>> : default_ {
  92         template <typename Xs>
  93         static constexpr auto apply(Xs&& xs) {
  94             return hana::unpack(static_cast<Xs&&>(xs), cartesian_product_impl{});
  95         }
  96
  97         template <typename ...Xs>
  98         constexpr auto operator()(Xs&& ...xs) const {
  99             using indices = detail::cartesian_product_indices<
 100                 decltype(hana::length(xs))::value...
 101             >;
 102             return indices::template create_product<S>(
 103                         std::make_index_sequence<indices::length>{},
 104                         static_cast<Xs&&>(xs)...);
 105         }
 106
 107         constexpr auto operator()() const {
 108             return hana::make<S>();
 109         }
 110     };
 111 BOOST_HANA_NAMESPACE_END
 112
 113 #endif // !BOOST_HANA_CARTESIAN_PRODUCT_HPP