boost/hana/permutations.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::permutations`.
   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_PERMUTATIONS_HPP
  11 #define BOOST_HANA_PERMUTATIONS_HPP
  12
  13 #include <boost/hana/fwd/permutations.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/length.hpp>
  22
  23 #include <cstddef>
  24 #include <utility>
  25
  26
  27 BOOST_HANA_NAMESPACE_BEGIN
  28     //! @cond
  29     template <typename Xs>
  30     constexpr auto permutations_t::operator()(Xs&& xs) const {
  31         using S = typename hana::tag_of<Xs>::type;
  32         using Permutations = BOOST_HANA_DISPATCH_IF(permutations_impl<S>,
  33             hana::Sequence<S>::value
  34         );
  35
  36     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  37         static_assert(hana::Sequence<S>::value,
  38         "hana::permutations(xs) requires 'xs' to be a Sequence");
  39     #endif
  40
  41         return Permutations::apply(static_cast<Xs&&>(xs));
  42     }
  43     //! @endcond
  44
  45     namespace detail {
  46         template <std::size_t N>
  47         struct permutation_indices {
  48             static constexpr auto value =
  49                 detail::array<std::size_t, N>{}.iota(0).permutations();
  50         };
  51     }
  52
  53     template <typename S, bool condition>
  54     struct permutations_impl<S, when<condition>> : default_ {
  55         template <std::size_t n, typename Xs, std::size_t ...i>
  56         static constexpr auto
  57         nth_permutation(Xs const& xs, std::index_sequence<i...>) {
  58             constexpr auto indices = detail::permutation_indices<sizeof...(i)>::value;
  59             (void)indices; // workaround GCC warning when sizeof...(i) == 0
  60             return hana::make<S>(hana::at_c<indices[n][i]>(xs)...);
  61         }
  62
  63         template <std::size_t N, typename Xs, std::size_t ...n>
  64         static constexpr auto
  65         permutations_helper(Xs const& xs, std::index_sequence<n...>) {
  66             return hana::make<S>(nth_permutation<n>(xs, std::make_index_sequence<N>{})...);
  67         }
  68
  69         template <typename Xs>
  70         static constexpr auto apply(Xs const& xs) {
  71             constexpr std::size_t N = decltype(hana::length(xs))::value;
  72             constexpr std::size_t total_perms = detail::factorial(N);
  73             return permutations_helper<N>(xs, std::make_index_sequence<total_perms>{});
  74         }
  75     };
  76 BOOST_HANA_NAMESPACE_END
  77
  78 #endif // !BOOST_HANA_PERMUTATIONS_HPP