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Editing: permutations.hpp

/*!
@file
Defines `boost::hana::permutations`.

@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_PERMUTATIONS_HPP
#define BOOST_HANA_PERMUTATIONS_HPP

#include <boost/hana/fwd/permutations.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/length.hpp>

#include <cstddef>
#include <utility>

BOOST_HANA_NAMESPACE_BEGIN
    //! @cond
    template <typename Xs>
    constexpr auto permutations_t::operator()(Xs&& xs) const {
        using S = typename hana::tag_of<Xs>::type;
        using Permutations = BOOST_HANA_DISPATCH_IF(permutations_impl<S>,
            hana::Sequence<S>::value
        );

#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
        static_assert(hana::Sequence<S>::value,
        "hana::permutations(xs) requires 'xs' to be a Sequence");
    #endif

return Permutations::apply(static_cast<Xs&&>(xs));
    }
    //! @endcond

namespace detail {
        template <std::size_t N>
        struct permutation_indices {
            static constexpr auto value =
                detail::array<std::size_t, N>{}.iota(0).permutations();
        };
    }

template <typename S, bool condition>
    struct permutations_impl<S, when<condition>> : default_ {
        template <std::size_t n, typename Xs, std::size_t ...i>
        static constexpr auto
        nth_permutation(Xs const& xs, std::index_sequence<i...>) {
            constexpr auto indices = detail::permutation_indices<sizeof...(i)>::value;
            (void)indices; // workaround GCC warning when sizeof...(i) == 0
            return hana::make<S>(hana::at_c<indices[n][i]>(xs)...);
        }

template <std::size_t N, typename Xs, std::size_t ...n>
        static constexpr auto
        permutations_helper(Xs const& xs, std::index_sequence<n...>) {
            return hana::make<S>(nth_permutation<n>(xs, std::make_index_sequence<N>{})...);
        }

template <typename Xs>
        static constexpr auto apply(Xs const& xs) {
            constexpr std::size_t N = decltype(hana::length(xs))::value;
            constexpr std::size_t total_perms = detail::factorial(N);
            return permutations_helper<N>(xs, std::make_index_sequence<total_perms>{});
        }
    };
BOOST_HANA_NAMESPACE_END

#endif // !BOOST_HANA_PERMUTATIONS_HPP

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