79 lines
2.6 KiB
C++
79 lines
2.6 KiB
C++
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/*!
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@file
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Defines `boost::hana::permutations`.
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@copyright Louis Dionne 2013-2016
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
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*/
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#ifndef BOOST_HANA_PERMUTATIONS_HPP
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#define BOOST_HANA_PERMUTATIONS_HPP
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#include <boost/hana/fwd/permutations.hpp>
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#include <boost/hana/at.hpp>
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#include <boost/hana/concept/sequence.hpp>
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#include <boost/hana/config.hpp>
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#include <boost/hana/core/dispatch.hpp>
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#include <boost/hana/core/make.hpp>
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#include <boost/hana/detail/array.hpp>
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#include <boost/hana/length.hpp>
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#include <cstddef>
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#include <utility>
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BOOST_HANA_NAMESPACE_BEGIN
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//! @cond
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template <typename Xs>
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constexpr auto permutations_t::operator()(Xs&& xs) const {
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using S = typename hana::tag_of<Xs>::type;
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using Permutations = BOOST_HANA_DISPATCH_IF(permutations_impl<S>,
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hana::Sequence<S>::value
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);
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#ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
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static_assert(hana::Sequence<S>::value,
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"hana::permutations(xs) requires 'xs' to be a Sequence");
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#endif
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return Permutations::apply(static_cast<Xs&&>(xs));
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}
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//! @endcond
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namespace detail {
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template <std::size_t N>
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struct permutation_indices {
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static constexpr auto value =
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detail::array<std::size_t, N>{}.iota(0).permutations();
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};
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}
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template <typename S, bool condition>
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struct permutations_impl<S, when<condition>> : default_ {
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template <std::size_t n, typename Xs, std::size_t ...i>
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static constexpr auto
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nth_permutation(Xs const& xs, std::index_sequence<i...>) {
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constexpr auto indices = detail::permutation_indices<sizeof...(i)>::value;
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(void)indices; // workaround GCC warning when sizeof...(i) == 0
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return hana::make<S>(hana::at_c<indices[n][i]>(xs)...);
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}
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template <std::size_t N, typename Xs, std::size_t ...n>
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static constexpr auto
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permutations_helper(Xs const& xs, std::index_sequence<n...>) {
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return hana::make<S>(nth_permutation<n>(xs, std::make_index_sequence<N>{})...);
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}
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template <typename Xs>
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static constexpr auto apply(Xs const& xs) {
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constexpr std::size_t N = decltype(hana::length(xs))::value;
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constexpr std::size_t total_perms = detail::factorial(N);
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return permutations_helper<N>(xs, std::make_index_sequence<total_perms>{});
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}
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};
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BOOST_HANA_NAMESPACE_END
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#endif // !BOOST_HANA_PERMUTATIONS_HPP
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