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/*!
@file
Defines `boost::hana::permutations`.
@copyright Louis Dionne 2013-2016
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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