boost/hana/reverse.hpp

0001 /*!
0002 @file
0003 Defines `boost::hana::reverse`.
0004
0005 Copyright Louis Dionne 2013-2022
0006 Distributed under the Boost Software License, Version 1.0.
0007 (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
0008  */
0009
0010 #ifndef BOOST_HANA_REVERSE_HPP
0011 #define BOOST_HANA_REVERSE_HPP
0012
0013 #include <boost/hana/fwd/reverse.hpp>
0014
0015 #include <boost/hana/at.hpp>
0016 #include <boost/hana/concept/sequence.hpp>
0017 #include <boost/hana/config.hpp>
0018 #include <boost/hana/core/dispatch.hpp>
0019 #include <boost/hana/core/make.hpp>
0020 #include <boost/hana/length.hpp>
0021
0022 #include <cstddef>
0023 #include <utility>
0024
0025
0026 namespace boost { namespace hana {
0027     //! @cond
0028     template <typename Xs>
0029     constexpr auto reverse_t::operator()(Xs&& xs) const {
0030         using S = typename hana::tag_of<Xs>::type;
0031         using Reverse = BOOST_HANA_DISPATCH_IF(reverse_impl<S>,
0032             hana::Sequence<S>::value
0033         );
0034
0035     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
0036         static_assert(hana::Sequence<S>::value,
0037         "hana::reverse(xs) requires 'xs' to be a Sequence");
0038     #endif
0039
0040         return Reverse::apply(static_cast<Xs&&>(xs));
0041     }
0042     //! @endcond
0043
0044     template <typename S, bool condition>
0045     struct reverse_impl<S, when<condition>> : default_ {
0046         template <typename Xs, std::size_t ...i>
0047         static constexpr auto reverse_helper(Xs&& xs, std::index_sequence<i...>) {
0048             return hana::make<S>(
0049                 hana::at_c<sizeof...(i) - i - 1>(static_cast<Xs&&>(xs))...
0050             );
0051         }
0052
0053         template <typename Xs>
0054         static constexpr auto apply(Xs&& xs) {
0055             constexpr std::size_t N = decltype(hana::length(xs))::value;
0056             return reverse_helper(static_cast<Xs&&>(xs), std::make_index_sequence<N>{});
0057         }
0058     };
0059 }} // end namespace boost::hana
0060
0061 #endif // !BOOST_HANA_REVERSE_HPP