ceph/src/boost/libs/hana/include/boost/hana/take_back.hpp

   1 /*!
   2 @file
   3 Defines `boost::hana::take_back`.
   4
   5 @copyright Louis Dionne 2013-2016
   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_TAKE_BACK_HPP
  11 #define BOOST_HANA_TAKE_BACK_HPP
  12
  13 #include <boost/hana/fwd/take_back.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concept/integral_constant.hpp>
  17 #include <boost/hana/concept/sequence.hpp>
  18 #include <boost/hana/config.hpp>
  19 #include <boost/hana/core/dispatch.hpp>
  20 #include <boost/hana/core/make.hpp>
  21 #include <boost/hana/integral_constant.hpp>
  22 #include <boost/hana/length.hpp>
  23
  24 #include <cstddef>
  25 #include <utility>
  26
  27
  28 BOOST_HANA_NAMESPACE_BEGIN
  29     //! @cond
  30     template <typename Xs, typename N>
  31     constexpr auto take_back_t::operator()(Xs&& xs, N const& n) const {
  32         using S = typename hana::tag_of<Xs>::type;
  33         using TakeBack = BOOST_HANA_DISPATCH_IF(take_back_impl<S>,
  34             hana::Sequence<S>::value &&
  35             hana::IntegralConstant<N>::value
  36         );
  37
  38 #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  39         static_assert(hana::Sequence<S>::value,
  40         "hana::take_back(xs, n) requires 'xs' to be a Sequence");
  41
  42         static_assert(hana::IntegralConstant<N>::value,
  43         "hana::take_back(xs, n) requires 'n' to be an IntegralConstant");
  44 #endif
  45
  46         return TakeBack::apply(static_cast<Xs&&>(xs), n);
  47     }
  48     //! @endcond
  49
  50     template <typename S, bool condition>
  51     struct take_back_impl<S, when<condition>> : default_ {
  52         template <std::size_t start, typename Xs, std::size_t ...n>
  53         static constexpr auto take_back_helper(Xs&& xs, std::index_sequence<n...>) {
  54             return hana::make<S>(hana::at_c<start + n>(static_cast<Xs&&>(xs))...);
  55         }
  56
  57         template <typename Xs, typename N>
  58         static constexpr auto apply(Xs&& xs, N const&) {
  59             constexpr std::size_t n = N::value;
  60             constexpr std::size_t size = decltype(hana::length(xs))::value;
  61             constexpr std::size_t start = n < size ? size - n : 0;
  62             return take_back_helper<start>(static_cast<Xs&&>(xs),
  63                         std::make_index_sequence<(n < size ? n : size)>{});
  64         }
  65     };
  66 BOOST_HANA_NAMESPACE_END
  67
  68 #endif // !BOOST_HANA_TAKE_BACK_HPP