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

   1 /*!
   2 @file
   3 Defines `boost::hana::slice` and `boost::hana::slice_c`.
   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_SLICE_HPP
  11 #define BOOST_HANA_SLICE_HPP
  12
  13 #include <boost/hana/fwd/slice.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concept/foldable.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/range.hpp>
  22 #include <boost/hana/unpack.hpp>
  23
  24 #include <cstddef>
  25 #include <utility>
  26
  27
  28 BOOST_HANA_NAMESPACE_BEGIN
  29     //! @cond
  30     template <typename Xs, typename Indices>
  31     constexpr auto slice_t::operator()(Xs&& xs, Indices&& indices) const {
  32         using S = typename hana::tag_of<Xs>::type;
  33         using Slice = BOOST_HANA_DISPATCH_IF(slice_impl<S>,
  34             hana::Sequence<S>::value &&
  35             hana::Foldable<Indices>::value
  36         );
  37
  38     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  39         static_assert(hana::Sequence<S>::value,
  40         "hana::slice(xs, indices) requires 'xs' to be a Sequence");
  41
  42         static_assert(hana::Foldable<Indices>::value,
  43         "hana::slice(xs, indices) requires 'indices' to be Foldable");
  44     #endif
  45
  46         return Slice::apply(static_cast<Xs&&>(xs), static_cast<Indices&&>(indices));
  47     }
  48     //! @endcond
  49
  50     namespace detail {
  51         template <typename Xs>
  52         struct take_arbitrary {
  53             Xs& xs;
  54             using S = typename hana::tag_of<Xs>::type;
  55
  56             template <typename ...N>
  57             constexpr auto operator()(N const& ...) const {
  58                 return hana::make<S>(hana::at_c<N::value>(xs)...);
  59             }
  60         };
  61     }
  62
  63     template <typename S, bool condition>
  64     struct slice_impl<S, when<condition>> : default_ {
  65         template <std::size_t from, typename Xs, std::size_t ...i>
  66         static constexpr auto from_offset(Xs&& xs, std::index_sequence<i...>) {
  67             return hana::make<S>(hana::at_c<from + i>(static_cast<Xs&&>(xs))...);
  68         }
  69
  70         template <typename Xs, typename T, T from, T to>
  71         static constexpr auto apply(Xs&& xs, hana::range<T, from, to> const&) {
  72             return slice_impl::from_offset<from>(
  73                 static_cast<Xs&&>(xs), std::make_index_sequence<to - from>{}
  74             );
  75         }
  76
  77         //! @todo
  78         //! Since we have the right to specify the same index more than once,
  79         //! we can't move from the elements of the source sequence even if it
  80         //! is a temporary object: we could end up double-moving. Perhaps it
  81         //! would be possible to determine the indices from which we can move
  82         //! without incurring a too large compile-time penalty?
  83         template <typename Xs, typename Indices>
  84         static constexpr auto apply(Xs const& xs, Indices const& indices) {
  85             return hana::unpack(indices, detail::take_arbitrary<Xs const>{xs});
  86         }
  87     };
  88
  89     template <std::size_t from, std::size_t to>
  90     struct slice_c_t {
  91         template <typename Xs>
  92         constexpr auto operator()(Xs&& xs) const {
  93             return hana::slice(static_cast<Xs&&>(xs),
  94                                hana::range_c<std::size_t, from, to>);
  95         }
  96     };
  97 BOOST_HANA_NAMESPACE_END
  98
  99 #endif // !BOOST_HANA_SLICE_HPP