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

   1 /*!
   2 @file
   3 Defines `boost::hana::cycle`.
   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_CYCLE_HPP
  11 #define BOOST_HANA_CYCLE_HPP
  12
  13 #include <boost/hana/fwd/cycle.hpp>
  14
  15 #include <boost/hana/at.hpp>
  16 #include <boost/hana/concat.hpp>
  17 #include <boost/hana/concept/integral_constant.hpp>
  18 #include <boost/hana/concept/monad_plus.hpp>
  19 #include <boost/hana/concept/sequence.hpp>
  20 #include <boost/hana/config.hpp>
  21 #include <boost/hana/core/dispatch.hpp>
  22 #include <boost/hana/core/make.hpp>
  23 #include <boost/hana/detail/array.hpp>
  24 #include <boost/hana/empty.hpp>
  25 #include <boost/hana/length.hpp>
  26
  27 #include <cstddef>
  28 #include <utility>
  29
  30
  31 BOOST_HANA_NAMESPACE_BEGIN
  32     //! @cond
  33     template <typename Xs, typename N>
  34     constexpr auto cycle_t::operator()(Xs&& xs, N const& n) const {
  35         using M = typename hana::tag_of<Xs>::type;
  36         using Cycle = BOOST_HANA_DISPATCH_IF(cycle_impl<M>,
  37             hana::MonadPlus<M>::value
  38         );
  39
  40     #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
  41         static_assert(hana::MonadPlus<M>::value,
  42         "hana::cycle(xs, n) requires 'xs' to be a MonadPlus");
  43
  44         static_assert(hana::IntegralConstant<N>::value,
  45         "hana::cycle(xs, n) requires 'n' to be an IntegralConstant");
  46     #endif
  47
  48         static_assert(N::value >= 0,
  49         "hana::cycle(xs, n) requires 'n' to be non-negative");
  50
  51         return Cycle::apply(static_cast<Xs&&>(xs), n);
  52     }
  53     //! @endcond
  54
  55     namespace detail {
  56         template <typename M, std::size_t n, bool = n % 2 == 0>
  57         struct cycle_helper;
  58
  59         template <typename M>
  60         struct cycle_helper<M, 0, true> {
  61             template <typename Xs>
  62             static constexpr auto apply(Xs const&)
  63             { return hana::empty<M>(); }
  64         };
  65
  66         template <typename M, std::size_t n>
  67         struct cycle_helper<M, n, true> {
  68             template <typename Xs>
  69             static constexpr auto apply(Xs const& xs)
  70             { return cycle_helper<M, n/2>::apply(hana::concat(xs, xs)); }
  71         };
  72
  73         template <typename M, std::size_t n>
  74         struct cycle_helper<M, n, false> {
  75             template <typename Xs>
  76             static constexpr auto apply(Xs const& xs)
  77             { return hana::concat(xs, cycle_helper<M, n-1>::apply(xs)); }
  78         };
  79     }
  80
  81     template <typename M, bool condition>
  82     struct cycle_impl<M, when<condition>> : default_ {
  83         template <typename Xs, typename N>
  84         static constexpr auto apply(Xs const& xs, N const&) {
  85             constexpr std::size_t n = N::value;
  86             return detail::cycle_helper<M, n>::apply(xs);
  87         }
  88     };
  89
  90     namespace detail {
  91         template <std::size_t N, std::size_t Len>
  92         struct cycle_indices {
  93             static constexpr auto compute_value() {
  94                 detail::array<std::size_t, N * Len> indices{};
  95                 // Avoid (incorrect) Clang warning about remainder by zero
  96                 // in the loop below.
  97                 std::size_t len = Len;
  98                 for (std::size_t i = 0; i < N * Len; ++i)
  99                     indices[i] = i % len;
 100                 return indices;
 101             }
 102
 103             static constexpr auto value = compute_value();
 104         };
 105     }
 106
 107     template <typename S>
 108     struct cycle_impl<S, when<Sequence<S>::value>> {
 109         template <typename Indices, typename Xs, std::size_t ...i>
 110         static constexpr auto cycle_helper(Xs&& xs, std::index_sequence<i...>) {
 111             constexpr auto indices = Indices::value;
 112             (void)indices; // workaround GCC warning when sizeof...(i) == 0
 113             return hana::make<S>(hana::at_c<indices[i]>(xs)...);
 114         }
 115
 116         template <typename Xs, typename N>
 117         static constexpr auto apply(Xs&& xs, N const&) {
 118             constexpr std::size_t n = N::value;
 119             constexpr std::size_t len = decltype(hana::length(xs))::value;
 120             using Indices = detail::cycle_indices<n, len>;
 121             return cycle_helper<Indices>(static_cast<Xs&&>(xs),
 122                                          std::make_index_sequence<n * len>{});
 123         }
 124     };
 125 BOOST_HANA_NAMESPACE_END
 126
 127 #endif // !BOOST_HANA_CYCLE_HPP