1/*!
2@file
3Defines `boost::hana::cycle`.
4
5Copyright Louis Dionne 2013-2022
6Distributed 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
31namespace boost { namespace hana {
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}} // end namespace boost::hana
126
127#endif // !BOOST_HANA_CYCLE_HPP
128

source code of boost/libs/hana/include/boost/hana/cycle.hpp