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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 |