]>
Commit | Line | Data |
---|---|---|
7c673cae FG |
1 | /*! |
2 | @file | |
3 | Defines `boost::hana::filter`. | |
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_FILTER_HPP | |
11 | #define BOOST_HANA_FILTER_HPP | |
12 | ||
13 | #include <boost/hana/fwd/filter.hpp> | |
14 | ||
15 | #include <boost/hana/at.hpp> | |
16 | #include <boost/hana/bool.hpp> | |
17 | #include <boost/hana/chain.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/algorithm.hpp> | |
24 | #include <boost/hana/detail/array.hpp> | |
25 | #include <boost/hana/detail/decay.hpp> | |
26 | #include <boost/hana/empty.hpp> | |
27 | #include <boost/hana/lift.hpp> | |
28 | #include <boost/hana/unpack.hpp> | |
29 | ||
30 | #include <cstddef> | |
31 | #include <utility> | |
32 | ||
33 | ||
34 | BOOST_HANA_NAMESPACE_BEGIN | |
35 | //! @cond | |
36 | template <typename Xs, typename Pred> | |
37 | constexpr auto filter_t::operator()(Xs&& xs, Pred&& pred) const { | |
38 | using M = typename hana::tag_of<Xs>::type; | |
39 | using Filter = BOOST_HANA_DISPATCH_IF(filter_impl<M>, | |
40 | hana::MonadPlus<M>::value | |
41 | ); | |
42 | ||
43 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
44 | static_assert(hana::MonadPlus<M>::value, | |
45 | "hana::filter(xs, pred) requires 'xs' to be a MonadPlus"); | |
46 | #endif | |
47 | ||
48 | return Filter::apply(static_cast<Xs&&>(xs), | |
49 | static_cast<Pred&&>(pred)); | |
50 | } | |
51 | //! @endcond | |
52 | ||
53 | namespace detail { | |
54 | template <typename Pred, typename M> | |
55 | struct lift_or_empty { | |
56 | template <typename X> | |
57 | static constexpr auto helper(X&& x, hana::true_) | |
58 | { return hana::lift<M>(static_cast<X&&>(x)); } | |
59 | ||
60 | template <typename X> | |
61 | static constexpr auto helper(X&&, hana::false_) | |
62 | { return hana::empty<M>(); } | |
63 | ||
64 | template <typename X> | |
65 | constexpr auto operator()(X&& x) const { | |
66 | constexpr bool cond = decltype(std::declval<Pred>()(x))::value; | |
67 | return helper(static_cast<X&&>(x), hana::bool_c<cond>); | |
68 | } | |
69 | }; | |
70 | } | |
71 | ||
72 | template <typename M, bool condition> | |
73 | struct filter_impl<M, when<condition>> : default_ { | |
74 | template <typename Xs, typename Pred> | |
75 | static constexpr decltype(auto) apply(Xs&& xs, Pred const&) { | |
76 | return hana::chain(static_cast<Xs&&>(xs), | |
77 | detail::lift_or_empty<Pred, M>{} | |
78 | ); | |
79 | } | |
80 | }; | |
81 | ||
82 | namespace detail { | |
83 | template <bool ...b> | |
84 | struct filter_indices { | |
85 | static constexpr auto compute_indices() { | |
86 | constexpr bool bs[] = {b..., false}; // avoid empty array | |
87 | constexpr std::size_t N = detail::count(bs, bs + sizeof(bs), true); | |
88 | detail::array<std::size_t, N> indices{}; | |
89 | std::size_t* keep = &indices[0]; | |
90 | for (std::size_t i = 0; i < sizeof...(b); ++i) | |
91 | if (bs[i]) | |
92 | *keep++ = i; | |
93 | return indices; | |
94 | } | |
95 | ||
96 | static constexpr auto indices = compute_indices(); | |
97 | }; | |
98 | ||
99 | template <typename Pred> | |
100 | struct make_filter_indices { | |
101 | Pred const& pred; | |
102 | template <typename ...X> | |
103 | auto operator()(X&& ...x) const -> filter_indices< | |
104 | static_cast<bool>(detail::decay< | |
105 | decltype(pred(static_cast<X&&>(x))) | |
106 | >::type::value)... | |
107 | > { return {}; } | |
108 | }; | |
109 | } | |
110 | ||
111 | template <typename S> | |
112 | struct filter_impl<S, when<Sequence<S>::value>> { | |
113 | template <typename Indices, typename Xs, std::size_t ...i> | |
114 | static constexpr auto filter_helper(Xs&& xs, std::index_sequence<i...>) { | |
115 | return hana::make<S>( | |
116 | hana::at_c<Indices::indices[i]>(static_cast<Xs&&>(xs))... | |
117 | ); | |
118 | } | |
119 | ||
120 | template <typename Xs, typename Pred> | |
121 | static constexpr auto apply(Xs&& xs, Pred const& pred) { | |
122 | using Indices = decltype( | |
123 | hana::unpack(static_cast<Xs&&>(xs), | |
124 | detail::make_filter_indices<Pred>{pred}) | |
125 | ); | |
126 | ||
127 | return filter_impl::filter_helper<Indices>( | |
128 | static_cast<Xs&&>(xs), | |
129 | std::make_index_sequence<Indices::indices.size()>{} | |
130 | ); | |
131 | } | |
132 | }; | |
133 | BOOST_HANA_NAMESPACE_END | |
134 | ||
135 | #endif // !BOOST_HANA_FILTER_HPP |