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1 | /*! |
2 | @file | |
3 | Defines `boost::hana::at_key`. | |
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_AT_KEY_HPP | |
11 | #define BOOST_HANA_AT_KEY_HPP | |
12 | ||
13 | #include <boost/hana/fwd/at_key.hpp> | |
14 | ||
15 | #include <boost/hana/accessors.hpp> | |
16 | #include <boost/hana/at.hpp> | |
17 | #include <boost/hana/concept/searchable.hpp> | |
18 | #include <boost/hana/concept/struct.hpp> | |
19 | #include <boost/hana/config.hpp> | |
20 | #include <boost/hana/core/dispatch.hpp> | |
21 | #include <boost/hana/detail/decay.hpp> | |
22 | #include <boost/hana/equal.hpp> | |
23 | #include <boost/hana/find.hpp> | |
24 | #include <boost/hana/find_if.hpp> | |
25 | #include <boost/hana/first.hpp> | |
26 | #include <boost/hana/functional/on.hpp> | |
27 | #include <boost/hana/length.hpp> | |
28 | #include <boost/hana/optional.hpp> | |
29 | #include <boost/hana/second.hpp> | |
30 | ||
31 | #include <cstddef> | |
32 | #include <utility> | |
33 | ||
34 | ||
35 | BOOST_HANA_NAMESPACE_BEGIN | |
36 | //! @cond | |
37 | template <typename Xs, typename Key> | |
38 | constexpr decltype(auto) at_key_t::operator()(Xs&& xs, Key const& key) const { | |
39 | using S = typename hana::tag_of<Xs>::type; | |
40 | using AtKey = BOOST_HANA_DISPATCH_IF(at_key_impl<S>, | |
41 | hana::Searchable<S>::value | |
42 | ); | |
43 | ||
44 | #ifndef BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS | |
45 | static_assert(hana::Searchable<S>::value, | |
46 | "hana::at_key(xs, key) requires 'xs' to be Searchable"); | |
47 | #endif | |
48 | ||
49 | return AtKey::apply(static_cast<Xs&&>(xs), key); | |
50 | } | |
51 | //! @endcond | |
52 | ||
53 | template <typename S, bool condition> | |
54 | struct at_key_impl<S, when<condition>> : default_ { | |
55 | template <typename Xs, typename Key> | |
56 | static constexpr auto apply(Xs&& xs, Key const& key) { | |
57 | return hana::find(static_cast<Xs&&>(xs), key).value(); | |
58 | } | |
59 | }; | |
60 | ||
61 | namespace at_key_detail { | |
62 | template <typename T> | |
63 | struct equal_to { | |
64 | T const& t; | |
65 | template <typename U> | |
66 | constexpr auto operator()(U const& u) const { | |
67 | return hana::equal(t, u); | |
68 | } | |
69 | }; | |
70 | ||
71 | //! @todo This causes an awful duplication of code with `find_if`. | |
72 | template <typename Xs, typename Pred, std::size_t i, std::size_t N, bool Done> | |
73 | struct advance_until; | |
74 | ||
75 | template <typename Xs, typename Pred, std::size_t i, std::size_t N> | |
76 | struct advance_until<Xs, Pred, i, N, false> | |
77 | : advance_until<Xs, Pred, i + 1, N, static_cast<bool>(detail::decay<decltype( | |
78 | std::declval<Pred>()(hana::at_c<i>(std::declval<Xs>())) | |
79 | )>::type::value)> | |
80 | { }; | |
81 | ||
82 | template <typename Xs, typename Pred, std::size_t N> | |
83 | struct advance_until<Xs, Pred, N, N, false> { | |
84 | template <typename Ys> | |
85 | static constexpr auto apply(Ys&&) = delete; | |
86 | }; | |
87 | ||
88 | template <typename Xs, typename Pred, std::size_t i, std::size_t N> | |
89 | struct advance_until<Xs, Pred, i, N, true> { | |
90 | template <typename Ys> | |
91 | static constexpr decltype(auto) apply(Ys&& ys) { | |
92 | return hana::at_c<i - 1>(static_cast<Ys&&>(ys)); | |
93 | } | |
94 | }; | |
95 | } | |
96 | ||
97 | template <typename S> | |
98 | struct at_key_impl<S, when<hana::Sequence<S>::value>> { | |
99 | template <typename Xs, typename Key> | |
100 | static constexpr decltype(auto) apply(Xs&& xs, Key const&) { | |
101 | constexpr std::size_t N = decltype(hana::length(xs))::value; | |
102 | using Pred = at_key_detail::equal_to<Key>; | |
103 | return at_key_detail::advance_until<Xs&&, Pred, 0, N, false>::apply( | |
104 | static_cast<Xs&&>(xs) | |
105 | ); | |
106 | } | |
107 | }; | |
108 | ||
109 | template <typename S> | |
110 | struct at_key_impl<S, when<hana::Struct<S>::value>> { | |
111 | template <typename X, typename Key> | |
112 | static constexpr decltype(auto) apply(X&& x, Key const& key) { | |
113 | auto accessor = hana::second(*hana::find_if(hana::accessors<S>(), | |
114 | hana::equal.to(key) ^hana::on^ hana::first | |
115 | )); | |
116 | return accessor(static_cast<X&&>(x)); | |
117 | } | |
118 | }; | |
119 | BOOST_HANA_NAMESPACE_END | |
120 | ||
121 | #endif // !BOOST_HANA_AT_KEY_HPP |