xref: /freebsd/contrib/llvm-project/libcxx/include/__ranges/drop_view.h (revision f126890ac5386406dadf7c4cfa9566cbb56537c5)
1 // -*- C++ -*-
2 //===----------------------------------------------------------------------===//
3 //
4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 // See https://llvm.org/LICENSE.txt for license information.
6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //
8 //===----------------------------------------------------------------------===//
9 
10 #ifndef _LIBCPP___RANGES_DROP_VIEW_H
11 #define _LIBCPP___RANGES_DROP_VIEW_H
12 
13 #include <__algorithm/min.h>
14 #include <__assert>
15 #include <__concepts/constructible.h>
16 #include <__concepts/convertible_to.h>
17 #include <__config>
18 #include <__functional/bind_back.h>
19 #include <__fwd/span.h>
20 #include <__fwd/string_view.h>
21 #include <__iterator/concepts.h>
22 #include <__iterator/distance.h>
23 #include <__iterator/iterator_traits.h>
24 #include <__iterator/next.h>
25 #include <__ranges/access.h>
26 #include <__ranges/all.h>
27 #include <__ranges/concepts.h>
28 #include <__ranges/empty_view.h>
29 #include <__ranges/enable_borrowed_range.h>
30 #include <__ranges/iota_view.h>
31 #include <__ranges/non_propagating_cache.h>
32 #include <__ranges/range_adaptor.h>
33 #include <__ranges/repeat_view.h>
34 #include <__ranges/size.h>
35 #include <__ranges/subrange.h>
36 #include <__ranges/view_interface.h>
37 #include <__type_traits/conditional.h>
38 #include <__type_traits/decay.h>
39 #include <__type_traits/is_nothrow_constructible.h>
40 #include <__type_traits/make_unsigned.h>
41 #include <__type_traits/remove_cvref.h>
42 #include <__utility/auto_cast.h>
43 #include <__utility/forward.h>
44 #include <__utility/move.h>
45 #include <cstddef>
46 
47 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
48 #  pragma GCC system_header
49 #endif
50 
51 _LIBCPP_PUSH_MACROS
52 #include <__undef_macros>
53 
54 _LIBCPP_BEGIN_NAMESPACE_STD
55 
56 #if _LIBCPP_STD_VER >= 20
57 
58 namespace ranges {
59   template<view _View>
60   class drop_view
61     : public view_interface<drop_view<_View>>
62   {
63     // We cache begin() whenever ranges::next is not guaranteed O(1) to provide an
64     // amortized O(1) begin() method. If this is an input_range, then we cannot cache
65     // begin because begin is not equality preserving.
66     // Note: drop_view<input-range>::begin() is still trivially amortized O(1) because
67     // one can't call begin() on it more than once.
68     static constexpr bool _UseCache = forward_range<_View> && !(random_access_range<_View> && sized_range<_View>);
69     using _Cache = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
70     _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
71     range_difference_t<_View> __count_ = 0;
72     _View __base_ = _View();
73 
74 public:
75     _LIBCPP_HIDE_FROM_ABI drop_view() requires default_initializable<_View> = default;
76 
77     _LIBCPP_HIDE_FROM_ABI
78     constexpr _LIBCPP_EXPLICIT_SINCE_CXX23 drop_view(_View __base, range_difference_t<_View> __count)
79       : __count_(__count)
80       , __base_(std::move(__base))
81     {
82       _LIBCPP_ASSERT_UNCATEGORIZED(__count_ >= 0, "count must be greater than or equal to zero.");
83     }
84 
85     _LIBCPP_HIDE_FROM_ABI constexpr _View base() const& requires copy_constructible<_View> { return __base_; }
86     _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
87 
88     _LIBCPP_HIDE_FROM_ABI
89     constexpr auto begin()
90       requires (!(__simple_view<_View> &&
91                   random_access_range<const _View> && sized_range<const _View>))
92     {
93       if constexpr (_UseCache)
94         if (__cached_begin_.__has_value())
95           return *__cached_begin_;
96 
97       auto __tmp = ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
98       if constexpr (_UseCache)
99         __cached_begin_.__emplace(__tmp);
100       return __tmp;
101     }
102 
103     _LIBCPP_HIDE_FROM_ABI
104     constexpr auto begin() const
105       requires random_access_range<const _View> && sized_range<const _View>
106     {
107       return ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
108     }
109 
110     _LIBCPP_HIDE_FROM_ABI
111     constexpr auto end()
112       requires (!__simple_view<_View>)
113     { return ranges::end(__base_); }
114 
115     _LIBCPP_HIDE_FROM_ABI
116     constexpr auto end() const
117       requires range<const _View>
118     { return ranges::end(__base_); }
119 
120     _LIBCPP_HIDE_FROM_ABI
121     static constexpr auto __size(auto& __self) {
122       const auto __s = ranges::size(__self.__base_);
123       const auto __c = static_cast<decltype(__s)>(__self.__count_);
124       return __s < __c ? 0 : __s - __c;
125     }
126 
127     _LIBCPP_HIDE_FROM_ABI
128     constexpr auto size()
129       requires sized_range<_View>
130     { return __size(*this); }
131 
132     _LIBCPP_HIDE_FROM_ABI
133     constexpr auto size() const
134       requires sized_range<const _View>
135     { return __size(*this); }
136   };
137 
138 template<class _Range>
139 drop_view(_Range&&, range_difference_t<_Range>) -> drop_view<views::all_t<_Range>>;
140 
141 template<class _Tp>
142 inline constexpr bool enable_borrowed_range<drop_view<_Tp>> = enable_borrowed_range<_Tp>;
143 
144 namespace views {
145 namespace __drop {
146 
147 template <class _Tp>
148 inline constexpr bool __is_empty_view = false;
149 
150 template <class _Tp>
151 inline constexpr bool __is_empty_view<empty_view<_Tp>> = true;
152 
153 template <class _Tp>
154 inline constexpr bool __is_passthrough_specialization = false;
155 
156 template <class _Tp, size_t _Extent>
157 inline constexpr bool __is_passthrough_specialization<span<_Tp, _Extent>> = true;
158 
159 template <class _CharT, class _Traits>
160 inline constexpr bool __is_passthrough_specialization<basic_string_view<_CharT, _Traits>> = true;
161 
162 template <class _Np, class _Bound>
163 inline constexpr bool __is_passthrough_specialization<iota_view<_Np, _Bound>> = true;
164 
165 template <class _Iter, class _Sent, subrange_kind _Kind>
166 inline constexpr bool __is_passthrough_specialization<subrange<_Iter, _Sent, _Kind>> =
167     !subrange<_Iter, _Sent, _Kind>::_StoreSize;
168 
169 template <class _Tp>
170 inline constexpr bool __is_subrange_specialization_with_store_size = false;
171 
172 template <class _Iter, class _Sent, subrange_kind _Kind>
173 inline constexpr bool __is_subrange_specialization_with_store_size<subrange<_Iter, _Sent, _Kind>> =
174     subrange<_Iter, _Sent, _Kind>::_StoreSize;
175 
176 template <class _Tp>
177 struct __passthrough_type;
178 
179 template <class _Tp, size_t _Extent>
180 struct __passthrough_type<span<_Tp, _Extent>> {
181   using type = span<_Tp>;
182 };
183 
184 template <class _CharT, class _Traits>
185 struct __passthrough_type<basic_string_view<_CharT, _Traits>> {
186   using type = basic_string_view<_CharT, _Traits>;
187 };
188 
189 template <class _Np, class _Bound>
190 struct __passthrough_type<iota_view<_Np, _Bound>> {
191   using type = iota_view<_Np, _Bound>;
192 };
193 
194 template <class _Iter, class _Sent, subrange_kind _Kind>
195 struct __passthrough_type<subrange<_Iter, _Sent, _Kind>> {
196   using type = subrange<_Iter, _Sent, _Kind>;
197 };
198 
199 template <class _Tp>
200 using __passthrough_type_t = typename __passthrough_type<_Tp>::type;
201 
202 struct __fn {
203   // [range.drop.overview]: the `empty_view` case.
204   template <class _Range, convertible_to<range_difference_t<_Range>> _Np>
205     requires __is_empty_view<remove_cvref_t<_Range>>
206   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
207   constexpr auto operator()(_Range&& __range, _Np&&) const
208     noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Range>(__range))))
209     -> decltype(      _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)))
210     { return          _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)); }
211 
212   // [range.drop.overview]: the `span | basic_string_view | iota_view | subrange (StoreSize == false)` case.
213   template <class _Range,
214             convertible_to<range_difference_t<_Range>> _Np,
215             class _RawRange = remove_cvref_t<_Range>,
216             class _Dist = range_difference_t<_Range>>
217     requires (!__is_empty_view<_RawRange> &&
218               random_access_range<_RawRange> &&
219               sized_range<_RawRange> &&
220               __is_passthrough_specialization<_RawRange>)
221   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
222   constexpr auto operator()(_Range&& __rng, _Np&& __n) const
223     noexcept(noexcept(__passthrough_type_t<_RawRange>(
224                               ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
225                               ranges::end(__rng)
226                               )))
227     -> decltype(      __passthrough_type_t<_RawRange>(
228                               // Note: deliberately not forwarding `__rng` to guard against double moves.
229                               ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
230                               ranges::end(__rng)
231                               ))
232     { return          __passthrough_type_t<_RawRange>(
233                               ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
234                               ranges::end(__rng)
235                               ); }
236 
237   // [range.drop.overview]: the `subrange (StoreSize == true)` case.
238   template <class _Range,
239             convertible_to<range_difference_t<_Range>> _Np,
240             class _RawRange = remove_cvref_t<_Range>,
241             class _Dist = range_difference_t<_Range>>
242     requires (!__is_empty_view<_RawRange> &&
243               random_access_range<_RawRange> &&
244               sized_range<_RawRange> &&
245               __is_subrange_specialization_with_store_size<_RawRange>)
246   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
247   constexpr auto operator()(_Range&& __rng, _Np&& __n) const
248     noexcept(noexcept(_RawRange(
249                               ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
250                               ranges::end(__rng),
251                               std::__to_unsigned_like(ranges::distance(__rng) -
252                                   std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
253                               )))
254     -> decltype(      _RawRange(
255                               // Note: deliberately not forwarding `__rng` to guard against double moves.
256                               ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
257                               ranges::end(__rng),
258                               std::__to_unsigned_like(ranges::distance(__rng) -
259                                   std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
260                               ))
261     {
262       // Introducing local variables avoids calculating `min` and `distance` twice (at the cost of diverging from the
263       // expression used in the `noexcept` clause and the return statement).
264       auto __dist = ranges::distance(__rng);
265       auto __clamped = std::min<_Dist>(__dist, std::forward<_Np>(__n));
266       return          _RawRange(
267                               ranges::begin(__rng) + __clamped,
268                               ranges::end(__rng),
269                               std::__to_unsigned_like(__dist - __clamped)
270                               );}
271 // clang-format off
272 #if _LIBCPP_STD_VER >= 23
273   // [range.drop.overview]: the `repeat_view` "_RawRange models sized_range" case.
274   template <class _Range,
275             convertible_to<range_difference_t<_Range>> _Np,
276             class _RawRange = remove_cvref_t<_Range>,
277             class _Dist     = range_difference_t<_Range>>
278     requires (__is_repeat_specialization<_RawRange> && sized_range<_RawRange>)
279   _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr auto operator()(_Range&& __range, _Np&& __n) const
280     noexcept(noexcept(views::repeat(*__range.__value_, ranges::distance(__range) - std::min<_Dist>(ranges::distance(__range), std::forward<_Np>(__n)))))
281     -> decltype(      views::repeat(*__range.__value_, ranges::distance(__range) - std::min<_Dist>(ranges::distance(__range), std::forward<_Np>(__n))))
282     { return          views::repeat(*__range.__value_, ranges::distance(__range) - std::min<_Dist>(ranges::distance(__range), std::forward<_Np>(__n))); }
283 
284   // [range.drop.overview]: the `repeat_view` "otherwise" case.
285   template <class _Range,
286             convertible_to<range_difference_t<_Range>> _Np,
287             class _RawRange = remove_cvref_t<_Range>,
288             class _Dist     = range_difference_t<_Range>>
289     requires (__is_repeat_specialization<_RawRange> && !sized_range<_RawRange>)
290   _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI
291   constexpr auto operator()(_Range&& __range, _Np&&) const
292     noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Range>(__range))))
293     -> decltype(      _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)))
294     { return          _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)); }
295 #endif
296 // clang-format on
297 
298   // [range.drop.overview]: the "otherwise" case.
299   template <class _Range, convertible_to<range_difference_t<_Range>> _Np,
300             class _RawRange = remove_cvref_t<_Range>>
301     // Note: without specifically excluding the other cases, GCC sees this overload as ambiguous with the other
302     // overloads.
303     requires (!(__is_empty_view<_RawRange> ||
304 #if _LIBCPP_STD_VER >= 23
305                 __is_repeat_specialization<_RawRange> ||
306 #endif
307                (__is_subrange_specialization_with_store_size<_RawRange> &&
308                sized_range<_RawRange> &&
309                 random_access_range<_RawRange>) ||
310                (__is_passthrough_specialization<_RawRange> &&
311                 sized_range<_RawRange> &&
312                 random_access_range<_RawRange>)
313              ))
314   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
315   constexpr auto operator()(_Range&& __range, _Np&& __n) const
316     noexcept(noexcept(drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n))))
317     -> decltype(      drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n)))
318     { return          drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n)); }
319 
320   template <class _Np>
321     requires constructible_from<decay_t<_Np>, _Np>
322   [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
323   constexpr auto operator()(_Np&& __n) const
324     noexcept(is_nothrow_constructible_v<decay_t<_Np>, _Np>)
325   { return __range_adaptor_closure_t(std::__bind_back(*this, std::forward<_Np>(__n))); }
326 };
327 
328 } // namespace __drop
329 
330 inline namespace __cpo {
331   inline constexpr auto drop = __drop::__fn{};
332 } // namespace __cpo
333 } // namespace views
334 
335 } // namespace ranges
336 
337 #endif // _LIBCPP_STD_VER >= 20
338 
339 _LIBCPP_END_NAMESPACE_STD
340 
341 _LIBCPP_POP_MACROS
342 
343 #endif // _LIBCPP___RANGES_DROP_VIEW_H
344