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