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/size.h>
34#include <__ranges/subrange.h>
35#include <__ranges/view_interface.h>
36#include <__utility/auto_cast.h>
37#include <__utility/forward.h>
38#include <__utility/move.h>
39#include <type_traits>
40
41#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
42#  pragma GCC system_header
43#endif
44
45_LIBCPP_PUSH_MACROS
46#include <__undef_macros>
47
48_LIBCPP_BEGIN_NAMESPACE_STD
49
50#if _LIBCPP_STD_VER > 17
51
52namespace ranges {
53  template<view _View>
54  class drop_view
55    : public view_interface<drop_view<_View>>
56  {
57    // We cache begin() whenever ranges::next is not guaranteed O(1) to provide an
58    // amortized O(1) begin() method. If this is an input_range, then we cannot cache
59    // begin because begin is not equality preserving.
60    // Note: drop_view<input-range>::begin() is still trivially amortized O(1) because
61    // one can't call begin() on it more than once.
62    static constexpr bool _UseCache = forward_range<_View> && !(random_access_range<_View> && sized_range<_View>);
63    using _Cache = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>;
64    _LIBCPP_NO_UNIQUE_ADDRESS _Cache __cached_begin_ = _Cache();
65    range_difference_t<_View> __count_ = 0;
66    _View __base_ = _View();
67
68public:
69    drop_view() requires default_initializable<_View> = default;
70
71    _LIBCPP_HIDE_FROM_ABI
72    constexpr drop_view(_View __base, range_difference_t<_View> __count)
73      : __count_(__count)
74      , __base_(std::move(__base))
75    {
76      _LIBCPP_ASSERT(__count_ >= 0, "count must be greater than or equal to zero.");
77    }
78
79    _LIBCPP_HIDE_FROM_ABI constexpr _View base() const& requires copy_constructible<_View> { return __base_; }
80    _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return std::move(__base_); }
81
82    _LIBCPP_HIDE_FROM_ABI
83    constexpr auto begin()
84      requires (!(__simple_view<_View> &&
85                  random_access_range<const _View> && sized_range<const _View>))
86    {
87      if constexpr (_UseCache)
88        if (__cached_begin_.__has_value())
89          return *__cached_begin_;
90
91      auto __tmp = ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
92      if constexpr (_UseCache)
93        __cached_begin_.__emplace(__tmp);
94      return __tmp;
95    }
96
97    _LIBCPP_HIDE_FROM_ABI
98    constexpr auto begin() const
99      requires random_access_range<const _View> && sized_range<const _View>
100    {
101      return ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_));
102    }
103
104    _LIBCPP_HIDE_FROM_ABI
105    constexpr auto end()
106      requires (!__simple_view<_View>)
107    { return ranges::end(__base_); }
108
109    _LIBCPP_HIDE_FROM_ABI
110    constexpr auto end() const
111      requires range<const _View>
112    { return ranges::end(__base_); }
113
114    _LIBCPP_HIDE_FROM_ABI
115    static constexpr auto __size(auto& __self) {
116      const auto __s = ranges::size(__self.__base_);
117      const auto __c = static_cast<decltype(__s)>(__self.__count_);
118      return __s < __c ? 0 : __s - __c;
119    }
120
121    _LIBCPP_HIDE_FROM_ABI
122    constexpr auto size()
123      requires sized_range<_View>
124    { return __size(*this); }
125
126    _LIBCPP_HIDE_FROM_ABI
127    constexpr auto size() const
128      requires sized_range<const _View>
129    { return __size(*this); }
130  };
131
132template<class _Range>
133drop_view(_Range&&, range_difference_t<_Range>) -> drop_view<views::all_t<_Range>>;
134
135template<class _Tp>
136inline constexpr bool enable_borrowed_range<drop_view<_Tp>> = enable_borrowed_range<_Tp>;
137
138namespace views {
139namespace __drop {
140
141template <class _Tp>
142inline constexpr bool __is_empty_view = false;
143
144template <class _Tp>
145inline constexpr bool __is_empty_view<empty_view<_Tp>> = true;
146
147template <class _Tp>
148inline constexpr bool __is_passthrough_specialization = false;
149
150template <class _Tp, size_t _Extent>
151inline constexpr bool __is_passthrough_specialization<span<_Tp, _Extent>> = true;
152
153template <class _CharT, class _Traits>
154inline constexpr bool __is_passthrough_specialization<basic_string_view<_CharT, _Traits>> = true;
155
156template <class _Np, class _Bound>
157inline constexpr bool __is_passthrough_specialization<iota_view<_Np, _Bound>> = true;
158
159template <class _Iter, class _Sent, subrange_kind _Kind>
160inline constexpr bool __is_passthrough_specialization<subrange<_Iter, _Sent, _Kind>> =
161    !subrange<_Iter, _Sent, _Kind>::_StoreSize;
162
163template <class _Tp>
164inline constexpr bool __is_subrange_specialization_with_store_size = false;
165
166template <class _Iter, class _Sent, subrange_kind _Kind>
167inline constexpr bool __is_subrange_specialization_with_store_size<subrange<_Iter, _Sent, _Kind>> =
168    subrange<_Iter, _Sent, _Kind>::_StoreSize;
169
170template <class _Tp>
171struct __passthrough_type;
172
173template <class _Tp, size_t _Extent>
174struct __passthrough_type<span<_Tp, _Extent>> {
175  using type = span<_Tp>;
176};
177
178template <class _CharT, class _Traits>
179struct __passthrough_type<basic_string_view<_CharT, _Traits>> {
180  using type = basic_string_view<_CharT, _Traits>;
181};
182
183template <class _Np, class _Bound>
184struct __passthrough_type<iota_view<_Np, _Bound>> {
185  using type = iota_view<_Np, _Bound>;
186};
187
188template <class _Iter, class _Sent, subrange_kind _Kind>
189struct __passthrough_type<subrange<_Iter, _Sent, _Kind>> {
190  using type = subrange<_Iter, _Sent, _Kind>;
191};
192
193template <class _Tp>
194using __passthrough_type_t = typename __passthrough_type<_Tp>::type;
195
196struct __fn {
197  // [range.drop.overview]: the `empty_view` case.
198  template <class _Range, convertible_to<range_difference_t<_Range>> _Np>
199    requires __is_empty_view<remove_cvref_t<_Range>>
200  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
201  constexpr auto operator()(_Range&& __range, _Np&&) const
202    noexcept(noexcept(_LIBCPP_AUTO_CAST(std::forward<_Range>(__range))))
203    -> decltype(      _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)))
204    { return          _LIBCPP_AUTO_CAST(std::forward<_Range>(__range)); }
205
206  // [range.drop.overview]: the `span | basic_string_view | iota_view | subrange (StoreSize == false)` case.
207  template <class _Range,
208            convertible_to<range_difference_t<_Range>> _Np,
209            class _RawRange = remove_cvref_t<_Range>,
210            class _Dist = range_difference_t<_Range>>
211    requires (!__is_empty_view<_RawRange> &&
212              random_access_range<_RawRange> &&
213              sized_range<_RawRange> &&
214              __is_passthrough_specialization<_RawRange>)
215  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
216  constexpr auto operator()(_Range&& __rng, _Np&& __n) const
217    noexcept(noexcept(__passthrough_type_t<_RawRange>(
218                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
219                              ranges::end(__rng)
220                              )))
221    -> decltype(      __passthrough_type_t<_RawRange>(
222                              // Note: deliberately not forwarding `__rng` to guard against double moves.
223                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
224                              ranges::end(__rng)
225                              ))
226    { return          __passthrough_type_t<_RawRange>(
227                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
228                              ranges::end(__rng)
229                              ); }
230
231  // [range.drop.overview]: the `subrange (StoreSize == true)` case.
232  template <class _Range,
233            convertible_to<range_difference_t<_Range>> _Np,
234            class _RawRange = remove_cvref_t<_Range>,
235            class _Dist = range_difference_t<_Range>>
236    requires (!__is_empty_view<_RawRange> &&
237              random_access_range<_RawRange> &&
238              sized_range<_RawRange> &&
239              __is_subrange_specialization_with_store_size<_RawRange>)
240  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
241  constexpr auto operator()(_Range&& __rng, _Np&& __n) const
242    noexcept(noexcept(_RawRange(
243                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
244                              ranges::end(__rng),
245                              std::__to_unsigned_like(ranges::distance(__rng) -
246                                  std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
247                              )))
248    -> decltype(      _RawRange(
249                              // Note: deliberately not forwarding `__rng` to guard against double moves.
250                              ranges::begin(__rng) + std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)),
251                              ranges::end(__rng),
252                              std::__to_unsigned_like(ranges::distance(__rng) -
253                                  std::min<_Dist>(ranges::distance(__rng), std::forward<_Np>(__n)))
254                              ))
255    {
256      // Introducing local variables avoids calculating `min` and `distance` twice (at the cost of diverging from the
257      // expression used in the `noexcept` clause and the return statement).
258      auto dist = ranges::distance(__rng);
259      auto clamped = std::min<_Dist>(dist, std::forward<_Np>(__n));
260      return          _RawRange(
261                              ranges::begin(__rng) + clamped,
262                              ranges::end(__rng),
263                              std::__to_unsigned_like(dist - clamped)
264                              );}
265
266  // [range.drop.overview]: the "otherwise" case.
267  template <class _Range, convertible_to<range_difference_t<_Range>> _Np,
268            class _RawRange = remove_cvref_t<_Range>>
269    // Note: without specifically excluding the other cases, GCC sees this overload as ambiguous with the other
270    // overloads.
271    requires (!(__is_empty_view<_RawRange> ||
272               (__is_subrange_specialization_with_store_size<_RawRange> &&
273               sized_range<_RawRange> &&
274                random_access_range<_RawRange>) ||
275               (__is_passthrough_specialization<_RawRange> &&
276                sized_range<_RawRange> &&
277                random_access_range<_RawRange>)
278             ))
279  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
280  constexpr auto operator()(_Range&& __range, _Np&& __n) const
281    noexcept(noexcept(drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n))))
282    -> decltype(      drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n)))
283    { return          drop_view(std::forward<_Range>(__range), std::forward<_Np>(__n)); }
284
285  template <class _Np>
286    requires constructible_from<decay_t<_Np>, _Np>
287  [[nodiscard]] _LIBCPP_HIDE_FROM_ABI
288  constexpr auto operator()(_Np&& __n) const
289    noexcept(is_nothrow_constructible_v<decay_t<_Np>, _Np>)
290  { return __range_adaptor_closure_t(std::__bind_back(*this, std::forward<_Np>(__n))); }
291};
292
293} // namespace __drop
294
295inline namespace __cpo {
296  inline constexpr auto drop = __drop::__fn{};
297} // namespace __cpo
298} // namespace views
299
300} // namespace ranges
301
302#endif // _LIBCPP_STD_VER > 17
303
304_LIBCPP_END_NAMESPACE_STD
305
306_LIBCPP_POP_MACROS
307
308#endif // _LIBCPP___RANGES_DROP_VIEW_H
309