libcxx/include/__ranges/drop_view.h

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