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 <__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