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 <__config> 13 #include <__iterator/concepts.h> 14 #include <__iterator/iterator_traits.h> 15 #include <__iterator/next.h> 16 #include <__ranges/access.h> 17 #include <__ranges/all.h> 18 #include <__ranges/concepts.h> 19 #include <__ranges/enable_borrowed_range.h> 20 #include <__ranges/non_propagating_cache.h> 21 #include <__ranges/size.h> 22 #include <__ranges/view_interface.h> 23 #include <__utility/move.h> 24 #include <concepts> 25 #include <type_traits> 26 27 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 28 #pragma GCC system_header 29 #endif 30 31 _LIBCPP_PUSH_MACROS 32 #include <__undef_macros> 33 34 _LIBCPP_BEGIN_NAMESPACE_STD 35 36 #if !defined(_LIBCPP_HAS_NO_RANGES) 37 38 namespace ranges { 39 template<view _View> 40 class drop_view 41 : public view_interface<drop_view<_View>> 42 { 43 // We cache begin() whenever ranges::next is not guaranteed O(1) to provide an 44 // amortized O(1) begin() method. If this is an input_range, then we cannot cache 45 // begin because begin is not equality preserving. 46 // Note: drop_view<input-range>::begin() is still trivially amortized O(1) because 47 // one can't call begin() on it more than once. 48 static constexpr bool _UseCache = forward_range<_View> && !(random_access_range<_View> && sized_range<_View>); 49 using _Cache = _If<_UseCache, __non_propagating_cache<iterator_t<_View>>, __empty_cache>; 50 [[no_unique_address]] _Cache __cached_begin_ = _Cache(); 51 range_difference_t<_View> __count_ = 0; 52 _View __base_ = _View(); 53 54 public: 55 drop_view() requires default_initializable<_View> = default; 56 57 _LIBCPP_HIDE_FROM_ABI 58 constexpr drop_view(_View __base, range_difference_t<_View> __count) 59 : __count_(__count) 60 , __base_(_VSTD::move(__base)) 61 { 62 _LIBCPP_ASSERT(__count_ >= 0, "count must be greater than or equal to zero."); 63 } 64 65 _LIBCPP_HIDE_FROM_ABI constexpr _View base() const& requires copy_constructible<_View> { return __base_; } 66 _LIBCPP_HIDE_FROM_ABI constexpr _View base() && { return _VSTD::move(__base_); } 67 68 _LIBCPP_HIDE_FROM_ABI 69 constexpr auto begin() 70 requires (!(__simple_view<_View> && 71 random_access_range<const _View> && sized_range<const _View>)) 72 { 73 if constexpr (_UseCache) 74 if (__cached_begin_.__has_value()) 75 return *__cached_begin_; 76 77 auto __tmp = ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_)); 78 if constexpr (_UseCache) 79 __cached_begin_.__set(__tmp); 80 return __tmp; 81 } 82 83 _LIBCPP_HIDE_FROM_ABI 84 constexpr auto begin() const 85 requires random_access_range<const _View> && sized_range<const _View> 86 { 87 return ranges::next(ranges::begin(__base_), __count_, ranges::end(__base_)); 88 } 89 90 _LIBCPP_HIDE_FROM_ABI 91 constexpr auto end() 92 requires (!__simple_view<_View>) 93 { return ranges::end(__base_); } 94 95 _LIBCPP_HIDE_FROM_ABI 96 constexpr auto end() const 97 requires range<const _View> 98 { return ranges::end(__base_); } 99 100 _LIBCPP_HIDE_FROM_ABI 101 static constexpr auto __size(auto& __self) { 102 const auto __s = ranges::size(__self.__base_); 103 const auto __c = static_cast<decltype(__s)>(__self.__count_); 104 return __s < __c ? 0 : __s - __c; 105 } 106 107 _LIBCPP_HIDE_FROM_ABI 108 constexpr auto size() 109 requires sized_range<_View> 110 { return __size(*this); } 111 112 _LIBCPP_HIDE_FROM_ABI 113 constexpr auto size() const 114 requires sized_range<const _View> 115 { return __size(*this); } 116 }; 117 118 template<class _Range> 119 drop_view(_Range&&, range_difference_t<_Range>) -> drop_view<views::all_t<_Range>>; 120 121 template<class _Tp> 122 inline constexpr bool enable_borrowed_range<drop_view<_Tp>> = enable_borrowed_range<_Tp>; 123 } // namespace ranges 124 125 #endif // !defined(_LIBCPP_HAS_NO_RANGES) 126 127 _LIBCPP_END_NAMESPACE_STD 128 129 _LIBCPP_POP_MACROS 130 131 #endif // _LIBCPP___RANGES_DROP_VIEW_H 132