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___ITERATOR_ADVANCE_H 11 #define _LIBCPP___ITERATOR_ADVANCE_H 12 13 #include <__assert> 14 #include <__concepts/assignable.h> 15 #include <__concepts/same_as.h> 16 #include <__config> 17 #include <__iterator/concepts.h> 18 #include <__iterator/incrementable_traits.h> 19 #include <__iterator/iterator_traits.h> 20 #include <__type_traits/enable_if.h> 21 #include <__type_traits/is_integral.h> 22 #include <__utility/convert_to_integral.h> 23 #include <__utility/declval.h> 24 #include <__utility/move.h> 25 #include <__utility/unreachable.h> 26 #include <limits> 27 28 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER) 29 # pragma GCC system_header 30 #endif 31 32 _LIBCPP_PUSH_MACROS 33 #include <__undef_macros> 34 35 _LIBCPP_BEGIN_NAMESPACE_STD 36 37 template <class _InputIter> 38 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void 39 __advance(_InputIter& __i, typename iterator_traits<_InputIter>::difference_type __n, input_iterator_tag) { 40 for (; __n > 0; --__n) 41 ++__i; 42 } 43 44 template <class _BiDirIter> 45 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void 46 __advance(_BiDirIter& __i, typename iterator_traits<_BiDirIter>::difference_type __n, bidirectional_iterator_tag) { 47 if (__n >= 0) 48 for (; __n > 0; --__n) 49 ++__i; 50 else 51 for (; __n < 0; ++__n) 52 --__i; 53 } 54 55 template <class _RandIter> 56 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void 57 __advance(_RandIter& __i, typename iterator_traits<_RandIter>::difference_type __n, random_access_iterator_tag) { 58 __i += __n; 59 } 60 61 template < class _InputIter, 62 class _Distance, 63 class _IntegralDistance = decltype(std::__convert_to_integral(std::declval<_Distance>())), 64 __enable_if_t<is_integral<_IntegralDistance>::value, int> = 0> 65 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17 void advance(_InputIter& __i, _Distance __orig_n) { 66 typedef typename iterator_traits<_InputIter>::difference_type _Difference; 67 _Difference __n = static_cast<_Difference>(std::__convert_to_integral(__orig_n)); 68 // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation. 69 _LIBCPP_ASSERT_PEDANTIC(__n >= 0 || __has_bidirectional_iterator_category<_InputIter>::value, 70 "Attempt to advance(it, n) with negative n on a non-bidirectional iterator"); 71 std::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category()); 72 } 73 74 #if _LIBCPP_STD_VER >= 20 75 76 // [range.iter.op.advance] 77 78 namespace ranges { 79 struct __advance { 80 private: 81 template <class _Ip> 82 _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_forward(_Ip& __i, iter_difference_t<_Ip> __n) { 83 while (__n > 0) { 84 --__n; 85 ++__i; 86 } 87 } 88 89 template <class _Ip> 90 _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_backward(_Ip& __i, iter_difference_t<_Ip> __n) { 91 while (__n < 0) { 92 ++__n; 93 --__i; 94 } 95 } 96 97 public: 98 // Preconditions: If `I` does not model `bidirectional_iterator`, `n` is not negative. 99 template <input_or_output_iterator _Ip> 100 _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, iter_difference_t<_Ip> __n) const { 101 // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation. 102 _LIBCPP_ASSERT_PEDANTIC( 103 __n >= 0 || bidirectional_iterator<_Ip>, "If `n < 0`, then `bidirectional_iterator<I>` must be true."); 104 105 // If `I` models `random_access_iterator`, equivalent to `i += n`. 106 if constexpr (random_access_iterator<_Ip>) { 107 __i += __n; 108 return; 109 } else if constexpr (bidirectional_iterator<_Ip>) { 110 // Otherwise, if `n` is non-negative, increments `i` by `n`. 111 __advance_forward(__i, __n); 112 // Otherwise, decrements `i` by `-n`. 113 __advance_backward(__i, __n); 114 return; 115 } else { 116 // Otherwise, if `n` is non-negative, increments `i` by `n`. 117 __advance_forward(__i, __n); 118 return; 119 } 120 } 121 122 // Preconditions: Either `assignable_from<I&, S> || sized_sentinel_for<S, I>` is modeled, or [i, bound_sentinel) 123 // denotes a range. 124 template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp> 125 _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, _Sp __bound_sentinel) const { 126 // If `I` and `S` model `assignable_from<I&, S>`, equivalent to `i = std::move(bound_sentinel)`. 127 if constexpr (assignable_from<_Ip&, _Sp>) { 128 __i = std::move(__bound_sentinel); 129 } 130 // Otherwise, if `S` and `I` model `sized_sentinel_for<S, I>`, equivalent to `ranges::advance(i, bound_sentinel - 131 // i)`. 132 else if constexpr (sized_sentinel_for<_Sp, _Ip>) { 133 (*this)(__i, __bound_sentinel - __i); 134 } 135 // Otherwise, while `bool(i != bound_sentinel)` is true, increments `i`. 136 else { 137 while (__i != __bound_sentinel) { 138 ++__i; 139 } 140 } 141 } 142 143 // Preconditions: 144 // * If `n > 0`, [i, bound_sentinel) denotes a range. 145 // * If `n == 0`, [i, bound_sentinel) or [bound_sentinel, i) denotes a range. 146 // * If `n < 0`, [bound_sentinel, i) denotes a range, `I` models `bidirectional_iterator`, and `I` and `S` model 147 // `same_as<I, S>`. 148 // Returns: `n - M`, where `M` is the difference between the ending and starting position. 149 template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp> 150 _LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip> 151 operator()(_Ip& __i, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const { 152 // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation. 153 _LIBCPP_ASSERT_PEDANTIC((__n >= 0) || (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>), 154 "If `n < 0`, then `bidirectional_iterator<I> && same_as<I, S>` must be true."); 155 // If `S` and `I` model `sized_sentinel_for<S, I>`: 156 if constexpr (sized_sentinel_for<_Sp, _Ip>) { 157 // If |n| >= |bound_sentinel - i|, equivalent to `ranges::advance(i, bound_sentinel)`. 158 // __magnitude_geq(a, b) returns |a| >= |b|, assuming they have the same sign. 159 auto __magnitude_geq = [](auto __a, auto __b) { return __a == 0 ? __b == 0 : __a > 0 ? __a >= __b : __a <= __b; }; 160 if (const auto __m = __bound_sentinel - __i; __magnitude_geq(__n, __m)) { 161 (*this)(__i, __bound_sentinel); 162 return __n - __m; 163 } 164 165 // Otherwise, equivalent to `ranges::advance(i, n)`. 166 (*this)(__i, __n); 167 return 0; 168 } else { 169 // Otherwise, if `n` is non-negative, while `bool(i != bound_sentinel)` is true, increments `i` but at 170 // most `n` times. 171 while (__n > 0 && __i != __bound_sentinel) { 172 ++__i; 173 --__n; 174 } 175 176 // Otherwise, while `bool(i != bound_sentinel)` is true, decrements `i` but at most `-n` times. 177 if constexpr (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>) { 178 while (__n < 0 && __i != __bound_sentinel) { 179 --__i; 180 ++__n; 181 } 182 } 183 return __n; 184 } 185 186 __libcpp_unreachable(); 187 } 188 }; 189 190 inline namespace __cpo { 191 inline constexpr auto advance = __advance{}; 192 } // namespace __cpo 193 } // namespace ranges 194 195 #endif // _LIBCPP_STD_VER >= 20 196 197 _LIBCPP_END_NAMESPACE_STD 198 199 _LIBCPP_POP_MACROS 200 201 #endif // _LIBCPP___ITERATOR_ADVANCE_H 202