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 class = __enable_if_t<is_integral<_IntegralDistance>::value> > 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 namespace __advance { 80 81 struct __fn { 82 private: 83 template <class _Ip> 84 _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_forward(_Ip& __i, iter_difference_t<_Ip> __n) { 85 while (__n > 0) { 86 --__n; 87 ++__i; 88 } 89 } 90 91 template <class _Ip> 92 _LIBCPP_HIDE_FROM_ABI static constexpr void __advance_backward(_Ip& __i, iter_difference_t<_Ip> __n) { 93 while (__n < 0) { 94 ++__n; 95 --__i; 96 } 97 } 98 99 public: 100 // Preconditions: If `I` does not model `bidirectional_iterator`, `n` is not negative. 101 template <input_or_output_iterator _Ip> 102 _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, iter_difference_t<_Ip> __n) const { 103 // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation. 104 _LIBCPP_ASSERT_PEDANTIC( 105 __n >= 0 || bidirectional_iterator<_Ip>, "If `n < 0`, then `bidirectional_iterator<I>` must be true."); 106 107 // If `I` models `random_access_iterator`, equivalent to `i += n`. 108 if constexpr (random_access_iterator<_Ip>) { 109 __i += __n; 110 return; 111 } else if constexpr (bidirectional_iterator<_Ip>) { 112 // Otherwise, if `n` is non-negative, increments `i` by `n`. 113 __advance_forward(__i, __n); 114 // Otherwise, decrements `i` by `-n`. 115 __advance_backward(__i, __n); 116 return; 117 } else { 118 // Otherwise, if `n` is non-negative, increments `i` by `n`. 119 __advance_forward(__i, __n); 120 return; 121 } 122 } 123 124 // Preconditions: Either `assignable_from<I&, S> || sized_sentinel_for<S, I>` is modeled, or [i, bound_sentinel) 125 // denotes a range. 126 template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp> 127 _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, _Sp __bound_sentinel) const { 128 // If `I` and `S` model `assignable_from<I&, S>`, equivalent to `i = std::move(bound_sentinel)`. 129 if constexpr (assignable_from<_Ip&, _Sp>) { 130 __i = std::move(__bound_sentinel); 131 } 132 // Otherwise, if `S` and `I` model `sized_sentinel_for<S, I>`, equivalent to `ranges::advance(i, bound_sentinel - 133 // i)`. 134 else if constexpr (sized_sentinel_for<_Sp, _Ip>) { 135 (*this)(__i, __bound_sentinel - __i); 136 } 137 // Otherwise, while `bool(i != bound_sentinel)` is true, increments `i`. 138 else { 139 while (__i != __bound_sentinel) { 140 ++__i; 141 } 142 } 143 } 144 145 // Preconditions: 146 // * If `n > 0`, [i, bound_sentinel) denotes a range. 147 // * If `n == 0`, [i, bound_sentinel) or [bound_sentinel, i) denotes a range. 148 // * If `n < 0`, [bound_sentinel, i) denotes a range, `I` models `bidirectional_iterator`, and `I` and `S` model 149 // `same_as<I, S>`. 150 // Returns: `n - M`, where `M` is the difference between the ending and starting position. 151 template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp> 152 _LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip> 153 operator()(_Ip& __i, iter_difference_t<_Ip> __n, _Sp __bound_sentinel) const { 154 // Calling `advance` with a negative value on a non-bidirectional iterator is a no-op in the current implementation. 155 _LIBCPP_ASSERT_PEDANTIC((__n >= 0) || (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>), 156 "If `n < 0`, then `bidirectional_iterator<I> && same_as<I, S>` must be true."); 157 // If `S` and `I` model `sized_sentinel_for<S, I>`: 158 if constexpr (sized_sentinel_for<_Sp, _Ip>) { 159 // If |n| >= |bound_sentinel - i|, equivalent to `ranges::advance(i, bound_sentinel)`. 160 // __magnitude_geq(a, b) returns |a| >= |b|, assuming they have the same sign. 161 auto __magnitude_geq = [](auto __a, auto __b) { return __a == 0 ? __b == 0 : __a > 0 ? __a >= __b : __a <= __b; }; 162 if (const auto __m = __bound_sentinel - __i; __magnitude_geq(__n, __m)) { 163 (*this)(__i, __bound_sentinel); 164 return __n - __m; 165 } 166 167 // Otherwise, equivalent to `ranges::advance(i, n)`. 168 (*this)(__i, __n); 169 return 0; 170 } else { 171 // Otherwise, if `n` is non-negative, while `bool(i != bound_sentinel)` is true, increments `i` but at 172 // most `n` times. 173 while (__i != __bound_sentinel && __n > 0) { 174 ++__i; 175 --__n; 176 } 177 178 // Otherwise, while `bool(i != bound_sentinel)` is true, decrements `i` but at most `-n` times. 179 if constexpr (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>) { 180 while (__i != __bound_sentinel && __n < 0) { 181 --__i; 182 ++__n; 183 } 184 } 185 return __n; 186 } 187 188 __libcpp_unreachable(); 189 } 190 }; 191 192 } // namespace __advance 193 194 inline namespace __cpo { 195 inline constexpr auto advance = __advance::__fn{}; 196 } // namespace __cpo 197 } // namespace ranges 198 199 #endif // _LIBCPP_STD_VER >= 20 200 201 _LIBCPP_END_NAMESPACE_STD 202 203 _LIBCPP_POP_MACROS 204 205 #endif // _LIBCPP___ITERATOR_ADVANCE_H 206