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
__advance(_InputIter & __i,typename iterator_traits<_InputIter>::difference_type __n,input_iterator_tag)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
__advance(_BiDirIter & __i,typename iterator_traits<_BiDirIter>::difference_type __n,bidirectional_iterator_tag)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
__advance(_RandIter & __i,typename iterator_traits<_RandIter>::difference_type __n,random_access_iterator_tag)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> >
advance(_InputIter & __i,_Distance __orig_n)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>
__advance_forward__fn84 _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>
__advance_backward__fn92 _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>
operator__fn102 _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>
operator__fn127 _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>
operator__fn153 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