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 <cstdlib>
27 #include <limits>
28
29 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
30 # pragma GCC system_header
31 #endif
32
33 _LIBCPP_BEGIN_NAMESPACE_STD
34
35 template <class _InputIter>
36 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17
__advance(_InputIter & __i,typename iterator_traits<_InputIter>::difference_type __n,input_iterator_tag)37 void __advance(_InputIter& __i, typename iterator_traits<_InputIter>::difference_type __n, input_iterator_tag) {
38 for (; __n > 0; --__n)
39 ++__i;
40 }
41
42 template <class _BiDirIter>
43 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17
__advance(_BiDirIter & __i,typename iterator_traits<_BiDirIter>::difference_type __n,bidirectional_iterator_tag)44 void __advance(_BiDirIter& __i, typename iterator_traits<_BiDirIter>::difference_type __n, bidirectional_iterator_tag) {
45 if (__n >= 0)
46 for (; __n > 0; --__n)
47 ++__i;
48 else
49 for (; __n < 0; ++__n)
50 --__i;
51 }
52
53 template <class _RandIter>
54 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17
__advance(_RandIter & __i,typename iterator_traits<_RandIter>::difference_type __n,random_access_iterator_tag)55 void __advance(_RandIter& __i, typename iterator_traits<_RandIter>::difference_type __n, random_access_iterator_tag) {
56 __i += __n;
57 }
58
59 template <
60 class _InputIter, class _Distance,
61 class _IntegralDistance = decltype(_VSTD::__convert_to_integral(std::declval<_Distance>())),
62 class = __enable_if_t<is_integral<_IntegralDistance>::value> >
63 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX17
advance(_InputIter & __i,_Distance __orig_n)64 void advance(_InputIter& __i, _Distance __orig_n) {
65 typedef typename iterator_traits<_InputIter>::difference_type _Difference;
66 _Difference __n = static_cast<_Difference>(_VSTD::__convert_to_integral(__orig_n));
67 _LIBCPP_ASSERT(__n >= 0 || __is_cpp17_bidirectional_iterator<_InputIter>::value,
68 "Attempt to advance(it, n) with negative n on a non-bidirectional iterator");
69 _VSTD::__advance(__i, __n, typename iterator_traits<_InputIter>::iterator_category());
70 }
71
72 #if _LIBCPP_STD_VER > 17
73
74 // [range.iter.op.advance]
75
76 namespace ranges {
77 namespace __advance {
78
79 struct __fn {
80 private:
81 template <class _Ip>
82 _LIBCPP_HIDE_FROM_ABI
__advance_forward__fn83 static constexpr void __advance_forward(_Ip& __i, iter_difference_t<_Ip> __n) {
84 while (__n > 0) {
85 --__n;
86 ++__i;
87 }
88 }
89
90 template <class _Ip>
91 _LIBCPP_HIDE_FROM_ABI
__advance_backward__fn92 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
operator__fn103 constexpr void operator()(_Ip& __i, iter_difference_t<_Ip> __n) const {
104 _LIBCPP_ASSERT(__n >= 0 || bidirectional_iterator<_Ip>,
105 "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) denotes a range.
125 template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
operator__fn126 _LIBCPP_HIDE_FROM_ABI constexpr void operator()(_Ip& __i, _Sp __bound_sentinel) const {
127 // If `I` and `S` model `assignable_from<I&, S>`, equivalent to `i = std::move(bound_sentinel)`.
128 if constexpr (assignable_from<_Ip&, _Sp>) {
129 __i = _VSTD::move(__bound_sentinel);
130 }
131 // Otherwise, if `S` and `I` model `sized_sentinel_for<S, I>`, equivalent to `ranges::advance(i, bound_sentinel - 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 `same_as<I, S>`.
147 // Returns: `n - M`, where `M` is the difference between the ending and starting position.
148 template <input_or_output_iterator _Ip, sentinel_for<_Ip> _Sp>
operator__fn149 _LIBCPP_HIDE_FROM_ABI constexpr iter_difference_t<_Ip> operator()(_Ip& __i, iter_difference_t<_Ip> __n,
150 _Sp __bound_sentinel) const {
151 _LIBCPP_ASSERT((__n >= 0) || (bidirectional_iterator<_Ip> && same_as<_Ip, _Sp>),
152 "If `n < 0`, then `bidirectional_iterator<I> && same_as<I, S>` must be true.");
153 // If `S` and `I` model `sized_sentinel_for<S, I>`:
154 if constexpr (sized_sentinel_for<_Sp, _Ip>) {
155 // If |n| >= |bound_sentinel - i|, equivalent to `ranges::advance(i, bound_sentinel)`.
156 // __magnitude_geq(a, b) returns |a| >= |b|, assuming they have the same sign.
157 auto __magnitude_geq = [](auto __a, auto __b) {
158 return __a == 0 ? __b == 0 :
159 __a > 0 ? __a >= __b :
160 __a <= __b;
161 };
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 > 17
200
201 _LIBCPP_END_NAMESPACE_STD
202
203 #endif // _LIBCPP___ITERATOR_ADVANCE_H
204