1 //===----------------------------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #ifndef _LIBCPP___ALGORITHM_SHIFT_RIGHT_H
10 #define _LIBCPP___ALGORITHM_SHIFT_RIGHT_H
11
12 #include <__algorithm/move.h>
13 #include <__algorithm/move_backward.h>
14 #include <__algorithm/swap_ranges.h>
15 #include <__config>
16 #include <__iterator/iterator_traits.h>
17 #include <__utility/swap.h>
18
19 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
20 # pragma GCC system_header
21 #endif
22
23 _LIBCPP_BEGIN_NAMESPACE_STD
24
25 #if _LIBCPP_STD_VER >= 20
26
27 template <class _ForwardIterator>
28 inline _LIBCPP_INLINE_VISIBILITY constexpr
29 _ForwardIterator
shift_right(_ForwardIterator __first,_ForwardIterator __last,typename iterator_traits<_ForwardIterator>::difference_type __n)30 shift_right(_ForwardIterator __first, _ForwardIterator __last,
31 typename iterator_traits<_ForwardIterator>::difference_type __n)
32 {
33 if (__n == 0) {
34 return __first;
35 }
36
37 if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
38 decltype(__n) __d = __last - __first;
39 if (__n >= __d) {
40 return __last;
41 }
42 _ForwardIterator __m = __first + (__d - __n);
43 return _VSTD::move_backward(__first, __m, __last);
44 } else if constexpr (__is_cpp17_bidirectional_iterator<_ForwardIterator>::value) {
45 _ForwardIterator __m = __last;
46 for (; __n > 0; --__n) {
47 if (__m == __first) {
48 return __last;
49 }
50 --__m;
51 }
52 return _VSTD::move_backward(__first, __m, __last);
53 } else {
54 _ForwardIterator __ret = __first;
55 for (; __n > 0; --__n) {
56 if (__ret == __last) {
57 return __last;
58 }
59 ++__ret;
60 }
61
62 // We have an __n-element scratch space from __first to __ret.
63 // Slide an __n-element window [__trail, __lead) from left to right.
64 // We're essentially doing swap_ranges(__first, __ret, __trail, __lead)
65 // over and over; but once __lead reaches __last we needn't bother
66 // to save the values of elements [__trail, __last).
67
68 auto __trail = __first;
69 auto __lead = __ret;
70 while (__trail != __ret) {
71 if (__lead == __last) {
72 _VSTD::move(__first, __trail, __ret);
73 return __ret;
74 }
75 ++__trail;
76 ++__lead;
77 }
78
79 _ForwardIterator __mid = __first;
80 while (true) {
81 if (__lead == __last) {
82 __trail = _VSTD::move(__mid, __ret, __trail);
83 _VSTD::move(__first, __mid, __trail);
84 return __ret;
85 }
86 swap(*__mid, *__trail);
87 ++__mid;
88 ++__trail;
89 ++__lead;
90 if (__mid == __ret) {
91 __mid = __first;
92 }
93 }
94 }
95 }
96
97 #endif // _LIBCPP_STD_VER >= 20
98
99 _LIBCPP_END_NAMESPACE_STD
100
101 #endif // _LIBCPP___ALGORITHM_SHIFT_RIGHT_H
102