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
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 (__has_random_access_iterator_category<_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 (__has_bidirectional_iterator_category<_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