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 #include <type_traits>
19 
20 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
21 #pragma GCC system_header
22 #endif
23 
24 _LIBCPP_BEGIN_NAMESPACE_STD
25 
26 #if _LIBCPP_STD_VER > 17
27 
28 template <class _ForwardIterator>
29 inline _LIBCPP_INLINE_VISIBILITY constexpr
30 _ForwardIterator
31 shift_right(_ForwardIterator __first, _ForwardIterator __last,
32             typename iterator_traits<_ForwardIterator>::difference_type __n)
33 {
34     if (__n == 0) {
35         return __first;
36     }
37 
38     if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
39         decltype(__n) __d = __last - __first;
40         if (__n >= __d) {
41             return __last;
42         }
43         _ForwardIterator __m = __first + (__d - __n);
44         return _VSTD::move_backward(__first, __m, __last);
45     } else if constexpr (__is_cpp17_bidirectional_iterator<_ForwardIterator>::value) {
46         _ForwardIterator __m = __last;
47         for (; __n > 0; --__n) {
48             if (__m == __first) {
49                 return __last;
50             }
51             --__m;
52         }
53         return _VSTD::move_backward(__first, __m, __last);
54     } else {
55         _ForwardIterator __ret = __first;
56         for (; __n > 0; --__n) {
57             if (__ret == __last) {
58                 return __last;
59             }
60             ++__ret;
61         }
62 
63         // We have an __n-element scratch space from __first to __ret.
64         // Slide an __n-element window [__trail, __lead) from left to right.
65         // We're essentially doing swap_ranges(__first, __ret, __trail, __lead)
66         // over and over; but once __lead reaches __last we needn't bother
67         // to save the values of elements [__trail, __last).
68 
69         auto __trail = __first;
70         auto __lead = __ret;
71         while (__trail != __ret) {
72             if (__lead == __last) {
73                 _VSTD::move(__first, __trail, __ret);
74                 return __ret;
75             }
76             ++__trail;
77             ++__lead;
78         }
79 
80         _ForwardIterator __mid = __first;
81         while (true) {
82             if (__lead == __last) {
83                 __trail = _VSTD::move(__mid, __ret, __trail);
84                 _VSTD::move(__first, __mid, __trail);
85                 return __ret;
86             }
87             swap(*__mid, *__trail);
88             ++__mid;
89             ++__trail;
90             ++__lead;
91             if (__mid == __ret) {
92                 __mid = __first;
93             }
94         }
95     }
96 }
97 
98 #endif // _LIBCPP_STD_VER > 17
99 
100 _LIBCPP_END_NAMESPACE_STD
101 
102 #endif // _LIBCPP___ALGORITHM_SHIFT_RIGHT_H
103