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_PUSH_MACROS
24 #include <__undef_macros>
25 
26 _LIBCPP_BEGIN_NAMESPACE_STD
27 
28 #if _LIBCPP_STD_VER >= 20
29 
30 template <class _ForwardIterator>
31 inline _LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
32 shift_right(_ForwardIterator __first,
33             _ForwardIterator __last,
34             typename iterator_traits<_ForwardIterator>::difference_type __n) {
35   if (__n == 0) {
36     return __first;
37   }
38 
39   if constexpr (__has_random_access_iterator_category<_ForwardIterator>::value) {
40     decltype(__n) __d = __last - __first;
41     if (__n >= __d) {
42       return __last;
43     }
44     _ForwardIterator __m = __first + (__d - __n);
45     return std::move_backward(__first, __m, __last);
46   } else if constexpr (__has_bidirectional_iterator_category<_ForwardIterator>::value) {
47     _ForwardIterator __m = __last;
48     for (; __n > 0; --__n) {
49       if (__m == __first) {
50         return __last;
51       }
52       --__m;
53     }
54     return std::move_backward(__first, __m, __last);
55   } else {
56     _ForwardIterator __ret = __first;
57     for (; __n > 0; --__n) {
58       if (__ret == __last) {
59         return __last;
60       }
61       ++__ret;
62     }
63 
64     // We have an __n-element scratch space from __first to __ret.
65     // Slide an __n-element window [__trail, __lead) from left to right.
66     // We're essentially doing swap_ranges(__first, __ret, __trail, __lead)
67     // over and over; but once __lead reaches __last we needn't bother
68     // to save the values of elements [__trail, __last).
69 
70     auto __trail = __first;
71     auto __lead  = __ret;
72     while (__trail != __ret) {
73       if (__lead == __last) {
74         std::move(__first, __trail, __ret);
75         return __ret;
76       }
77       ++__trail;
78       ++__lead;
79     }
80 
81     _ForwardIterator __mid = __first;
82     while (true) {
83       if (__lead == __last) {
84         __trail = std::move(__mid, __ret, __trail);
85         std::move(__first, __mid, __trail);
86         return __ret;
87       }
88       swap(*__mid, *__trail);
89       ++__mid;
90       ++__trail;
91       ++__lead;
92       if (__mid == __ret) {
93         __mid = __first;
94       }
95     }
96   }
97 }
98 
99 #endif // _LIBCPP_STD_VER >= 20
100 
101 _LIBCPP_END_NAMESPACE_STD
102 
103 _LIBCPP_POP_MACROS
104 
105 #endif // _LIBCPP___ALGORITHM_SHIFT_RIGHT_H
106