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