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
28template <class _ForwardIterator>
29inline _LIBCPP_INLINE_VISIBILITY constexpr
30_ForwardIterator
31shift_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