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1 //===----------------------------------------------------------------------===//
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2 //
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3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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4 // See https://llvm.org/LICENSE.txt for license information.
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5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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6 //
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7 //===----------------------------------------------------------------------===//
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8
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9 #ifndef _LIBCPP___ALGORITHM_SHIFT_H
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10 #define _LIBCPP___ALGORITHM_SHIFT_H
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11
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12 #include <__config>
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13 #include <__algorithm/move.h>
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14 #include <__iterator/iterator_traits.h>
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15
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16 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
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17 #pragma GCC system_header
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18 #endif
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19
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20 _LIBCPP_PUSH_MACROS
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21 #include <__undef_macros>
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22
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23 _LIBCPP_BEGIN_NAMESPACE_STD
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24
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25 // shift_left, shift_right
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26
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27 #if _LIBCPP_STD_VER > 17
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28
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29 template <class _ForwardIterator>
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30 inline _LIBCPP_INLINE_VISIBILITY constexpr
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31 _ForwardIterator
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32 shift_left(_ForwardIterator __first, _ForwardIterator __last,
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33 typename iterator_traits<_ForwardIterator>::difference_type __n)
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34 {
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35 if (__n == 0) {
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36 return __last;
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37 }
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38
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39 _ForwardIterator __m = __first;
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40 if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
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41 if (__n >= __last - __first) {
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42 return __first;
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43 }
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44 __m += __n;
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45 } else {
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46 for (; __n > 0; --__n) {
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47 if (__m == __last) {
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48 return __first;
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49 }
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50 ++__m;
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51 }
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52 }
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53 return _VSTD::move(__m, __last, __first);
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54 }
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55
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56 template <class _ForwardIterator>
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57 inline _LIBCPP_INLINE_VISIBILITY constexpr
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58 _ForwardIterator
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59 shift_right(_ForwardIterator __first, _ForwardIterator __last,
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60 typename iterator_traits<_ForwardIterator>::difference_type __n)
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61 {
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62 if (__n == 0) {
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63 return __first;
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64 }
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65
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66 if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
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67 decltype(__n) __d = __last - __first;
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68 if (__n >= __d) {
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69 return __last;
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70 }
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71 _ForwardIterator __m = __first + (__d - __n);
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72 return _VSTD::move_backward(__first, __m, __last);
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73 } else if constexpr (__is_cpp17_bidirectional_iterator<_ForwardIterator>::value) {
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74 _ForwardIterator __m = __last;
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75 for (; __n > 0; --__n) {
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76 if (__m == __first) {
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77 return __last;
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78 }
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79 --__m;
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80 }
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81 return _VSTD::move_backward(__first, __m, __last);
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82 } else {
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83 _ForwardIterator __ret = __first;
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84 for (; __n > 0; --__n) {
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85 if (__ret == __last) {
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86 return __last;
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87 }
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88 ++__ret;
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89 }
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90
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91 // We have an __n-element scratch space from __first to __ret.
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92 // Slide an __n-element window [__trail, __lead) from left to right.
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93 // We're essentially doing swap_ranges(__first, __ret, __trail, __lead)
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94 // over and over; but once __lead reaches __last we needn't bother
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95 // to save the values of elements [__trail, __last).
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96
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97 auto __trail = __first;
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98 auto __lead = __ret;
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99 while (__trail != __ret) {
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100 if (__lead == __last) {
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101 _VSTD::move(__first, __trail, __ret);
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102 return __ret;
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103 }
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104 ++__trail;
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105 ++__lead;
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106 }
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107
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108 _ForwardIterator __mid = __first;
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109 while (true) {
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110 if (__lead == __last) {
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111 __trail = _VSTD::move(__mid, __ret, __trail);
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112 _VSTD::move(__first, __mid, __trail);
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113 return __ret;
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114 }
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115 swap(*__mid, *__trail);
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116 ++__mid;
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117 ++__trail;
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118 ++__lead;
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119 if (__mid == __ret) {
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120 __mid = __first;
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121 }
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122 }
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123 }
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124 }
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125
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126 #endif // _LIBCPP_STD_VER > 17
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127
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128 _LIBCPP_END_NAMESPACE_STD
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129
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130 _LIBCPP_POP_MACROS
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131
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132 #endif // _LIBCPP___ALGORITHM_SHIFT_H
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