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
 30template <class _ForwardIterator>
 31inline _LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
 32shift_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