include/__algorithm/shift_right.h

//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

#ifndef _LIBCPP___ALGORITHM_SHIFT_RIGHT_H
#define _LIBCPP___ALGORITHM_SHIFT_RIGHT_H

#include <__algorithm/move.h>
#include <__algorithm/move_backward.h>
#include <__algorithm/swap_ranges.h>
#include <__config>
#include <__iterator/iterator_traits.h>
#include <__utility/swap.h>

#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
#  pragma GCC system_header
#endif

_LIBCPP_PUSH_MACROS
#include <__undef_macros>

_LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 20

template <class _ForwardIterator>
inline _LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
shift_right(_ForwardIterator __first,
            _ForwardIterator __last,
            typename iterator_traits<_ForwardIterator>::difference_type __n) {
  if (__n == 0) {
    return __first;
  }

  if constexpr (__has_random_access_iterator_category<_ForwardIterator>::value) {
    decltype(__n) __d = __last - __first;
    if (__n >= __d) {
      return __last;
    }
    _ForwardIterator __m = __first + (__d - __n);
    return std::move_backward(__first, __m, __last);
  } else if constexpr (__has_bidirectional_iterator_category<_ForwardIterator>::value) {
    _ForwardIterator __m = __last;
    for (; __n > 0; --__n) {
      if (__m == __first) {
        return __last;
      }
      --__m;
    }
    return std::move_backward(__first, __m, __last);
  } else {
    _ForwardIterator __ret = __first;
    for (; __n > 0; --__n) {
      if (__ret == __last) {
        return __last;
      }
      ++__ret;
    }

    // We have an __n-element scratch space from __first to __ret.
    // Slide an __n-element window [__trail, __lead) from left to right.
    // We're essentially doing swap_ranges(__first, __ret, __trail, __lead)
    // over and over; but once __lead reaches __last we needn't bother
    // to save the values of elements [__trail, __last).

    auto __trail = __first;
    auto __lead  = __ret;
    while (__trail != __ret) {
      if (__lead == __last) {
        std::move(__first, __trail, __ret);
        return __ret;
      }
      ++__trail;
      ++__lead;
    }

    _ForwardIterator __mid = __first;
    while (true) {
      if (__lead == __last) {
        __trail = std::move(__mid, __ret, __trail);
        std::move(__first, __mid, __trail);
        return __ret;
      }
      swap(*__mid, *__trail);
      ++__mid;
      ++__trail;
      ++__lead;
      if (__mid == __ret) {
        __mid = __first;
      }
    }
  }
}

#endif // _LIBCPP_STD_VER >= 20

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS

#endif // _LIBCPP___ALGORITHM_SHIFT_RIGHT_H