libcxx/include/__algorithm/shift_left.h

   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_LEFT_H
  10 #define _LIBCPP___ALGORITHM_SHIFT_LEFT_H
  11
  12 #include <__algorithm/move.h>
  13 #include <__config>
  14 #include <__iterator/iterator_traits.h>
  15
  16 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  17 #  pragma GCC system_header
  18 #endif
  19
  20 _LIBCPP_BEGIN_NAMESPACE_STD
  21
  22 #if _LIBCPP_STD_VER >= 20
  23
  24 template <class _ForwardIterator>
  25 inline _LIBCPP_INLINE_VISIBILITY constexpr
  26 _ForwardIterator
  27 shift_left(_ForwardIterator __first, _ForwardIterator __last,
  28            typename iterator_traits<_ForwardIterator>::difference_type __n)
  29 {
  30     if (__n == 0) {
  31         return __last;
  32     }
  33
  34     _ForwardIterator __m = __first;
  35     if constexpr (__has_random_access_iterator_category<_ForwardIterator>::value) {
  36         if (__n >= __last - __first) {
  37             return __first;
  38         }
  39         __m += __n;
  40     } else {
  41         for (; __n > 0; --__n) {
  42             if (__m == __last) {
  43                 return __first;
  44             }
  45             ++__m;
  46         }
  47     }
  48     return _VSTD::move(__m, __last, __first);
  49 }
  50
  51 #endif // _LIBCPP_STD_VER >= 20
  52
  53 _LIBCPP_END_NAMESPACE_STD
  54
  55 #endif // _LIBCPP___ALGORITHM_SHIFT_LEFT_H