libcxx/include/__cxx03/__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 <__cxx03/__algorithm/move.h>
  13 #include <__cxx03/__config>
  14 #include <__cxx03/__iterator/iterator_traits.h>
  15
  16 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
  17 #  pragma GCC system_header
  18 #endif
  19
  20 _LIBCPP_PUSH_MACROS
  21 #include <__cxx03/__undef_macros>
  22
  23 _LIBCPP_BEGIN_NAMESPACE_STD
  24
  25 #if _LIBCPP_STD_VER >= 20
  26
  27 template <class _ForwardIterator>
  28 inline _LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
  29 shift_left(_ForwardIterator __first,
  30            _ForwardIterator __last,
  31            typename iterator_traits<_ForwardIterator>::difference_type __n) {
  32   if (__n == 0) {
  33     return __last;
  34   }
  35
  36   _ForwardIterator __m = __first;
  37   if constexpr (__has_random_access_iterator_category<_ForwardIterator>::value) {
  38     if (__n >= __last - __first) {
  39       return __first;
  40     }
  41     __m += __n;
  42   } else {
  43     for (; __n > 0; --__n) {
  44       if (__m == __last) {
  45         return __first;
  46       }
  47       ++__m;
  48     }
  49   }
  50   return std::move(__m, __last, __first);
  51 }
  52
  53 #endif // _LIBCPP_STD_VER >= 20
  54
  55 _LIBCPP_END_NAMESPACE_STD
  56
  57 _LIBCPP_POP_MACROS
  58
  59 #endif // _LIBCPP___ALGORITHM_SHIFT_LEFT_H