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 <__config>
13 #include <__algorithm/move.h>
14 #include <__iterator/iterator_traits.h>
15 #include <type_traits> // swap
16 
17 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
18 #pragma GCC system_header
19 #endif
20 
21 _LIBCPP_PUSH_MACROS
22 #include <__undef_macros>
23 
24 _LIBCPP_BEGIN_NAMESPACE_STD
25 
26 #if _LIBCPP_STD_VER > 17
27 
28 template <class _ForwardIterator>
29 inline _LIBCPP_INLINE_VISIBILITY constexpr
30 _ForwardIterator
31 shift_left(_ForwardIterator __first, _ForwardIterator __last,
32            typename iterator_traits<_ForwardIterator>::difference_type __n)
33 {
34     if (__n == 0) {
35         return __last;
36     }
37 
38     _ForwardIterator __m = __first;
39     if constexpr (__is_cpp17_random_access_iterator<_ForwardIterator>::value) {
40         if (__n >= __last - __first) {
41             return __first;
42         }
43         __m += __n;
44     } else {
45         for (; __n > 0; --__n) {
46             if (__m == __last) {
47                 return __first;
48             }
49             ++__m;
50         }
51     }
52     return _VSTD::move(__m, __last, __first);
53 }
54 
55 #endif // _LIBCPP_STD_VER > 17
56 
57 _LIBCPP_END_NAMESPACE_STD
58 
59 _LIBCPP_POP_MACROS
60 
61 #endif // _LIBCPP___ALGORITHM_SHIFT_LEFT_H
62