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
56