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