[libcxx-commits] [libcxx] [libc++][ranges] implement `ranges::shift_left` (PR #83231)

[Nikolas Klauser via libcxx-commits]

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@@ -24,30 +26,40 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 20
 
-template <class _ForwardIterator>
-inline _LIBCPP_HIDE_FROM_ABI constexpr _ForwardIterator
-shift_left(_ForwardIterator __first,
-           _ForwardIterator __last,
-           typename iterator_traits<_ForwardIterator>::difference_type __n) {
+template <class _AlgPolicy, class _Iter, class _Sent>
+inline _LIBCPP_HIDE_FROM_ABI constexpr pair<_Iter, _Iter>
+__shift_left(_Iter __first, _Sent __last, typename iterator_traits<_Iter>::difference_type __n) {
+  _Iter __end = _IterOps<_AlgPolicy>::next(__first, __last);
+
   if (__n == 0) {
-    return __last;
+    return {std::move(__first), std::move(__end)};
   }
 
-  _ForwardIterator __m = __first;
-  if constexpr (__has_random_access_iterator_category<_ForwardIterator>::value) {
-    if (__n >= __last - __first) {
-      return __first;
+  _Iter __m = __first;
+  if constexpr (__has_random_access_iterator_category<_Iter>::value) {
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philnik777 wrote:

We should check `_IterOps<_AlgPolicy>::template __iterator_category`. We should also make sure `[__first, __last)` is a sized range to avoid having to iterate linearly through the range twice. Could you add a benchmark for these cases too?

https://github.com/llvm/llvm-project/pull/83231