[libcxx-commits] [libcxx] [libc++] Forward std::find_end with bidirectional iterators to std::search (PR #176125)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

<details>
<summary>Changes</summary>

This significantly reduces the amount of code duplication and reduces the likelihood that optimiazions are applied to only one of the two algorithms.


---
Full diff: https://github.com/llvm/llvm-project/pull/176125.diff


1 Files Affected:

- (modified) libcxx/include/__algorithm/find_end.h (+13-82) 


``````````diff
diff --git a/libcxx/include/__algorithm/find_end.h b/libcxx/include/__algorithm/find_end.h
index 84b43e31a3a59..3da15fa7d2611 100644
--- a/libcxx/include/__algorithm/find_end.h
+++ b/libcxx/include/__algorithm/find_end.h
@@ -12,9 +12,11 @@
 
 #include <__algorithm/comp.h>
 #include <__algorithm/iterator_operations.h>
+#include <__algorithm/search.h>
 #include <__config>
 #include <__functional/identity.h>
 #include <__iterator/iterator_traits.h>
+#include <__iterator/reverse_iterator.h>
 #include <__type_traits/invoke.h>
 #include <__utility/pair.h>
 
@@ -96,89 +98,18 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 pair<_Iter1, _Iter1> __find_
     bidirectional_iterator_tag) {
   auto __last1 = _IterOps<_AlgPolicy>::next(__first1, __sent1);
   auto __last2 = _IterOps<_AlgPolicy>::next(__first2, __sent2);
-  // modeled after search algorithm (in reverse)
-  if (__first2 == __last2)
-    return std::make_pair(__last1, __last1); // Everything matches an empty sequence
-  _Iter1 __l1 = __last1;
-  _Iter2 __l2 = __last2;
-  --__l2;
-  while (true) {
-    // Find last element in sequence 1 that matches *(__last2-1), with a mininum of loop checks
-    while (true) {
-      if (__first1 == __l1) // return __last1 if no element matches *__first2
-        return std::make_pair(__last1, __last1);
-      if (std::__invoke(__pred, std::__invoke(__proj1, *--__l1), std::__invoke(__proj2, *__l2)))
-        break;
-    }
-    // *__l1 matches *__l2, now match elements before here
-    _Iter1 __match_last = __l1;
-    _Iter1 __m1         = __l1;
-    _Iter2 __m2         = __l2;
-    while (true) {
-      if (__m2 == __first2) // If pattern exhausted, __m1 is the answer (works for 1 element pattern)
-        return std::make_pair(__m1, ++__match_last);
-      if (__m1 == __first1) // Otherwise if source exhaused, pattern not found
-        return std::make_pair(__last1, __last1);
 
-      // if there is a mismatch, restart with a new __l1
-      if (!std::__invoke(__pred, std::__invoke(__proj1, *--__m1), std::__invoke(__proj2, *--__m2))) {
-        break;
-      } // else there is a match, check next elements
-    }
-  }
-}
-
-template <class _AlgPolicy,
-          class _Pred,
-          class _Iter1,
-          class _Sent1,
-          class _Iter2,
-          class _Sent2,
-          class _Proj1,
-          class _Proj2>
-_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_Iter1, _Iter1> __find_end_impl(
-    _Iter1 __first1,
-    _Sent1 __sent1,
-    _Iter2 __first2,
-    _Sent2 __sent2,
-    _Pred& __pred,
-    _Proj1& __proj1,
-    _Proj2& __proj2,
-    random_access_iterator_tag,
-    random_access_iterator_tag) {
-  typedef typename iterator_traits<_Iter1>::difference_type _D1;
-  auto __last1 = _IterOps<_AlgPolicy>::next(__first1, __sent1);
-  auto __last2 = _IterOps<_AlgPolicy>::next(__first2, __sent2);
-  // Take advantage of knowing source and pattern lengths.  Stop short when source is smaller than pattern
-  auto __len2 = __last2 - __first2;
-  if (__len2 == 0)
-    return std::make_pair(__last1, __last1);
-  auto __len1 = __last1 - __first1;
-  if (__len1 < __len2)
-    return std::make_pair(__last1, __last1);
-  const _Iter1 __s = __first1 + _D1(__len2 - 1); // End of pattern match can't go before here
-  _Iter1 __l1      = __last1;
-  _Iter2 __l2      = __last2;
-  --__l2;
-  while (true) {
-    while (true) {
-      if (__s == __l1)
-        return std::make_pair(__last1, __last1);
-      if (std::__invoke(__pred, std::__invoke(__proj1, *--__l1), std::__invoke(__proj2, *__l2)))
-        break;
-    }
-    _Iter1 __last_match = __l1;
-    _Iter1 __m1         = __l1;
-    _Iter2 __m2         = __l2;
-    while (true) {
-      if (__m2 == __first2)
-        return std::make_pair(__m1, ++__last_match);
-      // no need to check range on __m1 because __s guarantees we have enough source
-      if (!std::__invoke(__pred, std::__invoke(__proj1, *--__m1), std::__invoke(__proj2, *--__m2))) {
-        break;
-      }
-    }
-  }
+  auto __result = std::__search_impl(
+      reverse_iterator<_Iter1>(__last1),
+      reverse_iterator<_Iter1>(__first1),
+      reverse_iterator<_Iter2>(__last2),
+      reverse_iterator<_Iter2>(__first2),
+      __pred,
+      __proj1,
+      __proj2);
+  if (__result.first.base() == __first1)
+    return pair<_Iter1, _Iter1>(__last1, __last1);
+  return pair<_Iter1, _Iter1>(__result.second.base(), __result.first.base());
 }
 
 template <class _ForwardIterator1, class _ForwardIterator2, class _BinaryPredicate>

``````````

</details>


https://github.com/llvm/llvm-project/pull/176125