[libcxx-commits] [libcxx] [libc++] Optimize search_n (PR #171389)

[Louis Dionne via libcxx-commits]

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@@ -68,44 +64,63 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 pair<_Iter, _Iter> __search_
   }
 }
 
-template <class _AlgPolicy, class _Pred, class _Iter, class _Sent, class _SizeT, class _Type, class _Proj, class _DiffT>
+// Finds the last element in [__first, __last) that doesn't satisfy __pred.
+template <class _RAIter, class _Pred, class _Proj, class _ValueT>
+_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _RAIter
+__find_last_not(_RAIter __first, _RAIter __last, const _ValueT& __value, _Pred& __pred, _Proj& __proj) {
+  auto __orig_last = __last;
+  while (true) {
+    if (__first == __last)
+      return __orig_last;
+
+    if (!std::__invoke(__pred, std::__invoke(__proj, *--__last), __value)) {
+      return __last;
+    }
+  }
+}
+
+template <class _AlgPolicy, class _Pred, class _Iter, class _SizeT, class _Type, class _Proj, class _DiffT>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 std::pair<_Iter, _Iter> __search_n_random_access_impl(
-    _Iter __first, _Sent __last, _SizeT __count, const _Type& __value, _Pred& __pred, _Proj& __proj, _DiffT __size1) {
-  using difference_type = typename iterator_traits<_Iter>::difference_type;
+    _Iter __first, _SizeT __count_in, const _Type& __value, _Pred& __pred, _Proj& __proj, _DiffT __size) {
+  auto __last  = __first + __size;
+  auto __count = static_cast<_DiffT>(__count_in);
+
   if (__count == 0)
     return std::make_pair(__first, __first);
-  if (__size1 < static_cast<_DiffT>(__count)) {
-    _IterOps<_AlgPolicy>::__advance_to(__first, __last);
-    return std::make_pair(__first, __first);
-  }
+  if (__size < __count)
+    return std::make_pair(__last, __last);
+
+  // [__match_start, __matched_until) is the longest subrange where all elements are known to match at any given point
+  // in time.
+  // [__matched_until, __match_start + __count) is the subrange where we don't know whether the elements match.
+  // [__match_start, __match_start + __count) is the subrange which we currently check whether it only contains matching
+  // elements. This subrange is returned in case all the elements match.
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ldionne wrote:

Let's present this one first.

https://github.com/llvm/llvm-project/pull/171389