[libcxx-commits] [PATCH] D141205: [libcxx] nth_element use introselect to avoid quadratic behavior

Mon Jan 9 05:33:19 PST 2023

danlark added inline comments.

================
Comment at: libcxx/include/__algorithm/nth_element.h:104
+        {
+            // fall back to heap select.
+            std::__heap_select<_AlgPolicy, _Compare>(__first, __nth, __last, __comp);
----------------
Backl1ght wrote:
> danlark wrote:
> > Mordante wrote:
> > > Can you add comment why this is done? Currently the comment and the code tell the same thing.
> > > For maintainability it would be very good to know why we switch the algorithm.
> > Even though I am in favor of this patch, this does not fully fix the issue. With the comparator that changes the inputs on the fly, it's possible to get into a O(n log n) average case.
> > 
> > Otherwise LGTM, thanks!
> I think the average time complexity of introselect should be O(n), since the average time complexity of quickselect is O(n), and according to the original paper of introselect:
> 
> > If as in introsort we limit the depth to a logarithmic bound the average time remains linear but the worst case is O(N log N). 
The example from the link https://github.com/llvm/llvm-project/issues/52747#issuecomment-996916909 will still be of average case O(N log N)

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  https://reviews.llvm.org/D141205/new/

https://reviews.llvm.org/D141205