Hacker News Reader

Comment by beagle3 | Hacker News Reader

Depends on how you interact with the quicksort, really.

If you have control of the comparison routine, there's Doug McIlroy's classic "quicksort killer"[0]. If you have information about the state of the random number generator used to pick the pivot, then you can do the same without having to actually interact through the comparison routine. Many libraries use an LCG[1] with 32 to 64 bit state, which is trivial to reconstruct through a small sample of its output not much longer than the internal state.

I don't know about you, but requiring my sort() routine to have access to a cryptographically secure random number generator doesn't seem right; I much prefer any algorithm (e.g. HeapSort, Mergesort or TimSort) that can guarantee n\*log(n) behavior with deterministic (and especially, no secure random generator requirement!) behavior.

[0] http://www.cs.dartmouth.edu/~doug/mdmspe.pdf

[1] http://en.wikipedia.org/wiki/Linear_congruential_generator

View on HN

You might be interested in my new sorting algorithm, Pattern-defeating Quicksort (pdqsort): https://github.com/orlp/pdqsort

It's fully deterministic, as fast as Quicksort in the average case, O(n) in the best case (all equal, but it scales smoothly as the number of unique elements decreases), is optimized for common inputs like ascending/descending (both O(n)) and uses a novel idea of guaranteeing O(n log n) worst case, that guarantees the worst case is no slower than three times the average case, assuming heapsort is twice as slow as Quicksort.

I'm still tweaking the final details and working on the paper, so it's not ready for a full release yet, but a sneak peek here and there is nice :)

This is a benchmark comparing pdqsort against introsort (std::sort), heap sort and timsort: http://i.imgur.com/TSvXnG5.png .

Which timsort implementation did you compare to? (your profile.cpp on github doesn't include it).

Heapsort implementation vary widely in their cache coherence. The vast majority are extremely simple but result in essentially complete incoherence. I was once able to speed a heapsort about 2x by rearranging the scan order into a cache-oblivious one (Mostly the heap building part; I'm not aware of a way to make the extraction part cache-friendy).

Heapsorts, quicksorts and bubble sorts have the ability to sort just the "top-n" and stop there (unlike mergesort), which is often useful, and a significat speed up (goes from nlog(n) to n+mlog(n) to get top m, or from n^2 to n times m for bubble sort) - I wish library routines actually provided that as part of a standard interface. Perhaps you could be the torchbearer in your publication?

Personally, I think just about every standard library except APL/J/K got sorting wrong. The primary sort operation should be "compute stable ordering permutation", with "sort this array" (basically only available operation in most libraries) being at most a shortcut when you don't care about the permutation.

I'm sorry, the profiling code for timsort wasn't included in my remote repository yet. I profiled against https://github.com/gfx/cpp-TimSort .

Also, C++ has always had the "top-n" interface called partial sort: http://en.cppreference.com/w/cpp/algorithm/partial_sort .

I wrote a partial quicksort for Newzbin - once I saw how simple it was to add I was surprised it wasn't more widespread: https://github.com/Freaky/pqsort

>pdqsort

OK. But if you have control of the comparison routine, you can easily force mergesort to perform a quadratic number of comparisons by having the comparison function call itself an appropriate number of times. It feels a little unfair to call that a vulnerability in quicksort but not in mergesort.

Control comes in many flavors - e.g. you might be able to influence random element selection but not general code flow. (e.g - you might be able to set the seed before the call to quicksort).

And I only mentioned control because that's what McIlroy's example required to use as-is. But as I already stated, it is enough to know in advance the output of the (pseudo) random used to select a pivot to make it go quadratic, which is usually easy -- unless it is cryptographically secure (I've never seen qsort use one. have you?)

So, no, it is completely fair: There is no way to cause mergesort to go quadratic. However, it is quite easy to cause quicksort to go quadratic if you know how it selects it pivots (whether or not you can directly influence it during runtime or not).

Reply on news.ycombinator.com