## digitalmars.D - The Quick Mom Algorithm

- Andrei Alexandrescu (58/58) Jan 29 2016 http://dpaste.dzfl.pl/05a82699acc8
- Ivan Kazmenko (26/35) Feb 02 2016 The code does not compile when I try to use topN, as medianOfUpTo
- Andrei Alexandrescu (5/18) Feb 03 2016 Thanks for looking into this. I've been working a lot more on this and

http://dpaste.dzfl.pl/05a82699acc8 So over the past few days I've been in the zone working on a smooth implementation of "Median of Medians" (https://en.wikipedia.org/wiki/Median_of_medians). Its performance is much better compared to the straightforward implementation. However, in practice it's very hard to get it to beat simple heuristics (such as median of five, random pivot etc). These heuristics run `n` comparisons for an input of length `n`, whereas the MoM needs over twice as many comparisons. Also it does a bunch of swapping around the data. Overall I got it within 2.5x of the heuristic-based topN for most data sizes up to tens of millions. While thinking of MoM and the core reasons of its being slow (adds nice structure to its input and then "forgets" most of it when recursing), I stumbled upon a different algorithm. It's much simpler, also deterministic and faster than MoM for many (most?) inputs. But it's not guaranteed to be linear. After having pounded at this for many hours, it is clear that I am in need of some serious due destruction. I call it a "quick median of medians" or in short "quick mom". Consider the algorithm defined as follows over a range `r` of length `n`. It returns an index to an element likely to be in the second tertile of r. (A tertile is a third of the range. So the expectation is that the returned index x is such that e<=r[x] for at least one third of the range, and r[x]<=e again for at least one third of the range.) 0. If n<=3, compute median by rote and return its index. 1. Divide r in three equal adjacent subranges r0 = r[0 .. $/3], r1 = r[$/3 .. $*2/3], r2 = r[$*2/3 .. $]. 2. Recurse to get the median of these three, call them m0, m1, m2. 3. Return the median of m0, m1, m2. Note that no data has been written yet; we only have an estimate of a pivot. On random inputs it's expected that the median thus gotten is greater than 1/6 + 1/6 = 1/3 elements, and less than the same fraction. The algorithm completes in linear time. However, the pivot obtained is just an approximation. In the worst case it's possible that e.g. all recursions return the leftmost of the allowed index, so the bound deteriorates by 1/3 for each recursion depth, or generally to (1/3)^^log3(n). Not good! That's where the quick mom pays attention. After it partitions the data using the pivot obtained by the quick mom method, the partitioning stage checks whether the resulting pivot falls within bounds. If not, it runs a precise selection method (such as proper median of medians - "thorough mom") to bring the pivot where it needs to be. The data patterns that cause the quick mom to fail systematically are rather odd but worst case is what it is. Overall the partitioning either succeeds with the quick mom pivot in linear time, or does one more linear pass if "unlucky". So overall partition is linear. (Micro-optimization: not all data needs to be repartitioned, only that outside the not-so-good pivot.) Unlike the quick mom, the partition guarantees a pivot in the mid tertile. After partitioning the classic quickselect algorithm may be implemented. There's one more _really_ juicy detail. In fact, the quick mom may finish without looking at all data. Look at the implementation - there are two overloads of quickMom. The second gets the bounds and works as follows: if you've already computed two elements of a median, the third may be chosen only if in between the two. Otherwise, you only care whether it's larger or smaller than the other two. This allows the algorithm to finish certain recursion branches early. Destroy! Andrei

Jan 29 2016

On Friday, 29 January 2016 at 22:47:44 UTC, Andrei Alexandrescu wrote:http://dpaste.dzfl.pl/05a82699acc8 While thinking of MoM and the core reasons of its being slow (adds nice structure to its input and then "forgets" most of it when recursing), I stumbled upon a different algorithm. It's much simpler, also deterministic and faster than MoM for many (most?) inputs. But it's not guaranteed to be linear. After having pounded at this for many hours, it is clear that I am in need of some serious due destruction. I call it a "quick median of medians" or in short "quick mom".The code does not compile when I try to use topN, as medianOfUpTo is not present. Please include it. Anyway, for now, I just wanted to quickly check whether my previous attack on topN would have any effect here. I've replaced the line if (r.length <= 5) return medianOfUpTo!(5, lp)(r); with a quick fix: if (r.length == 2) { if (!less (r.front, r.back)) swap (r.front, r.back); return 0; } if (r.length == 1) return 0; Here is the code (just glued together the two snippets): http://dpaste.dzfl.pl/f648a600a11d Turns out like maybe it does have its effect (size = 5 million): building Element array: 2442 msecs checking Element array: 2432 msecs checking int array: 388 msecs checking random int array: 140 msecs checking increasing int array: 41 msecs checking decreasing int array: 42 msecs The int array built by the attack makes the program run slower than a random array, and the ratio increases with size.

Feb 02 2016

On 02/02/2016 05:14 PM, Ivan Kazmenko wrote:On Friday, 29 January 2016 at 22:47:44 UTC, Andrei Alexandrescu wrote:Thanks for looking into this. I've been working a lot more on this and have an algorithm in testing that would be very interesting to develop an attack input for. Please give me to the end of the week and I'll send it along. Thx! -- Andreihttp://dpaste.dzfl.pl/05a82699acc8 While thinking of MoM and the core reasons of its being slow (adds nice structure to its input and then "forgets" most of it when recursing), I stumbled upon a different algorithm. It's much simpler, also deterministic and faster than MoM for many (most?) inputs. But it's not guaranteed to be linear. After having pounded at this for many hours, it is clear that I am in need of some serious due destruction. I call it a "quick median of medians" or in short "quick mom".The code does not compile when I try to use topN, as medianOfUpTo is not present. Please include it.

Feb 03 2016