Sunday, April 16, 2023

Google Code Jam 2023 in Lisp

Since 2009, I have occasionally participated in Google's Code Jam competitions, usually programming in Lisp. Google decided to discontinue it along with their other coding competitions. So this year there was a "Farewell Round" held on a single day (yesterday), actually four rounds (A, B, C, D) held during the same long (four-hour) timeslot. Because I didn't feel very competitive or confident, I mainly tackled the problems from the "A" round, assuming that those were the easiest ones.

Thanks to the generous timeslot, I managed to solve all five problems in Round A.  You can find them on the gcj2023 project on my personal GitLab server.

Colliding Encoding: The task is to check whether there are collisions (duplicates) in a list of word after encoding with a lossy cipher function (mapping 26 alphabetic letters to 10 decimal digits). I map the strings, sort them, and walk through the result comparing neighboring entries—iff there's a match, then there are collisions. Code: a/1.lisp

Illumination Optimization: Given a set of lampposts at positions Xi along a road, how many lightbulbs of illumination radius R are needed to light the entire stretch of road from 0 to M? You go through the lamppost positions in order and skip those that are already illuminated by the previous lamp. My solution (a/2.lisp) tries both directions, in case one is "more optimal" than the other :-)

Rainbow Sort: On first read, I found it a bit confusing, because it talks about colors, card positions, and ordering numbers, all represented as integers; so I put it off until after the others. In the end it turned out to be quite easy. My solution (a/3.lisp) is a simple recursive function using a hash table to store colors that have already been seen.

ASCII Art: I quickly figured out that there must be a solution that involves just computation and no searching or construction. But it took me quite a bit of time (easily more than an hour) and trial/error until I removed all off-by-one errors. The solution (a/4.lisp) ended up simple enough, and solved all sets on the first attempt.

Untie: Here again, I quickly figured out that I can simply ignore all sub-sequences that have no repetitions. For every repeated sequences of length N, you can always fix the tie by flipping ⌊N/2⌋ symbols. I convinced myself that this is always possible without creating additional collision, thanks to the fact that you have two symbols to choose from. The trickiest part is that we're dealing with a circle rather than a sequence with start and end. My approach was to start at a point where the symbol changes—if there is no such point, then this is a special situation of a circular sequence of the total length of the sequence, and in this case actually you need to flip ⌈N/2⌉ symbols. Other than that, my recursive solution (a/5.lisp) looks relatively simple, and solved all cases on the first attempt.

By solving all problem sets in time, I ended up in rank 734 for that round, which made me quite happy! With some (but too little) time remaining, I tried my hand at two problems from more difficult rounds, but didn't manage to solve either of them in time.

Game Sort (Round C): This didn't look that hard, but my code consistently solved only the tiny sample set, and failed at even the small set on all my attempts. Maybe I take my broken code (c/1.lisp) up and try to find and fix what's wrong with it—or probably rather with my initial understanding of the problem and its possible solution.

Spacious Sets (Round B): My first iteration of code solved the small set, but ran into a timeout on the large set. I had to revise it to avoid quadratic complexity. I first thought of "memoization" to optimize away repeated iterations; in the process of implementing that, I discovered a nice linear initial pass (which has to be done in both directions—the problem has some structural similarities with Illumination Optimization above) that allowed the computation for a given pivot element to be done in constant time (adding two pre-computed numbers and 1). To my surprise, even with this probably very close to optimal solution, my program ran out of time! It turned out that I had another O(N²) issue in an innocuous part of the code that simply constructs a mapping from indexes in the original unordered sequence to the position in the sorted sequence that I use for the computation. I had to replace the trivial code using POSITION with my own FAST-POSITION trivially implementing binary search on the sorted output vector. The code (b/3.lisp) doesn't look great, but it works! I only really started after the end of the competition, though; otherwise I might have gotten four points for the small set...

So that was Google Code Jam's Farewell Round! Thanks a lot to Google for organizing those events over twenty years, and especially to the many Googlers for coming up with great problems, ironing them out, and providing tons of challenging data sets! I had a lot of fun—along with quite a bit of frustration; but that goes along with the challenge, I guess.

Thursday, June 10, 2021

AI/ML Hype: If even Google doesn't get it right, then...?

 I'm a happy and frequent user of Google Photos, which I use to back up, share, and edit the many photos that I take on my smartphone.

This morning, the mobile app greeted me with a new feature: It suggested that six of my photos were incorrectly oriented, and offered me to fix that by rotating them.

I checked, and out of those six photos, five were totally fine as they were. (I'm a bit pedantic and usually fix any wrong orientation quickly by hand, which Google Photos makes quite easy.)

The sixth was indeed misoriented, but the rotation suggested by the tool was also wrong.

First I laughed. If Google—of all people!—sends me twelve predictions via a highly popular app, and gets eleven of those wrong, how can anybody really expect that Artificial Intelligence/Machine Learning will solve real problems and eventually pay back all the investments being made in it? And note that the problem class here is basically image classification, which is one of the few narrow domains where ML has been particularly effective.

And then I dutifully corrected Google's mispredictions by rotating the proposed images until they were correct (correct again, in five of the six cases). So maybe if a few million other nice/gullible people help poor Google out, then maybe one day, it will actually become helpful even to pedants like me.

This suddenly recalled a question Prof. Mireille Hildebrandt asked at a large EU event last week ("Leading the Digital Decade", video):

What is a digitally skilled population? Is it a population capable of using AI and other digital systems? That's the usual way to talk about citizens: users. Or is it, perhaps, a population that is open to be used by these systems as data engines?

Sunday, May 03, 2020

Google Code Jam 2020 in Lisp—Round 1C

Well, that was slightly embarrassing and frustrating...

After a relatively promising attempt at round 1B, I entered round 1C yesterday as the last chance to advance to round 2. Again, three nice problems, with the third one looking too hard for me at first sight, at least for the general solution. My "plan" after reading the problem descriptions was roughly: Solve problems 1 and 2 and, time permitting, submit a solution for the easy first input set of problem 3. That would have been a good plan, except I took a bit too long to code problem 1, and I got completely stuck trying to debug my Lisp solution for problem 2.

Problem 1: Overexcited Fan

The problem turned out to be even easier than I first thought. I should have spent more time designing the algorithm before starting to code it. I had an OK solution, but it was more complex than necessary and took me 53 minutes. That's much longer than it should have, given the overall time limit of 2 hours 30 for the three tasks. After the round, I simplified this to a more elegant solution.

Problem 2: Overrandomized

Here I had the correct inspiration to work with letter frequencies, in particular first-digit letter frequencies. Unfortunately my Lisp solution would always result in "RE" (runtime error) when submitted, even though it ran fine both against the sample data set and against some additional data sets that I have generated. At this point I ran out of time.
After the round, I tried for some time to get my Lisp program working, but without any possibility to get diagnostic output, I didn't find where the problem lied. I even started from scratch using a different implementation technique, but the result was the same: My program would run fine against the sample and my self-generated tests, but throw "RE" when faced with the competition/training data. Very frustrating. If anyone finds the error(s), please let me know!
At one point I reimplemented my solution in C++, and it successfully solved all data sets as soon as I got it to compile. I then "backported" it to C, which simplified it further (in the C++ version I used both "traditional" arrays and "library" vectors—for sorting—which makes that version quite ugly.

Problem 3: Oversized Pancake Choppers

After reading the problem description, I assumed (probably correctly) that a full solution would beyond my capabilities for solving, at least under this time pressure. But the easy set seemed quite tractable: If there are only 2 or 3 diners, then the set of solution possibilities is quite bounded. Indeed it took me only (competitive coders can laugh now) about 15 minutes to write a stupid solution that worked for this limited case. But that's academic because I had run out of time in problem 2, so this happened after the round was over.


With only the first and easiest problem solved in time, I ranked in the 8000s, much below my rank in 1B—and hopelessly below the cut-off at 1500.
In problem 1 and (my constrained/stupid solution for) problem 3 I had no problems with Common Lisp as a coding language. But I failed to get my problem 2 Lisp solution working, while later attempts in C++ and C worked right away (and showed that I had analyzed the problem correctly).

Sunday, April 19, 2020

Google Code Jam 2020 in Lisp—Round 1B

Although I had quite a bit of trouble surviving this year's qualification round, I was allowed to participate in online round 1, which, as usual, is held as three different sub-rounds to accommodate participants from various time zones.  The first (1A) was in the middle of the night in my time zone, so I skipped it and didn't even get around to reading the problems.  But today I attempted  round 1B.  The problems were again very cute and interesting, and while I struggled with the limited time, I ended up being quite satisfied, even though I missed qualification this time.

Problem 1: Expogo

I wrote a relatively simple implementation using search with memoization (caching) and a bit of pruning of implausible search regions, and that was sufficient for all three data sets.  According to the after-round summary, there's also a constructive solution.  I had suspected so, but couldn't find it.

Problem 2: Blindfolded Bullseye

This is a nice search problem that involves some geometry.  I think I started with a reasonable search strategy, but then ran out of time when it came to doing the geometry... So I gave up trying to find a "good" solution, and coded the trivial brute-force solution for solving the easiest data set.  That was worth only 3 points, but those 3 points improved my final ranking by more than 1'000.

Problem 3: Join the Ranks

This didn't look too impossible, but given the high amounts of points awarded for this, I suspected that this would be beyond my skills and decided to focus on the other two problems.


With a full solution for problem 1, a half(?)-done good solution for problem 2, and the few extra points for the trivial data set of problem 2, I finally ranked #1996 [edited for final result].  As only the first 1500 contestants qualify for round 2, this was not sufficient.  But I'm quite happy with how it went, and am looking forward to round 1C, about two weeks from now.  And unlike during the qualification round, I didn't run into any issues with coding in Lisp—even though I haven't been using it professionally for several years now.

Tuesday, April 07, 2020

Google Code Jam 2020 in Lisp—Qualification Round

This year I registered for Google Code Jam again. The new platform supports Common Lisp (SBCL) again, so I started writing my solutions for the Qualification Round in SBCL. Although I managed to qualify for Online Round 1, it didn't work as well as I'd hoped.
The Qualification Round this year had five problems, which took me a while to notice—I had to scroll horizontally to see the fifth problem. I attempted all five, but only managed to write working solutions for three of them.

Problem 1: Vestigium

That was an easy one, but for some reason I didn't manage to code it correctly in Common Lisp. Even though my code looked totally straightforward, it only produced an RE (Runtime Error) when run against the data set. Finally I rewrote the solution in C++, and that worked on first try.

Problem 2: Nesting Depth

The task was to generate appropriate numbers of correctly nested parentheses. Obviously this type of problem is familiar to Lispers, and this time my straightforward code actually worked. My sloppiness resulted in an invalid attempt, as I initially left out the (solve) call at the end of the script.

Problem 3: Parenting Partnering Returns

It occurred to me relatively early that the problem maps to the problem of partitioning (two-coloring) the interval graph. However, all my attempts of coding this simple graph traversal algorithm in Common Lisp ended in REs (Runtime Exceptions) again. Very frustrating!

Problem 4: ESAb ATAd

This was my favorite problem, and I came up with a simple and near-optimal solution pretty quickly. It would have worked the first time, had I not (again) forgotten to add the correct (solve) call after the development phase.

Problem 5: Indicium

That one was definitely too hard for me! After the deadline, I read the analysis and took it up again. I managed to code a super-efficient method to generate latin squares with arbitrary possible traces at will, based on "circulating" matrices. The only cases that I cannot solve in this way are odd-sized with traces of is n+2 or n2-2.


Finally I advanced with 49 points (from 100, with 30 required), and ranked #6318 of about 44000 participants. My Lisp development and graph algorithm skills have become a bit rusty, and I'm not very optimistic about being able to survive Online Round 1.