This is that time of the year once again, when coders gather to take part in some good programming fun, the Facebook Hacker Cup. It’s only the first qualifier round, and while I hoped it will go better than last year, well, it didn’t. Not that I’m really surprised.

After that 72 hours results are in, as is the explanation and example source code for the solutions. It’s good that the example solutions are in Python, I might even learn a trick or too.

Alphabet Soup

The problem setting is easy enough. Funny thought that no matter how many times I counted the letters in the world “HACKERCUP”, I didn’t notice that there are two Cs. I mean, duh! As usual, the example input set was designed such that it wouldn’t trigger the bug of miscounted Cs. This carelessness is one thing that comes up quite often in my programming, probably should take better care of it.

Billboards

This problem actually worked, which means I’m in Round 2, but I guess it can be improved quite a bit, make it more efficient or come up with some heuristics. Or maybe it doesn’t matter much.

Auction

This problem was on a whole different level. While the first two had apparently more than 5000 and 3000 correct solutions respectively, this had only 28… I was thinking about it for quite a while, drawing diagrams, trying to use my intuition and imagination to see where the trick is since the naive O(N^2) algorithm is definitely unusable on the N~10^18 level. On the other hand, I might have tricked myself. Reading the solution the trick was completely different than I expected – I thought there’s some weakness in the random number generator that can be used to express everything analytically, while it is actually just about keeping good track of things. There’s no fancy algorithm to break this problem, just pure logical thinking. Now that’ll teach me as well…

Looking out

This of course means that I have a lot more to learn, and most likely I’m not cut out to be a Facebook caliber hacker. That’s no problem, but good to know. Whenever I think about it, the picture that comes to me is the hacking competition scene from The Social Network, where they hire their first employee. I’d love to be in the middle of such brainfest, such intense creation, such inspired learning from one another while having an an amazing time. Well, fortunately there are other places where I can have that experience, like the Startup Bus. And maybe, I can also set out to create that environment over here in Taiwan.

But first, let’s get ready for round 2, should make that one better.

Last night’s Hacker Cup round, which was a second try of the first round after last week’s disaster, didn’t go quite as well for me as I hoped. Probably not that much for the others either, as even if 1000 people will advance, less than that have submitted code.  Can be disappointment due to last week’s failure, I too was hesitating to take part, but also because the problems were I think at least one level up. Which is not a problem, just observation.

Got my coffee at 1am, set in to mood and started off right on time at 2am.  In the next 3 hours I finished one of the problems, and got halfway with another one, which is below par, but what can I do. Need to learn more. One thing that I’ve noticed is if one just searched the web with the right keywords, Problem 1 and 3 could be handed over on a silver plate, more or less… Which is kind of random for a programming competition.  Did they want people who can find the right solutions or maybe someone who knows the solution for all of these problems alread? The first one begs the question whether there will be  Wikipedia access in the final. The second one begs the question: who can be such a person? And if neither – why choose such problems that cannot possible be solved in the allotted time without prior knowledge or external input?

Well, I wonder what would be the right keywords for Problem 2, which I could not solve yet.

I’m also bothered, that it seems that the input file one had to download for Problem 3 does not conform to the input specs. Why? Especially when if you download the file, the system gives you 6 minutes to submit. This is rather underhanded….

I will certainly try to check out the other two sub-rounds as well, even if they are 5am next Wednesday and next Sunday local time. Not doing this for the glory but for the education anyway. :)

Notes on solutions

Warning: Spoilers ahead. Don’t read if you want to solve them yourself.

Almost all my code for this can be found in this Github repo, together with other rounds’ code.

1) Wine Tasting

This one is more mathematics than anything. Let’s have G glasses of wine and minimum C guesses to be right. For i = C to  G calculate the number of : how many ways one can have good guesses, multiplied by how many ways none of the remaining guesses are right. Sum these numbers up, and that’s your result.

I was thinking for a long time how to calculate the number of ways none of the guesses are right (i.e. the number of permutations where non of the elements are at their original place). Finally I found there’s a name for that: derangement. And it is not at all simple, so I guess I wouldn’t have figured it out myself.

In the end, what we have to return is:
$$\mathrm{result} = \sum_{i = C}^{G}{G \choose i} (G – i)! \sum_{j = 0}^{(G-i)} \frac{(-1)^{j}}{j!}$$

Now that it actually makes sence, it turned out this very expression has a name too: recontres numbers.

Update: fixed my program and now should be correct.

This can be a very large number due to the factorials, so got to use the right types. In my python code I had to use 64bit numbers, but it’s rather ad-hoc, just choose the ones not to break. I do have to revisit this again and fix the types.  (And don’t forget the modulus).

Also, I used SciPy to get the results, but only needed the factorial. If I quickly write a fast factorial subroutine then SciPy is not needed – another thing to fix in my code.

It turned out that my code has failed, so basically this round is 0 out of 3. Nevertheless, wanted to see what was wrong, so I rewrote most of the math code. After messing around for a while, it turns out that all my problems came from

from __future__ import division

which is basically the newer, py3k-style integer division code. If I excluded that, suddenly everything was fine. I really have to remember this, because I used this include in practically every math-related code I wrote lately. Besides this change, I kept the other upgrades: fast factorial and choose. My output for the given input file has md5sum of d17dfc9e9fef637f771da3d693d7920b

2) Diversity Number

This one I failed on, and seems very few people actually succeeded in general (less than 60 people in the whole round). I start to understand bits and pieces of the possible solution, but I have nothing complete yet.  I wonder if there’s such an insight into this one as in the previous problem with the derangement.

Anyway, one thing I know: if there’s a list A, sorted, with length n and index starting at 0, then the diversity number is
$$\mathrm{diversity} = \prod_{i=0}^{n-1} \left(A_{i} – i\right)$$
or in Python

prod = lambda array: reduce(lambda x, y: x*y, array)
diverse = lambda array: prod([array[i]-i for i in xrange(len(array))]) if len(array) > 0 else 0

Now just have to figure out, what to do, not to enumerate all the $$2^{100} > 10^{30}$$ subsequences in the worse case scenario. :-?

(I’ll update this when I’ve found the solution).

3) Turn on the Lights

This one is pretty much straight out of the game Lights Out, with the only twist that one has to turn on all the lights instead of turning off. This I found only well after the round ended but this makes the puzzle a bit of a duh for me, nevertheless it is worth doing, since there are some optimization choices that are not trivial.

The method I used to get the solution, which follows most of the steps as outlined on the above link. Let’s call the button in row i and column j as [i, j].

1. While importing the input, turn ever “on” into “off” to get the right setting for this problem.
2. Turn off every light down to the bottom row. This is done by pressing button at [i, j] if [i-1, j] is on. Save all the buttons that are pressed.
3. Check the state of last row at that point and keep it as a “target”.
4. Starting from a blank puzzle (all lights off), turn on a single line in the first row and propagate it down just like in step 2. Save the last row. Do this for all of the lights in the first row, which will create the “base vectors” of the solutions we seek.
5. We have to find the combination of those base vectors that equals to the above “target”. This can be done (i suppose) with a Gaussian elimination kind of method, but that’s just too much to implement for this little puzzle. According to the specs, the maximum number of colums is 18, thus there are $$2^{18}=262144$$ sett different configurations of lights at max.  That is not too bad, if there’s a fast comparison method, one can do a brute force search. That’s what I did. Also, I made use of binary expression of the light settings, e.g. switching lights can be done with XOR-ing the light pattern and the switch pattern.
6. If found a match in the previous step then do the swicthing in the first line according to that result and do the propagation again. The final result should be an all-off board. Save all the button presses again.
7. Check the pressed buttons. Since pressing a button twice is equal to not pressing it at all, if a button is pressed N times it is equivalent to N mod 2. Sum up all those presses, and that should be less than (rows * columns).

This is probably not the quickest method due to the brute force search, but it should be good enough for a programming competition where the speed of programming is more important the the speed of the program.

I do have one big complaint that I mentioned int the intro: the test file downloaded from Facebook did not follow the input specifications. In the specs every puzzle is a single line, the rows of the puzzle separated by whitespace. In the downloaded file for every puzzle every row is a new line.

The md5sum of my solution for the downloaded input file is 1e38422048a6aa9aeb007955d8b66f46