## Thursday, June 7, 2012

### Overall

IPSC this year had an interesting mix of normal algorithm programs and some more unique ones. A breakdown of some of the more interesting problem is given below. Overall, we did much better than last year, finishing in 213th place out of 644 teams who scored points, with a final total of 11 points (7 easy problems and 2 hard ones). With IPSC out of the way, we can set our eyes on ICFP, our all-time favorite competition, but more on that at a later date. Without further ado, our breakdown of the problems:

### Problems

#### A - Abundance of Sweets

As usual, Problem A was not much of a challenge. The easy solution required simply iterating through a matrix and outputting the number of 'o's found. The harder challenge required checking the surrounding values of all the 'o's, but could still be easily done with a brute force method.

#### B - (blank)

This was one of the more interesting problems of the challenge this year. Given no problem statement, we moved on from this challenge, though in hindsight we should have attacked this problem earlier on. When we did get around to it, we tried a random submission and immediately saw how this problem was laid out when we received a hint in response. Following the hints, we eventually understood what the solution was -- a sequence of 6 or more numbers forming a sequence such that each number was a multiple of the one before, and all of them totaled 123456789. Eventually we managed to use the prime factorization and some educated guessing to come up with a solution that worked. The better solution was much simpler -- just take the binary representation to get a sequence of powers of 2.

The hard counterpart for this problem used a similar hint upon submission method, though required more submissions to move towards the answer. We made good progress on this solution, but unfortunately ran out of time just as we were only 1 or 2 tries away from completion.

#### C - Card game

This problem dealt with probability, not one of our strong points. After a few wrong tries, we were able to get the first solution, but decided to work on other problems instead of attempting the hard part.

#### D - Data mania

This problem was more of an aggregate of problems, consisting of several data analysis problems on last year's IPSC user statistics. Each problem in the easy set was relatively easy, and we arrived at an answer, but were judged incorrectly. After looking through it, we were unable to find our mistake, so we moved on.

#### E - Evil matching

We looked over evil matching some, but didn't come up with any good methods to approach it, and ultimately weren't able to code any solutions.

#### F - Fair coin toss

This was again a problem focusing more on probability. After thinking through the problem, we came to the correct conclusion for the easy problem that only having a 1/2 probability coin would do it.

For the hard problem it took some thinking through, but it wasn't too hard to come up with a formula for the number of subsets of coins which had at least one probability 1/2 coin in it.

#### G - Gems in the maze

The easy sub-problem for this problem was easily brute forced. Simply looping through all initial chambers and following the paths was enough to pass.

For the hard test case, we initially though that dynamic programming was the solution, but realized that the possible looping made this impractical. After some thought, we realized that all chamber sequences were some sequence followed by some loop (either of which could be of zero length). In addition, once you found this sequence, all chambers on this sequence were determined, thus giving a linear time algorithm. Implementing this bug-free was somewhat of a challenge, and we ultimately ran out of time on this problem.

#### H - Harvesting potatoes

For the easy case, we realized that casework would get the job done. Unfortunately as we coded the solution, more and more edge cases came to mind, and the code became complicated quickly. Looking at the scores, we noticed that none of the top teams had finished the problem, so we moved on. At the end we didn't have enough time, and didn't end up finishing either part of the problem.

#### I - Invert the you-know-what

This was again a fun problem consisting of several username:password hashes which you were tasked with the challenge of decrypting. Luckily the easy subproblem only required decrypting one of the passwords, and passing them through a simple MD5 decrypter got us several passwords. We were stuck for a little bit because the salt prepended to the password threw us off. When we realized to remove the salt then submit we got the easy case.

We weren't sure how to tackle the hard sub-problem, so we moved on.

#### J - Jukebox

This problem was also a very unique one. As input, you were given MP3 files and the same files in text format, which described waveforms. Unfortunately, we realized analyzing these would require a Fast Fourier Transform, so instead we decided to listen to the pieces and simply transcribe them by ear (at least for the easy case). After a few mistakes, we got a correct submission.

For the hard sub-problem this was clearly not possible, and we didn't have time to figure out writing a Fast Fourier Transform, so we moved on.

#### K - Keys and locks

In hindsight this problem was very tractable, but we didn't have an good ideas at first and so didn't attempt this problem for very long.

#### L - Light in a room

After reading this problem, we knew it would take a while and moved on to easier problems.

#### M - Matrix nightmare

After reading through the wise words at the beginning of the problem statement and quickly scanning through, we again realized this was beyond our level and moved on.

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