inamerrata

ajtowns – Scamming my way onto “Team Samba” (“hey, I use it!”) was a good idea. Winners! #lca2010 #hackoff

Wellington Perl Mongers were awesome enough to run the Hackoff during LCA 2010. It consisted of a couple of hours of team hacking to decode craftily hidden eight character tokens. I’d seen Rusty carefully putting the “Samba Team” earlier (“Here’s what’ll happen: I’ll put together a team so awesome that they won’t want me on it”), so when I wandered in with Biella to check out what was going on and found myself sticking around to see how it went, it seemed obvious which team I needed to latch onto.

The first four problems went down pretty well, albeit with a chunk of brute force rather than any elegance. The first question was some text munging of an HTML document, pretty much perfectly designed for a perl solution. Personally, I got stuck on trying to get the damn thing to display on my underpowered netbook, but that’s what teams are for, right? Problem two was pretty similar — it had an ascii punched tape that you had to treat as binary (holes are one, untouched is zero), translate to hex, translate to ascii, then actually read the resulting text rather than just plugging in the promising look eight characters from the end. I think I abused iprint and shell to get that one out — see: users can be contributors too! Number three was one for the spreadsheet mungers: a fixed width text database where you had to pull out various bits of information. We solved this one as a team: people worked on different queries however they liked; Andrew Bartlett and I imported it into openoffice.org and used its sorting facilities; other people did cleverer things. The fourth problem, which was the last one anyone got out before the organisers had to call time (and that after an extension), was an OpenOffice document inside a tarfile with various letters highlighted; putting the highlighted letters together gives you the answer. I think it was Jelmer who managed to pull the document apart and programmatically extract the answer from that, and with it the win.

The next question we got was somewhat horrific. It was an animated gif, consisting of 1300 odd frames of green text falling down the screen Matrix-style, with one or two characters randomly adorned with a blue or red square. The instructions on the first frame suggests choosing the “red pill” (the rabbit hole one) or the “blue pill”, with little pointers at the different coloured frames. So the answer seems to be go through every frame and pull out the characters highlighted in red, and see what they end up saying.

We were kinda stymied by this, and after some back and forth, pulling the gif apart, converting to png, and basically getting nowhere ended up just assigned chunks of 1000 frames to different team members. We got a bunch of frames done, but were still a ways off an answer when it got shutdown. Then it was off to the nearby pub for dinner and beers (and Tridge’s analysis of the the “Harry Read Me” climate data stuff).

(We’d only looked at the red squares, trusting in fandom to not let us down; one of the other teams had actually looked at the blue highlighted letters though — turns out they were just DEADBEEF repeating. Nice.)

After heading back I spent a bit of time playing with my pygame project; I think getting from just having a triangular grid, to having little moving circles on top of the grid or something equally compelling. But that then inspired me to have another poke at that problem again. So I pulled out pygame, and the 1349 frames converted into PNM, and set about automatically extracting the relevant characters.

I took a few assumptions: that the characters would be laid out on a nice rectangular grid, that all the characters would look the same (every A is the same combination of pixels as every other A), and that the highlights would be in a predictable position relative to the character grid. I pulled up a frame in gimp to work out the pixel width of each character and the offsets for the frames, at which point it was just a simple matter of programming.

And rather than explicate that further here, I’ll point at my junkcode page for the explanation of that programming and the prettified source instead. All in all, pretty neat, I thought.

Anyway, submitting that (sitting on the pavement outside the venue; a security guard questioned me about stealing the conference wifi, and a group of party girls walked past and commented “He’s facebooking!” — sadly, they were right) then allowed us access to the final problem they’d prepared, which was a midi file disguised as a spreadsheet where the answer was encoded as errors in a repeating tune. Beyond listening to it for a bit, I didn’t make an attempt at that point: graphics and OCR were scary enough; audio? Please no. I see now that a couple of other teams actually got that one out too eventually. Kudos!

(Tridge and Jelmer had a go at solving it independently too; they did it in C, loading each PNM into a structure in memory directly, scanning that for the red square, copying it into a separate structure, writing that to a new file, and running gocr over the file; then doing the next frame. PNM is a particularly great format for doing that in C.)