I managed to solve 5 problems and to rank 145th out of 1890 contestants, spending about 12h in total during the weekend. There were a few technical quirks in the process, but all in all I really enjoyed the sprint and was pleasantly surprised by the quality and the complexity of the problems.

In this post I will explain how I solved those 5 problems; it’s mostly for myself, to straighten the thoughts out, as it was a bit chaotic and stressful during the contest.

If you are interested in solutions make sure to check out the official CodeSprint website, they should have them available in a couple of days.

Picking Cards

Links: problem, solution

Sort cards by their number, then take them one by one. On step n the number of ways is multiplied by the number of cards at the beginning of the deck with c_i ≤ n. If this number is 0, it’s impossible to pick up all the cards.

The brute-force approach is O(N²) and could be too slow. To speed it up, we keep track of the last m : c_m ≤ n and start from there. As m is never decreased the overall complexity is O(N).

Coin Tosses

Links: problem, solution

On each toss we either have a head or a tail, so the expected number of remaining tosses T(n, m) = 1 + ½ × [T(n, m+1) + T(n, 0)]. We could try a dynamic programming approach, but the formula is cyclic.

However, for m = 0 the expected number of tosses can be expressed analytically: T(n, 0) = 2ⁿ⁺¹ – 2 (proof). Add the boundary condition T(n, n) = 0 and memoisation, and you have a solution.

Fraud Prevention

Links: problem, solution

One of the company-sponsored problems. We want to normalise email and street addresses and to keep two hashes with the combination of the normalised values and the deal IDs as keys. For each key we keep a list of credit cards along with order IDs. Then we process orders one by one and check all possible cases (see the source code comments).

I found this problem a bit uninteresting for a contest — it’s tedious and, uhm, un-algorithmic. However it would probably make a decent interview questions with all its practicalities.

Subsequence Weighting

Links: problem, solution

Generalisation of the Longest increasing subsequence problem. The algorithm is very similar: take each value one by one and maintain values (and cumulative weights) of the last elements of subsequences of certain weight. After we process all values, the last element will have the maximal weight.

One complication is that we need a data structure with efficient insertion, deletion, search and traversal times. One possibility is to use a red-black tree (as implemented by std::set) and augment it so that all nodes form a doubly-linked list. Also, unlike with the LIS algorithm, each insertion can lead to many deletions (see lines 74-82).

With such a data structure, the running time is still at O(n log n).

That was my favourite question, and the one I spent the most time on, even though in retrospect it doesn’t look all that complex.

Quora Nearby

Links: problem, solution

Another company-sponsored problem. A pity N was quite small and the brute-force approach actually worked. Higher limits would require a clever data structure, such as the k-d tree and would make the problem much harder and more interesting.

First we read all topics and questions and create a vector of all topics (ID and co-ordinates) and a map of topic IDs to the lists of question IDs.

Topic queries are straight-forward: we sort topics by distance (and IDs, in case the distance is the same) and print the first m IDs.

For queries it’s a bit more involving: again, we sort topics by distance, then check all associated questions using our map. The complication is that we need to check all questions for topics with the same distance, and include those with higher IDs.

Please never mind the code for this problem, my solution was accepted literally 5 minutes before the contest ended, as you can imagine I was a bit in a hurry.

Big thanks to the Interviewstreet team for a great weekend!

Today I’m happy to announce the availability of the muspy API. The API allows you to create and modify user accounts, to manage the list of artists you want to follow and to receive their releases.

Someone is already working on an iPhone app that will use the API, expect more news on this front soon!

To make it easier to populate the list of artists you want to follow, muspy allows to import top artists from your Last.fm account. Today this function became more flexible: in addition to getting overall top artists, it can now import most frequently listened artists in the last 12, 6 and 3 months and 1 week.

I also lifted the limit on the number of artists that can be imported from 200 to 500, and increased the number selected by default from 50 to 100.

Do you have a feature that you want to see in muspy? Let me know! Alternatively, feel free to fork muspy on GitHub and to send your pull requests.

If you are a music lover and never tried muspy before, give it a go! With muspy you will not miss an album release ever again.

muspy is an album release notification service, you give it a list of your favourite artists and it sends you a notice (by email or RSS) as soon as they have new releases.

I wrote muspy 3 years ago to scratch a personal itch — I was spending too much time online checking if bands I’m into have something new; but was still missing many releases.

muspy was initially developed for Google App Engine, which was the hot new thing back then. In retrospect, while working with App Engine was extremely educational and a lot of fun, it wasn’t a very good fit. The recent announcement on the price increase was the last straw — I decided to re-write it in vanilla Django and to host it myself.

I’m also releasing the source code under GNU AGPL in the hope that it will be beneficial for the service and for its users.

Major changes since the previous version:

• Track release groups instead of releases

  This was the most frequently requested feature, there were too many duplicate releases for some artists, release groups make everything much cleaner. This change was also the reason why the new version is almost a complete rewrite.

• Stars are off by default for new releases

  muspy allows to star individual releases, starred releases show up on top of the sorted list of releases. This feature wasn’t used by many which resulted in all releases being starred for most users. Now stars are off for new releases, if you want to star, you have to do it manually on the website. I migrated stars only for users with S < min(R, 40) / 2, where S is the number of starred releases and R is the total number of releases for all artists you follow.

• Speed

  It used to take more than a week to check all artists for new releases, now the checking cycle is much shorter. Things like importing from Last.fm or adding a comma-separated list of artists should also be significantly faster.

• Blog

  I will be blogging about muspy here instead of on the website itself. Feel free to subscribe to the full feed or just to the muspy category.

Other than that, muspy remains pretty much the same. I migrated all the data but I encourage you to go the old site at muspy.appspot.com and to check if everything was migrated correctly. Please note that the old site’s background processes are not running and it will be taken down in a month or two.

Now that muspy is free and open-source, don’t hesitate to look at the code, tweak it and suggest improvements. Git and GitHub make it too damn easy!

And if you never used muspy before, give it a try!

I recently took over xmonad-log-applet maintainership from Adam Wick, and today I’m happy to announce the release of version 2.0.0.

Changes since the previous release:

• Migrated the GNOME 2 applet from deprecated libbonobo API to the new D-Bus based API.
• GNOME 3 panel support (in fallback mode).
• Xfce 4 panel support.
• Revamped the build system.
• Dropped GConf dependency which was used to specify the applet width; instead fill all available space (like the window list applet) and ellipsise when necessary.
• Simplified background transparency handling.
• Fixed install locations.
• Updated sample xmonad.hs.

To install get and unpack the tarball or clone the repo, then run:

% ./configure --with-panel=gnome2
% make
% sudo make install

Substitute `gnome2` with `gnome3` or `xfce4` if that’s what you use. If you cloned the git repo, use `./autogen.sh` instead of `./configure`. After restarting the panel you should be able to add the applet.

Use the provided sample xmonad.hs file to bind it to Xmonad. It depends on the DBus package, which currently doesn’t compile with GHC 7.x, but it’s easy to work around:

% cabal update
% cabal unpack DBus
% cd DBus-0.4
% $EDITOR DBus/Internal.hsc

Replace `import Control.Exception` with `import Control.OldException`, then:

% cabal configure
% cabal build
% cabal install

After this, your xmonad.hs should compile.

EDIT: With GHC 7.4, you also need to edit DBus/Message.hsc and prepend `Foreign.` to `unsafePerformIO`.

Happy Xmonading!

Spek is now fully compatible with the recently released Mac OS X Lion. The bug that prevented it from starting up is now fixed, thanks to John Ralls and the awesome GTK-OSX project.

Get your copy of Spek today!

This version features multi-lingual support and a much better OS X integration. It also includes performance tweaks and many bug fixes.

Read the NEWS file for a full change log.

Download links and installation instructions are on the Spek website, get it while it’s hot!

Spek is now translatable, the next version will feature at least these translations: Dutch, German, Russian, Swedish and Ukrainian. We use Transifex to manage translations. Feel free to translate it into your language or to improve an existing translation. Transifex is a very easy to use tool, you don’t have to know anything about programming to translate.

Some progress has been made on the packaging front, Spek packages are now available in Arch Linux, FreeBSD and Gentoo repositories.

There’s also an official Ubuntu PPA for Spek containing packages for Maverick and Natty. I will keep the PPA up-to-date with future Spek and Ubuntu releases.

I also created a package for Debian which needs a sponsor. If you are a Debian Developer I would appreciate a review and eventually an upload. The package can be found on mentors.debian.net.

Last but not least, the next version will show some OS X love. Many OS X specific bugs have already been fixed or are scheduled to be fixed for the next release.

I expect to release Spek 0.7 sometime this spring. If you are feeling adventurous, try Spek from git master, compilation instructions are on the wiki.

At the new job I will mostly be dealing with Perl and, considering it’s not the language I’m most familiar with, I was looking for a small project to practice it.

Thus, updating.versia.com was born. It’s a web feed that keeps track of the /usr/ports/UPDATING file. I personally find it much easier to use a news reader than manually checking the file each time I want to update my ports.

Like most Perl scripts, the one generating the feed is short, uses a lot of CPAN modules, and is a bit ugly :) You can check it on GitHub.

P.S. As requested on freebsd-ports@ I also added feeds to head/UPDATING, stable-7/UPDATING and stable-8/UPDATING. Subscribe on updating.versia.com

This version is about 3 times faster than 0.5 thanks to the lightning-fast FFmpeg decoders and the new multi-threaded analysis algorithm.

Spek 0.6 also features dramatically reduced size of the Windows installer (from 17.1 MiB to 9.8 MiB) and OS X bundle (from 10.5 MiB to 6.1 MiB)

Read the NEWS file for a complete change log.