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Elo rankings for foosball
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Elo Rating Tracking for Foosball

After getting a Foosball Table for the Gaslight Software office, it didn't take long for talk of who was the better player and thoughts of a tournament to begin. Getting everyone to agree on the format of a tournament is quite another thing.

However, a bit of research turned up the Elo Rating system as modified by Bonzini USA. This little Rails application implements this scoring system for anyone in the office who plays. It's working so far and the app is becoming a place where we at Gaslight are able to explore some newer gems and techniques.

Brief Summary of Elo

The Elo rating system is a method for calculating the relative skill levels of players in two-player games such as chess. It is named after its creator Arpad Elo, a Hungarian-born American physics professor.

The heart of the Elo ranking is the "Win Expectancy" expressed as a probability that one player will beat another based on the difference between their rankings. The Win Expectancy is defined as:

We = 1/(10^(-D/F) + 1)

Where D equals the difference between the two players' rantings and F is the "rating interval scale weight factor". Bonzini set the ranking interval sizes to 500 and the weight factor to 1000.

In the Bonzini system, if you win your rating goes up by an interval constant times the We. The loser's rating goes down by an equal amount. The Elo system is a zero sum system. The only way to add points to the league is to add more players.

We've slightly modified the Bonzini system to give fractional rating increases and decreases based on the percentage of points won or lost. This has the effect of causing the winner of the game to potentially lose ranking if the margin by which they won doesn't exceed the Win Expectancy.


@cdmwebs added some new-fangled developer stuff using per-app pg instances with foreman. You can read more about it in this super long tweet by @pvh.

Here's how to get it running:

initdb pg
foreman start -f
createdb elo_dev

Pretty nifty, huh? Now you have your very own postgres instance running just for this application. No daemons in the background opening all your ports and eating cycles for no reason. You can see all the logs in one place and you're even keeping your data in the same location, too!

Now you can move on to the regular old Rails setup.

rake db:setup
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