Elo rating system

Very simple Ruby implementation of the Elo rating system, a method for calculating the relative skill levels of players in zero-sum games such as chess.

It contains two functions: expected(player_a, player_b), to calculate the expected score of player_a in a match against player_b; and elo(old, exp, score, k_factor = 32), to calculate the new score for each player.

Add this line to your application's Gemfile:

gem 'elo_rating_system'

And then execute:

$ bundle

Or install it yourself as:

$ gem install elo_rating_system

In a five-round tournament, player A, with a rating of 1613, plays against opponents with the following ratings: 1609, 1477, 1388, 1586, 1720.

The expected score is therefore:

exp  = EloRatingSystem.expected(1613, 1609)
exp += EloRatingSystem.expected(1613, 1477)
exp += EloRatingSystem.expected(1613, 1388)
exp += EloRatingSystem.expected(1613, 1586)
exp += EloRatingSystem.expected(1613, 1720)

exp.round(3) # => 2.867

Player A lost match #1, draw match #2, wins match #3 and #4, and loses match #5. Therefore the player's actual score is (0 + 0.5 + 1 + 1 + 0) = 2.5.

We can now use this to calculate the new Elo rating for player A:

EloRatingSystem.elo(1613, 2.867, 2.5).round # => 1601

The code is available as open source under the terms of the MIT License.

This gem is maintained by Sashite.

With some lines of code, let's share the beauty of Chinese, Japanese and Western cultures through the game of chess!