module EloRating

Calculates the expected score of a player given their rating (player_rating) and their opponent’s rating (opponent_rating).

Returns a float between 0 and 1 where 0.999 represents high certainty of the first player winning.

# File lib/elo_rating.rb, line 52
def self.expected_score(player_rating, opponent_rating)
  1.0/(1 + (10 ** ((opponent_rating - player_rating)/400.0)))
end

Calls the K-factor function with the provided rating.

# File lib/elo_rating.rb, line 17
def self.k_factor(rating = nil)
  @k_factor.call(rating)
end

Sets the K-factor to a number.

# File lib/elo_rating.rb, line 41
def self.k_factor=(k_factor)
  @k_factor = Proc.new do
    k_factor
  end
end

Calculates the amount a player’s rating should change.

Arguments¶ ↑

• expected_score: a float between 0 and 1, representing the probability of

the player winning

• actual_score: 0, 0.5, or 1, whether the outcome was a loss, draw, or win

(respectively)

• rating (optional): the rating of the player, used by the K-factor function

• k_factor (optional): the K-factor to use for this calculation to be used

instead of the normal K-factor or K-factor function

Returns a positive or negative float representing the amount the player’s rating should change.

# File lib/elo_rating.rb, line 69
def self.rating_adjustment(expected_score, actual_score, rating: nil, k_factor: nil)
  k_factor ||= k_factor(rating)
  k_factor * (actual_score - expected_score)
end

Sets the K-factor by providing a block that optionally takes a rating argument:

EloRating::set_k_factor do |rating|
  if rating && rating > 2500
    24
  else
    16
  end
end

Raises an ArgumentError if an exception is encountered when calling the provided block with nil as the argument

# File lib/elo_rating.rb, line 33
def self.set_k_factor(&k_factor)
  k_factor.call(nil)
  @k_factor = k_factor
rescue => e
  raise ArgumentError, "Error encountered in K-factor block when passed nil rating: #{e}"
end