rank-king/lib/rank_king.rb

require_relative "rank_king/models"
require_relative "rank_king/open_skill"

module RankKing
  OS = OpenSkill.new
  Match = Data.define(:a, :b)

  # rates all possible 1v1 games to find which generally decreases sigma
  # regardless of the winner/loser - is this mathematically sound? I have
  # no idea
  def self.suggest_game(axis)
    items = axis.pool.items
    ratings = items.to_h { [_1.id, Rating.first(axis:, item: _1) || OS.rating] }

    items.combination(2).sort_by {|combo|
      game = combo.map { ratings.fetch(_1.id) }
      Math.sqrt(
        [game, game.reverse]
          .map {|game| [game, OS.rate(*game).flatten] }
          .sum {|pre,post| pre.zip(post).sum { (_1.sigma - _2.sigma)**2 }}
      )
    }.last
  end

  def self.rank(axis, winner:, loser:)
    fail unless [winner, loser].all? { _1.pool == axis.pool }

    game = [winner, loser].map { Rating.first(axis:, item: _1) || OS.rating }
    result = OS.rate(*game).flatten

    DB.transaction do
      Ranking.create(axis:, winner:, loser:)
      Rating.update_or_create({axis:, item: winner}, result.fetch(0).to_h)
      Rating.update_or_create({axis:, item: loser}, result.fetch(1).to_h)
    end
  end

end