OpenSkill

A Ruby implementation of the OpenSkill rating system for multiplayer games. OpenSkill is a Bayesian skill rating system that can handle teams of varying sizes, asymmetric matches, and complex game scenarios.

Features

• 🎮 Multiplayer Support: Handle 2+ teams of any size
• ⚖️ Asymmetric Teams: Teams don't need equal player counts
• 🎯 Multiple Ranking Methods: Use ranks or scores
• 📊 Prediction Methods: Predict win probabilities, draws, and final rankings
• 🔢 Player Weights: Account for partial participation or contribution
• 📈 Score Margins: Factor in impressive wins
• 🔄 Tie Handling: Properly handle drawn matches
• ⚡ Fast: Efficient Ruby implementation
• 🧪 Well Tested: Comprehensive test suite matching reference implementation

Installation

Add this line to your application's Gemfile:

gem 'openskill'

And then execute:

bundle install

Or install it yourself as:

gem install openskill

Quick Start

require 'openskill'

# Create a model (Plackett-Luce by default)
model = OpenSkill::Models::PlackettLuce.new

# Create player ratings
alice = model.create_rating(name: "Alice")
bob = model.create_rating(name: "Bob")
charlie = model.create_rating(name: "Charlie")
dave = model.create_rating(name: "Dave")

# Simple 1v1 match (alice wins)
team1 = [alice]
team2 = [bob]
new_ratings = model.calculate_ratings([team1, team2])
alice, bob = new_ratings.flatten

puts "Alice: #{alice.mu.round(2)} ± #{alice.sigma.round(2)}"
puts "Bob: #{bob.mu.round(2)} ± #{bob.sigma.round(2)}"

Usage

Creating Ratings

model = OpenSkill::Models::PlackettLuce.new

# Create with defaults (mu=25, sigma=8.333)
player = model.create_rating

# Create with custom values
player = model.create_rating(mu: 30.0, sigma: 5.0, name: "Alice")

# Load from database [mu, sigma]
player = model.load_rating([28.5, 7.2], name: "Bob")

Calculating New Ratings

Simple Match (Team 1 wins)

team1 = [alice, bob]
team2 = [charlie, dave]

updated_teams = model.calculate_ratings([team1, team2])

Match with Explicit Ranks

Lower rank = better performance (0 is best)

teams = [[alice], [bob], [charlie]]
# Charlie wins, Bob second, Alice third
updated = model.calculate_ratings(teams, ranks: [2, 1, 0])

Match with Scores

Higher score = better performance

teams = [[alice, bob], [charlie, dave]]
# Team 2 wins 100-80
updated = model.calculate_ratings(teams, scores: [80, 100])

Match with Ties

teams = [[alice], [bob], [charlie]]
# Alice and Charlie tie for first, Bob comes third
updated = model.calculate_ratings(teams, ranks: [0, 2, 0])

Player Contribution Weights

When players contribute different amounts:

teams = [
  [alice, bob],      # Alice contributed more
  [charlie, dave]    # Dave carried the team
]

updated = model.calculate_ratings(
  teams,
  weights: [[2.0, 1.0], [1.0, 2.0]]
)

Score Margins (Impressive Wins)

Factor in score differences:

model = OpenSkill::Models::PlackettLuce.new(margin: 5.0)

# Large score difference means more rating change
updated = model.calculate_ratings(
  [[alice], [bob]],
  scores: [100, 20]  # Alice dominated
)

Predictions

Win Probability

teams = [[alice, bob], [charlie, dave], [eve]]
probabilities = model.predict_win_probability(teams)
# => [0.35, 0.45, 0.20] (sums to 1.0)

Draw Probability

Higher values mean more evenly matched:

probability = model.predict_draw_probability([[alice], [bob]])
# => 0.25

Rank Prediction

teams = [[alice], [bob], [charlie]]
predictions = model.predict_rank_probability(teams)
# => [[1, 0.504], [2, 0.333], [3, 0.163]]
# Format: [predicted_rank, probability]

Rating Display

The ordinal method provides a conservative rating estimate:

player = model.create_rating(mu: 30.0, sigma: 5.0)

# 99.7% confidence (3 standard deviations)
puts player.ordinal  # => 15.0 (30 - 3*5)

# 99% confidence
puts player.ordinal(z: 2.576)  # => 17.12

# For leaderboards
players.sort_by(&:ordinal).reverse

Model Options

model = OpenSkill::Models::PlackettLuce.new(
  mu: 25.0,           # Initial mean skill
  sigma: 25.0 / 3,    # Initial skill uncertainty
  beta: 25.0 / 6,     # Performance variance
  kappa: 0.0001,      # Minimum variance (regularization)
  tau: 25.0 / 300,    # Skill decay per match
  margin: 0.0,        # Score margin threshold
  limit_sigma: false, # Prevent sigma from increasing
  balance: false      # Emphasize rating outliers in teams
)

Advanced Features

Prevent Rating Uncertainty from Growing

# Useful for active players
updated = model.calculate_ratings(teams, limit_sigma: true)

Balance Outliers in Teams

model = OpenSkill::Models::PlackettLuce.new(balance: true)
# Gives more weight to rating differences within teams

Custom Tau (Skill Decay)

# Higher tau = more rating volatility
updated = model.calculate_ratings(teams, tau: 1.0)

How It Works

OpenSkill uses a Bayesian approach to model player skill as a normal distribution:

• μ (mu): The mean skill level
• σ (sigma): The uncertainty about the skill level

After each match:

1. Compute team strengths from individual player ratings
2. Calculate expected outcomes based on team strengths
3. Update ratings based on actual vs expected performance
4. Reduce uncertainty (sigma) as more matches are played

The ordinal value (μ - 3σ) provides a conservative estimate where the true skill is 99.7% likely to be higher.

Why OpenSkill?

vs Elo

• ✅ Handles multiplayer (3+ players/teams)
• ✅ Works with team games
• ✅ Accounts for rating uncertainty
• ✅ Faster convergence to true skill

vs TrueSkill

• ✅ Open source (MIT license)
• ✅ Faster computation
• ✅ Similar accuracy
• ✅ More flexible (weights, margins, custom parameters)

API Design Philosophy

This Ruby implementation uses idiomatic Ruby naming conventions:

Python API	Ruby API
model.rating()	model.create_rating
model.create_rating([25, 8.3])	model.load_rating([25, 8.3])
model.rate(teams)	model.calculate_ratings(teams)
model.predict_win(teams)	model.predict_win_probability(teams)
model.predict_draw(teams)	model.predict_draw_probability(teams)
model.predict_rank(teams)	model.predict_rank_probability(teams)

Examples

2v2 Team Game

model = OpenSkill::Models::PlackettLuce.new

# Create players
alice = model.create_rating(name: "Alice")
bob = model.create_rating(name: "Bob")
charlie = model.create_rating(name: "Charlie")
dave = model.create_rating(name: "Dave")

# Match: Alice + Bob vs Charlie + Dave (Team 1 wins)
teams = [[alice, bob], [charlie, dave]]
updated = model.calculate_ratings(teams)

# Updated ratings
updated[0].each { |p| puts "#{p.name}: #{p.ordinal.round(1)}" }
updated[1].each { |p| puts "#{p.name}: #{p.ordinal.round(1)}" }

Free-for-All (5 players)

players = 5.times.map { model.create_rating }

# Player 3 wins, 1 second, 4 third, 0 fourth, 2 fifth
updated = model.calculate_ratings(
  players.map { |p| [p] },
  ranks: [3, 1, 4, 0, 2]
)

Tracking Player Progress

class Player
  attr_accessor :name, :mu, :sigma

  def initialize(name, model)
    @name = name
    rating = model.create_rating
    @mu = rating.mu
    @sigma = rating.sigma
  end

  def to_rating(model)
    model.load_rating([@mu, @sigma], name: @name)
  end

  def update_from_rating!(rating)
    @mu = rating.mu
    @sigma = rating.sigma
  end

  def ordinal(z: 3.0)
    @mu - z * @sigma
  end
end

# Usage
model = OpenSkill::Models::PlackettLuce.new
alice = Player.new("Alice", model)
bob = Player.new("Bob", model)

# Play match
teams = [[alice.to_rating(model)], [bob.to_rating(model)]]
updated = model.calculate_ratings(teams)

# Update players
alice.update_from_rating!(updated[0][0])
bob.update_from_rating!(updated[1][0])

Testing

bundle install
bundle exec rake test

Development

This gem follows the OpenSkill specification and maintains compatibility with the Python reference implementation.

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

License

MIT License. See LICENSE for details.

References

• OpenSkill Python Implementation
• OpenSkill Documentation
• Original Paper: A Bayesian Approximation Method for Online Ranking by Ruby C. Weng and Chih-Jen Lin

Acknowledgments

This Ruby implementation is based on the excellent openskill.py Python library by Vivek Joshy.

The Plackett-Luce model implemented here is based on the work by Weng and Lin (2011), providing a faster and more accessible alternative to Microsoft's TrueSkill system.