A modular, deterministic chess engine built around immutable objects. Cleanly expresses chess concepts in code and designed for easy integration with any UI.

⚠️ Note: This is not a competitive chess engine like Stockfish. While AI features could be added in the future, the core purpose of this project is to provide a ruby gem for cleanly representing chess in code.

• UI-agnostic design
• Fully immutable game state representation
• Modular: components can be used separately or coordinated via the Engine class
• Chess concepts map cleanly to code: squares, pieces, board, rules, etc
• Pluggable notation systems for both parsing and formatting: a custom notation called ERAN is the default, but parsers & formatters for any other notation system(SAN, LAN, etc) can be implemented and plugged in instead
• FEN import and export

require 'chess_engine_rb'

engine = ChessEngine::Engine.new
engine.new_game
engine.play_turn('P e2-e4') # play a move
puts engine.status.board # prints the board
puts engine.to_fen # prints FEN
engine.from_fen('rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1') # Load from FEN

require 'chess_engine_rb'

class Listener
  FORMATTER = ChessEngine::Formatters::ERANShortFormatter

  def on_game_update(update)
    if update.failure?
      puts "Update failed: #{update.error}"
    elsif update.game_ended?
      puts "Game over! Reason: #{update.endgame_status.cause}"
    elsif update.offered_draw
      puts 'Draw has been offered'
    else
      puts "Move accepted: #{FORMATTER.call(update.event)}"
    end
  end
end

engine = ChessEngine::Engine.new
engine.add_listener(Listener.new)
engine.play_turn('P e7-e6') # => Move accepted: P e7-e6
engine.play_turn('HA!') # => Update failed: invalid_notation
engine.offer_draw # => Draw has been offered
engine.resign # => Game over! Reason: resignation

For more complete examples, see the examples folder.

• Install Ruby (version >= 3.4.1) if you haven't already (on Windows, you can use RubyInstaller)
• Install the gem: gem install chess_engine_rb
• If you want to see the engine in action, download the example CLI examples/chess_cli.rb and run it: ruby chess_cli.rb

• Make sure Ruby (version >= 3.4.1) and Bundler are installed
• Clone the repository and run bundle install from the project's root directory

• See the architectural overview
• Browse the docs and examples for more information

• Move undo
• SAN and LAN parsers/formatters
• PGN import/export
• Support for chess variants
• Performance improvements
• Comprehensive perft testing
• Basic AI

This project is licensed under the MIT License.