Tic Tac Toe in Ruby

game.rb

class Game
  WINNING_PATTERNS = [
    [0, 1, 2],
    [3, 4, 5],
    [6, 7, 8],
    [0, 3, 6],
    [1, 4, 7],
    [2, 5, 8],
    [0, 4, 8],
    [2, 4, 6]
  ]
  private_constant :WINNING_PATTERNS

  def next_turn(&block)
    @next_turn_callback = block
  end

  def win(&block)
    @win_callback = block
  end

  def draw(&block)
    @draw_callback = block
  end

  def play
    board = Array.new(9)
    [:player1, :player2].cycle do |player|
      selection = @next_turn_callback.call(board, player)
      # This failure should be guarded against in @next_turn_callback
      fail if selection > 8 || board[selection]
      board[selection] = player
      if winner?(board)
        @win_callback.call(board, player)
        break
      elsif board.none?(&:nil?)
        @draw_callback.call(board)
        break
      end
    end
  end

  private

  def winner?(board)
    WINNING_PATTERNS.any? do |pattern|
      values = pattern.map { |i| board[i] }
      values.none?(&:nil?) && values.uniq.length == 1
    end
  end
end

main.rb

require_relative 'game'

PLAYER_NAMES = {
  player1: "Player 1",
  player2: "Player 2"
}

def print_board(board)
  board.each_slice(3) do |row|
    row.each do |cell|
      print \
        case cell
        when :player1 then 'O'
        when :player2 then 'X'
        when nil then '_'
        end
      print ' '
    end
    puts
  end
end

puts "Tic Tac Toe"
game = Game.new

game.next_turn do |board, next_player|
  print_board(board)
  print "#{PLAYER_NAMES[next_player]}, pick a cell: "
  # TODO guard against invalid selections
  Integer(gets) - 1
end

game.win do |board, winner|
  print_board(board)
  puts "#{PLAYER_NAMES[winner]} wins!"
end

game.draw do |board|
  print_board(board)
  puts 'Draw!'
end

game.play