Interactive tic tac toe game

tictactoe.rb

class TicTacToeApp
     
    NIL_GAME = [
        [nil, nil, nil], 
        [nil, nil, nil],
        [nil, nil, nil]
      ]
    
    def initialize
      @game = NIL_GAME
      @turn = 'X'
      @turn_count = 0
    end
    
    def run
        done = false

        while ! done do
            print_game
            print "#{@turn}'s turn. What position would you like to occupy: "
            pos = gets
            
            if quit?(pos)
              done = true
              next
            elsif !valid?(pos)
              puts "INVALID MOVE! Try again..."
              next
            end
            
            occupy(pos)
            
            done = true if game_over?(pos)
            if someone_won?
              puts "We have a winner: #{@turn}!!!"
              print_game
            elsif game_over?(pos)
              puts "CATS!!!!!!!!"
              print_game
            end
            next_turn
        end 
    end
    
    def occupy(pos)
      a, b = pos.split(',').map(&:to_i)
      
      board_pos = @game[a][b]
      if board_pos.nil?
          @game[a][b] = @turn
      else
        puts "SPACE ALREADY TAKEN! Try again..."
      end
    end
    
    def valid?(pos)
      pos =~ /(quit|[0-2],[0-2])/i
    end
    
    def game_over?(pos)
       @turn_count > 7 || someone_won?
    end
    
    def someone_won?
       horizontal_winner? ||
       vertical_winner? ||
       diagonal_winner?
    end
    
    def horizontal_winner?
        @game.map {|row| 
            row.all? {|val| 
                val == @turn 
            } 
        }.any? { |x| x }
    end
    
    def vertical_winner?
        winning_positions = [
            [0,3,6],
            [1,4,7],
            [2,5,8]
        ]
        winning_positions.any? do |wp|
            chars = []
            wp.each do |pos|
                chars << @game[pos]
            end
            chars.uniq.size == 1
        end
    end
    
    def diagonal_winner?
       (@turn == @game[0][0] && @turn == @game[1][1] && @turn == @game[2][2]) ||
       (@turn == @game[2][0] && @turn == @game[1][1] && @turn == @game[0][2])
    end
    
    def quit?(pos)
      pos =~ /quit/i
    end
    
    def next_turn
      @turn = (@turn == 'X' ? 'O' : 'X')
      @turn_count += 1
    end
 
    def print_game
      @game.each_with_index do |row, i|
        puts row.map { |x| x.nil? ? ' ' : x }.join(" | ")
        puts '-' * 9 unless i == 2
      end
      return
    end
end

game = TicTacToeApp.new
game.run

This code is a result of the first experiment in Chaos Driven Development (CDD) with @tehcurtis @scally @skscally and @jyunderwood