tic_tac_toe.rb

# move positions
#
#  0 | 1 | 2
# ---+---+---
#  3 | 4 | 5
# ---+---+---
#  6 | 7 | 8

class TicTacToe
  module Patterns
    Winning = 
      [[/ OO....../,0],[/O..O.. ../,6],
       [/......OO /,8],[/.. ..O..O/,2],
       [/ ..O..O../,0],[/...... OO/,6],
       [/..O..O.. /,8],[/OO ....../,2],
       [/ ...O...O/,0],[/..O.O. ../,6],
       [/O...O... /,8],[/.. .O.O../,2],
       [/O O....../,1],[/O.. ..O../,3],
       [/......O O/,7],[/..O.. ..O/,5],
       [/. ..O..O./,1],[/... OO.../,3],
       [/.O..O.. ./,7],[/...OO .../,5]]
    Blocking = 
      [[/  X . X  /,1],[/ XX....../,0],[/X..X.. ../,6],
       [/......XX /,8],[/.. ..X..X/,2],[/ ..X..X../,0],
       [/...... XX/,6],[/..X..X.. /,8],[/XX ....../,2],
       [/ ...X...X/,0],[/..X.X. ../,6],[/X...X... /,8],
       [/.. .X.X../,2],[/X X....../,1],[/X.. ..X../,3],
       [/......X X/,7],[/..X.. ..X/,5],[/. ..X..X./,1],
       [/... XX.../,3],[/.X..X.. ./,7],[/...XX .../,5],
       [/ X X.. ../,0],[/ ..X.. X /,6],[/.. ..X X /,8],
       [/ X ..X.. /,2],[/  XX.. ../,0],[/X.. .. X /,6],
       [/.. .XX   /,8],[/X  ..X.. /,2],[/ X  ..X../,0],
       [/ ..X..  X/,6],[/..X..  X /,8],[/X  ..X.. /,2]]
    Won = 
      [[/OOO....../,:O], [/...OOO.../,:O],
       [/......OOO/,:O], [/O..O..O../,:O],
       [/.O..O..O./,:O], [/..O..O..O/,:O],
       [/O...O...O/,:O], [/..O.O.O../,:O],
       [/XXX....../,:X], [/...XXX.../,:X],
       [/......XXX/,:X], [/X..X..X../,:X],
       [/.X..X..X./,:X], [/..X..X..X/,:X],
       [/X...X...X/,:X], [/..X.X.X../,:X]] 
  end
  
  def initialize
    @board = [].fill(0, 9) { " " }
    @players = { :X => 'X', :O => 'O' }
    @turn = :X
  end
  
  def play
    winner_flag = false
    9.times do
      if @turn === :X
        display
        player_move
      else
        cpu_move
      end				
      winner = someone_win?
      unless winner.nil?
        display
        print "\n#{winner} is the winner!\n"
        winner_flag = true
        break
      end
      @turn = (@turn === :X) ? :O : :X
    end
    if (!winner_flag)
      display
      print "\nGame is a draw.\n"
    end	
  end
  
  private
  def player_move
    print "Enter your move [0-8]: "
    move_pos = gets.chomp.to_i
    unless (0..8) === move_pos
      print "\nInvalid move: #{move_pos}. Please re-enter.\n"
      player_move
      return
    end
    if @board.at(move_pos) != ' '
      print "\nSpace is already occupied. Please re-enter.\n"
      player_move
      return
    end
    move move_pos, 'X'
  end
  
  def cpu_move
    move_pos = get_winning_pattern_move
    if move_pos.nil?
      move_pos = get_blocking_pattern_move
      if move_pos.nil?
        move_pos = get_first_available_move
      end
    end
    move move_pos, 'O'
  end
  
  def move(pos, piece)
    @board.delete_at(pos)
    @board.insert(pos, piece)
  end
  
  def display
    print "\n\n"
    print " #{@board[0]} |"
    print " #{@board[1]} |"
    print " #{@board[2]}"		
    print "\n---+---+---\n"
    print " #{@board[3]} |"
    print " #{@board[4]} |"
    print " #{@board[5]}"
    print "\n---+---+---\n"
    print " #{@board[6]} |"
    print " #{@board[7]} |"
    print " #{@board[8]}"
    print "\n\n"
  end
  
  def someone_win?
    symbol = nil
    array = Patterns::Won.find { |p| p.first =~ @board.join }
    unless array.nil?
      symbol = (array.last === :X) ? 'X' : 'O'
    end
    symbol
  end		
  
  def get_winning_pattern_move
    move_pos = nil
    array = Patterns::Winning.find { |p| p.first =~ @board.join }
    unless array.nil?
      move_pos = array.last
    end
    move_pos
  end
  
  def get_blocking_pattern_move
    move_pos = nil
    array = Patterns::Blocking.find { |p| p.first =~ @board.join }
    unless array.nil?
      move_pos = array.last
    end
    move_pos
  end
  
  def get_first_available_move
    if @board.at(4) == ' '
      move_pos = 4
    else
      move_pos = @board.index(' ')
    end
    move_pos
  end
end

if $0 == __FILE__
  print "\n\nYou are X.  Please go first."
  TicTacToe.new.play
end