def optimal_move(board)
  # Needs to return a space that will return an optimal move using minimax
  player = current_player(board)
  possible_moves = open_squares(board)
  best_move = nil
  best_score = nil
  possible_moves.each do |square_num|
    new_board = board.slice(0..-1)
    new_board[square_num - 1] = player
    score = minimax(new_board)
    if best_score.nil? || score > best_score
      best_score = score
      best_move = square_num
    end
  end
  best_move
end