e10_direct_answer.rb

def calculate(actions)
  if actions.all?{ |c| c == 'O'}
    return 2 ** (actions.count + 2) - 4
  elsif actions.all?{ |c| c == 'X'}
    if actions.count == 1
      return 5
    else
      return 4 ** (actions.count - 1) + 1
    end
  else
    num_of_xs = actions.select{ |c| c == 'X'}.size
    num_of_ys = actions.size - num_of_xs
    if actions.last == 'X'
      if num_of_xs >= 2
        4 ** (num_of_xs - 1) * 5 ** num_of_ys + 1
      else
        5 ** num_of_ys + 5
      end
    else
      # last is 'O'
      last, *rest = actions.reverse
      calculate(rest.reverse) + 4 * (2 ** (num_of_ys - 1))
    end
  end
end

def test(actions_str, expected)
  actions = actions_str.split("")
  result = calculate(actions)
  if result != expected.to_i
    puts "#{actions_str} : #{result} vs #{expected}"
  end
end

File.readlines("input_samples").map{ |x| x.chomp.split(",") }.each do |x|
  test(x[0], x[1])
end