Bulls and Cows

1_bulls_cows.rb

answer = 0.upto(9).to_a.sample(4).join

while true
  puts 'wrong format, should be 4 digits.' until (guess = gets.chop!) =~ /^\d{4}$/

  a = b = 0
  guess.each_char.each_with_index do |c1, idx1|
    answer.each_char.each_with_index do |c2, idx2|
      if c1 == c2
        idx1 == idx2 ? a += 1 : b += 1
        break
      end
    end
  end
  if a == 4
    puts 'Bingo!'
    break
  else
    puts "#{a}a#{b}b"
  end
end

2_bulls_cows.rb

answers = '0123456789'.chars.permutation(4).to_a.map!(&:join)

while answers.length > 1
  answer = answers.sample
  puts %Q{Is it #{answer}?}

  puts 'wrong format, soubld be "{number}a{number}b"' until gets.chop! =~ /^(\d)[Aa](\d)[Bb]$/

  answers.select! do |possible|
    a = b = 0
    possible.each_char.each_with_index do |c1, idx1|
      answer.each_char.each_with_index do |c2, idx2|
        if c1 == c2
          idx1 == idx2 ? a += 1 : b += 1
          break
        end
      end
    end
    $1.to_i == a && $2.to_i == b
  end

  if answers.length == 1
    puts "The answer is #{answers.first}!"
  elsif answers.empty?
    puts 'You cheat!'
  end
end