mastermind.rb

class Mind
  def initialize
    @random = Random.new
    generate_code
  end

  def play
    puts "Can you guess the code?\n\n"

    10.times do
      print 'Enter your guess: '
      guess = gets.chomp

      if guess.match(/exit|quit/)
        puts 'Oh, okay then...'
        return
      end

      while !guess.match(/^[1-6]{4}$/)
        puts "Your guess should be 4 digits between 1 and 6. Try again."
        guess = gets.chomp
      end

      if guess == @code
        puts "\nYou win!"
        return
      else
        response = guess_code(guess)
        puts response
      end
    end

    puts "\nYou lose!"
    puts "The code was #{@code}."
  end

  private
  def generate_code
    @code = ''
    @code_digits = Array.new(7)

    7.times do |i|
      @code_digits[i] = Hash.new(0)
    end

    4.times do
      digit = @random.rand(1..6)
      @code_digits[digit][:count] += 1
      @code << digit.to_s
    end
  end

  def guess_code(guess)
    correct = misplaced = 0

    guess.split('').each_with_index do |digit, i|
      if @code[i] == digit
        @code_digits[digit.to_i][:correct] += 1
      elsif @code.match(digit)
        @code_digits[digit.to_i][:present] += 1
      end
    end

    @code_digits.each do |h|
      correct += h[:correct]
      misplaced += [h[:count] - h[:correct], h[:present]].min
      h[:correct] = h[:present] = 0
    end

    "#{'o' * correct + 'x' * misplaced}".ljust(4, '-')
  end
end

puts "You have 10 guesses.\nThe code is 4 digits between 1 and 6.\n" +
  "o: A digit matches  x: A digit matches, but is in the wrong place)\n\n"

while true
  mastermind = Mind.new
  mastermind.play
  puts "Press enter to play again.\n"
  gets
end

Try it out here!

Sample play:

You have 10 guesses.
The code is 4 digits between 1 and 6.
o: A digit matches  x: A digit matches, but is in the wrong place)

Can you guess the code?

Enter your guess: 1111
o---
Enter your guess: 1222
x---
Enter your guess: 3133
ox--
Enter your guess: 4314
xx--
Enter your guess: 5531
ox--
Enter your guess: 6613
oxxx
Enter your guess: 6136
xxxx
Enter your guess: 1366
oxxx
Enter your guess: 1663
ooxx
Enter your guess: 3661

You win!
Press enter to play again.

@mattantonelli, dude your code looks great.

@TrevorS Thanks, dude!

The only thing I'm really unhappy with is this. I would really like to find a cleaner way of clearing out the guess each round. @code_digits.dup didn't seem to work like I wanted.