A command-line mastermind game

Mastermind.rb

class Mastermind
  COLOURS = 'A'.upto('Z').to_a

  def initialize(colour_count = 6, size = 4)
    fail "Size must be <= #{COLOURS.size}" unless size <= COLOURS.size
    @size = size
    @colours = COLOURS.first(colour_count)
    @code = size.times.map { @colours.sample }
    @moves = 0
    puts "Guess the code, size #{@size}, letters are (#{@colours.join(', ')})"
  end

  def check(input)
    chars = input.chars.select { |c| @colours.include?(c) }
    unless chars.size == @size
      puts "Input must be #{@size} letters in (#{@colours.join(', ')})"
      return
    end
    @moves += 1

    rest_code = []
    rest_input = []
    exact = 0
    input.chars.each_with_index do |c, i|
      if c == @code[i]
        exact += 1
      else
        rest_code << @code[i]
        rest_input << c
      end
    end

    if exact == @size
      puts "\tCongratz, you beat the game in #{@moves} tries!"
      exit 0
    end

    misplaced = (rest_code & rest_input).size
    puts "\t#{exact} in the right place, #{misplaced} in the wrong place\n"
  end
end

size, colours = ARGV[0], ARGV[1]
if ARGV.size != 2
  puts <<~HELP
    Usage: ruby mastermind.rb <size> <alphabet_size>

      size: the length of the code you want to guess
      alphabet_size: number of letters in your alphabet (max #{Mastermind::COLOURS.size})
  HELP
  exit 0
end

m = Mastermind.new(colours.to_i, size.to_i)

while true
  m.check($stdin.gets.strip)
end