mastermind.rb

class Mastermind
  attr_reader :answer

  def initialize(answer)
    @answer = answer
  end

  def match(picks)
    clues = []
    picks.each_with_index do |pick, p_index|
      if pick == answer[p_index]
        clues[p_index] = :exact
      elsif pos = (0...answer.count).detect do |a_index|
                    (pick == answer[a_index]) && # correct color
                    !clues[a_index]  # answer index not used yet
                  end
        clues[pos] = :right_color
      end
    end
    clues.compact.sort_by(&:to_s)
  end
end

# tests

class Object
  def should(value)
    puts(self == value ? true : "#{self.inspect} is not #{value.inspect}")
  end
end

m = Mastermind.new \
        [:b, :a, :f, :a]

m.match([:a, :d, :e, :a]).should [:exact, :right_color]
m.match([:b, :b, :a, :b]).should [:exact, :right_color]
m.match([:c, :b, :f, :a]).should [:exact, :exact, :right_color]
m.match([:a, :b, :a, :e]).should [:right_color, :right_color, :right_color]
m.match([:c, :a, :a, :e]).should [:exact, :right_color]
m.match([:f, :f, :e, :e]).should [:right_color]