def print_dates(dates, answer_text)
  puts "\nAnswers left:" if answer_text
  puts dates.join("; ")
  print "\n"
end

class Day
  attr_accessor :month, :day
  def initialize(month, day)
    @day = day;
    @month = month;
  end

  def to_s
    "#{@month.to_s.upcase}, #{@day}"
  end
end

possible_dates = [
  Day.new(:may, 15), Day.new(:may, 16), Day.new(:may, 19),
  Day.new(:june, 17), Day.new(:june, 18),
  Day.new(:july, 14), Day.new(:july, 16),
  Day.new(:august, 14), Day.new(:august, 15), Day.new(:august, 17)
]

puts "Possible dates are"
print_dates(possible_dates, false)

puts "Albert: “I don’t know when your birthday is, but I know Bernard doesn’t know, either.”"
# Albert doesn't know, so we take out the months that appear only once in the possible dates.
answer = possible_dates - possible_dates.select{|a| possible_dates.map(&:month).count(a.month) == 1}

# Albert knows Bernard doesn't know, so we take out the all answers that have
# the same month of the answers that their day appears only once.
answer -= answer.select{|a| answer.select{|b| answer.count{|c| c.day == b.day} == 1}.map(&:month).include?(a.month) }

print_dates(answer, true)

puts "Bernard: “I didn’t know originally, but now I do.”"
# Bernard knows the answer, because the day is unique in the answers left. Remove repeated days.
answer -= answer.select{|a| answer.map(&:day).count(a.day) > 1 }

print_dates(answer, true)

puts "Albert: “Well, now I know, too!”"
# Albert knows cause he knows the month, so it must be a month that is not repeated in the answers.
answer -= answer.select{|a| answer.map(&:month).count(a.month) > 1 }

print_dates(answer, true)