cheryl.rb

DATES = [
          "May 15",    "May 16",    "May 19",
          "June 17",   "June 18",
          "July 14",   "July 16",
          "August 14", "August 15", "August 17"
        ]

def month(date); date.split[0]; end
def day(date);   date.split[1]; end

def tell(part, possible_dates=DATES)
  possible_dates.select{|d| d.include? part}
end

def know(possible_dates)
  possible_dates.length == 1
end

def albert_doesnt_know_but_knows_bernard_doesnt_know_either(date)
  possible_dates = tell(month(date))
  !know(possible_dates) && possible_dates.none?{|d| know(tell(day(d)))}
end

def at_first_bernard_doesnt_know_but_knows_now(date)
  at_first = tell(day(date))
  !know(at_first) && know(at_first.select{|d| albert_doesnt_know_but_knows_bernard_doesnt_know_either(d)})
end

def then_albert_also_knows(date)
  know(tell(month(date)).select{|d| at_first_bernard_doesnt_know_but_knows_now(d)})
end

def cheryls_birthday(possible_dates=DATES)
  possible_dates.select do |d|
    albert_doesnt_know_but_knows_bernard_doesnt_know_either(d) &&
    at_first_bernard_doesnt_know_but_knows_now(d) &&
    then_albert_also_knows(d)
  end
end

puts cheryls_birthday # => July 16