Cheryl's Birthday in Ruby

cheryl.rb

# http://nbviewer.ipython.org/url/norvig.com/ipython/Cheryl.ipynb

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)
  # When Cheryl tells part of her birthday, return a new list of possible dates.
  possible_dates.select { |date| date if date.include? part }
end

def know(possible_dates)
  # If there is only one possible date left, we know Cheryl's birthday.
  possible_dates.length == 1
end

def cheryls_birthday(possible_dates=DATES)
  # Return a list of the possible dates for which statements 3 to 5 are true.
  possible_dates.select { |date| date if statements3to5(date) }
end

def statements3to5(date)
  statement3(date) && statement4(date) && statement5(date)
end

def statement3(date)
  # Albert: I don't know when Cheryl's birthday is,
  possible_dates = tell(month(date))
  clause1 = (not know(possible_dates))

  # but I know that Bernard does not know too.
  clause2 = possible_dates.all? { |d| not know(tell(day(d)))}

  clause1 && clause2
end

def statement4(date)
  # At first, Bernard doesn't know.
  at_first = tell(day(date))
  clause1 = (not know(at_first))

  # But after just considering dates for which statement 3 is true, he knows.
  clause2 = know(at_first.select { |d| statement3(d) } )

  # check: DATES.select { |d| statement3(d) }.select{ |d| statement4(d) }
  clause1 && clause2
end

def statement5(date)
  # After hearing the month and statement 4, Albert knows
  know(tell(month(date)).select { |d| statement4(d) })
end