"Cheryl's Birthday" solution in Scala, based on the solution from Peter Norvig at http://nbviewer.ipython.org/url/norvig.com/ipython/Cheryl.ipynb

CherylsBirthday.scala

package com.douglasjose.tech

/**
 * "Cheryl's Birthday" solution in Scala, based on the solution from Peter Norvig at
 *  http://nbviewer.ipython.org/url/norvig.com/ipython/Cheryl.ipynb
 *
 * @author Douglas José (@douglasjose)
 */
class CherylsBirthday {

  type Date = (Symbol, Int)

  type Part = Either[Symbol, Int]

  val dates:List[Date] = List(
    ('may, 15), ('may, 16), ('may, 19),
    ('jun, 17), ('jun, 18),
    ('jul, 14), ('jul, 16),
    ('aug, 14), ('aug, 15), ('aug, 17)
  )

  def month(d: Date):Part = Left(d._1)
  def day(d: Date):Part = Right(d._2)

  def tell(part: Part, possibleDates: List[Date] = dates) =
    possibleDates.filter {d => part match {
      case Left(month) => d._1 == month
      case Right(day) => d._2 == day
      }
    }

  def know(possibleDates: List[Date]) = possibleDates.length == 1

  // Albert: I don't know when Cheryl's birthday is, but I know that Bernard does not know too.
  def statement3(date: Date) = {
    val possibleDates = tell(month(date))
    !know(possibleDates) && possibleDates.forall(d => !know(tell(day(d))))
  }

  // Bernard: At first I don't know when Cheryl's birthday is, but I know now.
  def statement4(date: Date) = {
    val atFirst = tell(day(date))
    !know(atFirst) && know(atFirst.filter(statement3))
  }

  // Albert: Then I also know when Cheryl's birthday is.
  def statement5(date: Date) = know(tell(month(date)).filter(statement4))

  def statements3to5(date: Date) = statement3(date) && statement4(date) && statement5(date)

  def cherylsBirthday(possibleDates: List[Date] = dates) = possibleDates.filter(statements3to5)

}