import scala.io.Source

val input = Source.fromFile("C:/work/advent06/input4").getLines.toList

case class Room(name: String, id: Int, checksum: String)

val roomRegex = """((?:[a-z]+-)+)(\d+)\[([a-z]{5})\]""".r

val rooms = input.map {
  case roomRegex(name, id, checksum) => Room(name, id.toInt, checksum)
}

def isReal(room: Room) = {
  val nameChars = room.name.toList.filter(_.isLower)
  val charsByFrequency = nameChars
    .groupBy(identity)
    .mapValues(_.size)
    .toList
    .sortBy(_._1).sortBy(-_._2)
  val checksum = charsByFrequency.take(5).map(_._1).mkString
  checksum == room.checksum
}

val realRooms = rooms.filter(isReal)

val sumOfRealRoomIds = realRooms.map(_.id).sum

def decrypt(room: Room) = {
  val shift = room.id % 26

  def decryptChar(c: Char) = {
    val afterShift = (c + shift).toChar
    if (afterShift > 'z') (afterShift - 26).toChar else afterShift
  }

  room.copy(name = room.name.toList.map(decryptChar).mkString)
}

val decryptedRooms = realRooms.map(decrypt)

val targetRoom = decryptedRooms.find(_.name.startsWith("north"))