AoC day 4

import scala.io.Source

object day4 extends App {

  def checksum(s: String) = s.reverse.slice(1, 6).reverse
  def sectorId(s: String): Int = s.reverse.slice(7, 10).reverse.toInt
  def name(s: String) = (s take s.length - 11) filterNot (_ == '-')
  def occurrences(s: String): List[Char] = s.groupBy(identity)
    .map(x => (x._1, x._2.length)).toList
    .sortWith((a, b) => if (a._2 == b._2) a._1 < b._1 else a._2 > b._2) map (_._1)
  def generateChecksum(s: String): String = {
    (occurrences(name(s)) take 5).mkString
  }
  def isValid(s: String): Boolean = checksum(s) == generateChecksum(s)

  def decrypt(name: String, sectorId: Int): String = {
    val alphabet = "abcdefghijklmnopqrstuvwxyz"
    def rotateLeft(seq: Seq[Char], i: Int): Seq[Char] = {
        val (first, last) = seq.splitAt(i % seq.size)
        last ++ first
    }
    name.map(c => rotateLeft(alphabet, sectorId)(alphabet.indexOf(c)))
  }

  val codes = Source.fromFile("day4.txt").getLines.toList
  val sectorIds = codes filter isValid map sectorId
  println(sectorIds.foldLeft(0)(_ + _))
  val nr = codes.find(c => decrypt(name(c), sectorId(c)) startsWith("northpole"))
  println(sectorId(nr.get))

}