gistfile1.txt

Takes ages:

object Day10 extends App {
  def input = "3113322113"

  def charGroups(s: String): List[(Char, Int)] = s.foldLeft(List[(Char,Int)]())((acc, c) => acc match {
    case Nil => List((c,1))
    case v @ ((char, counted)) :: rest  =>
      if (char == c) (char, counted + 1) :: rest
      else (c, 1) :: v
  }).reverse

  def groupToString(groups: List[(Char,Int)]): String = {
    groups.foldLeft("")((acc,g) => g match {
      case (char,count) => acc + count + char
    })
  }

  def lookAndSay = charGroups _ andThen groupToString

  val part1 = Function.chain(List.fill(40)(lookAndSay))(input)
  println(part1.length)
  val part2 = Function.chain(List.fill(10)(lookAndSay))(part1)
  println(part2.length)
}

Terminates quickly:

(ns advent.day10)

(defn count-encode [x]
  (apply str
         (mapcat 
          (juxt count first)
          (partition-by identity x))))

;; part 1 and part 2
#_(count (nth (iterate count-encode "1321131112") #_40 50))