Source code in the scala programming language

import scala.annotation.tailrec

object LookAndSay extends App {

  loop(10, "1")

  @tailrec
  private def loop(n: Int, num: String): Unit = {
    println(num)
    if (n <= 0) () else loop(n - 1, lookandsay(num))
  }

  private def lookandsay(number: String): String = {
    val result = new StringBuilder

    @tailrec
    def loop(numberString: String, repeat: Char, times: Int): String =
      if (numberString.isEmpty) result.toString()
      else if (numberString.head != repeat) {
        result.append(times).append(repeat)
        loop(numberString.tail, numberString.head, 1)
      } else loop(numberString.tail, numberString.head, times + 1)

    loop(number.tail + " ", number.head, 1)
  }

}

def lookAndSay(seed: BigInt) = {
  val s = seed.toString
  ( 1 until s.size).foldLeft((1, s(0), new StringBuilder)) {
    case ((len, c, sb), index) if c != s(index) => sb.append(len); sb.append(c); (1, s(index), sb)
    case ((len, c, sb), _) => (len + 1, c, sb)
  } match {
    case (len, c, sb) => sb.append(len); sb.append(c); BigInt(sb.toString)
  }
}

def lookAndSayIterator(seed: BigInt) = Iterator.iterate(seed)(lookAndSay)

object Main extends App {

  def lookAndSay(previous: List[BigInt]): Stream[List[BigInt]] = {

    def next(num: List[BigInt]): List[BigInt] = num match {
      case Nil => Nil
      case head :: Nil => 1 :: head :: Nil
      case head :: tail =>
        val size = (num takeWhile (_ == head)).size
        List(BigInt(size), head) ::: next(num.drop(size))
    }
    val x = next(previous)
    x #:: lookAndSay(x)
  }

  (lookAndSay(1 :: Nil) take 10).foreach(s => println(s.mkString("")))
}