Analyse the frequency of repeated chars in a given string

ATGC.scala

import scala.util.Random
import scala.collection.immutable.SortedMap

object ATGC extends App {
  def randomString(size: Int) = List.fill(size)(randomChar).mkString
  def randomChar = "ATGC"(Random.nextInt(4))

  def analyze(s: String): Map[String, Int] = {
    val (_, _, res) = (s + "Z").tail.foldLeft((s.head, 1, SortedMap.empty[String, Int] withDefaultValue 0)) {
      case ((last, count, map), current) if current == last => (last, count + 1, map)
      case ((last, count, map), current) if current != last =>
        val key = last.toString * count
        val value = map(key) + 1
        (current, 1, map.updated(key, value))
    }
    res
  }

  val s = randomString(10)

  println(s)
  println(analyze(s))
}