benjaminBoboul/workbench-getting-started-with-scala

package generator

object stringCombination {
  def generate(size_span: Iterable[Int]): Iterable[String] = {
    val chars = 'a' to 'd'
    size_span
      .flatMap(
        x => List.fill(x)(chars)
        .flatten
        .combinations(x)
        .flatMap(_.permutations)
        .map(_.mkString(""))
      )
  }
}