clusterBy

clusterBy.scala

import scala.annotation._

val l = 0 until 30 toList

def div(i:Int) = i / 2

def cluster[T,U](xs:Seq[T], f:T => U): Seq[(U, Seq[T])] = {
  @tailrec 
  def tr_cluster(acc: Seq[(U, Seq[T])], xs:Seq[T]): Seq[(U, Seq[T])] = {
    if (xs.isEmpty) {
      acc 
    } else {
      val u = f(xs.head)
      xs.span(x => f(x) == u) match {
        case (h, t) => tr_cluster(acc :+ (u ->h ), t)
      }
    }
  }
  tr_cluster(Nil, xs)
}

val res = cluster(l, div)