def avgTime(message: String, f:  => Any) {
      var avg = 0L
      val c = 42
      1 to c foreach {
        _ =>
          val t0 = System.nanoTime()
          f
          val t1 = System.nanoTime()
          avg += t1 - t0
      }
      println(message + " avg time is " + (avg/c/1000000) + " ms")
    }

    def sumByKeys[A](tuples: List[(A, Long)]) = {
      tuples.groupBy(_._1).mapValues(_.map(_._2).sum).toList
    }

    import annotation.tailrec

    @tailrec
    final def tailSum[A](tuples: List[(A, Long)], acc: Map[A, Long] = Map.empty[A, Long]): List[(A, Long)] = tuples match {
      case (k, v) :: tail => tailSum(tail, acc + (k -> (v + acc.get(k).getOrElse(0L))))
      case Nil => acc.toList
    }

    def foldLeftSum[A](tuples: List[(A, Long)]) = tuples.foldLeft(Map.empty[A, Long])({
      case (acc, (k, v)) => acc + (k -> (v + acc.get(k).getOrElse(0L)))
    }).toList

    def mutableSum[A](tuples: List[(A, Long)])  = {
      val m = scala.collection.mutable.Map.empty[A, Long].withDefault(_ => 0L)
      for ((k, v) <- tuples) m += (k -> (v + m(k)))
      m.toList
    }

    val tuples = (0L to 242000L).map(l => l % 10 -> l).toList

    avgTime("default", sumByKeys(tuples))
    avgTime("tailrec", tailSum(tuples))
    avgTime("foldleft", foldLeftSum(tuples))
    avgTime("mutableSum", mutableSum(tuples))