private def precisionAtK(k: Int, actual: Array[Int], predicted: Array[Int]): Double = {
    val pred = predicted.take(k)
    val a = actual.length
    actual.intersect(pred).length / a : Double
  }

  def averagePrecisionAtK(k: Int) = {
    new Score[Array[Int]] with PerformanceScore {
      //TODO
      override def evaluate(data: DataSet[(Array[Int], Array[Int])]): DataSet[Double] = {
        val perUser = data.map { trueAndPred => precisionAtK(k, trueAndPred._1, trueAndPred._2)
          val pred = trueAndPred._2.take(k)
          val a = trueAndPred._1.length
          trueAndPred._1.intersect(pred).length / a : Double}
        }
          //val perUser = data.map(trueAndPred => 2)
        perUser.mean()
      }
    }
  }