correction of ndcg

ndcg

 def ndcgAt(k: Int): Double = {
    require(k > 0, "ranking position k should be positive")
    predictionAndLabels.map { case (pred, lab) =>
      val labSet = lab.toSet

      if (labSet.nonEmpty) {
        val labSetSize = labSet.size
        val predSize = pred.size
        val n = math.min(math.max(pred.length, labSetSize), k)
        var maxDcg = 0.0
        var dcg = 0.0
        var i = 0
        while (i < n) {
          val gain = 1.0 / math.log(i + 2)
          if (i < predSize) {
            if (labSet.contains(pred(i))) {
              dcg += gain
            }
          }
          if (i < labSetSize) {
            maxDcg += gain
          }
          i += 1
        }
        dcg / maxDcg
      } else {
        logWarning("Empty ground truth set, check input data")
        0.0
      }
    }.mean
  }