Github user smurching commented on a diff in the pull request:

    https://github.com/apache/spark/pull/19666#discussion_r149237212
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/ml/tree/impl/RandomForest.scala ---
    @@ -741,17 +678,43 @@ private[spark] object RandomForest extends Logging {
               (splits(featureIndex)(bestFeatureSplitIndex), 
bestFeatureGainStats)
             } else if (binAggregates.metadata.isUnordered(featureIndex)) {
               // Unordered categorical feature
    -          val leftChildOffset = 
binAggregates.getFeatureOffset(featureIndexIdx)
    -          val (bestFeatureSplitIndex, bestFeatureGainStats) =
    -            Range(0, numSplits).map { splitIndex =>
    -              val leftChildStats = 
binAggregates.getImpurityCalculator(leftChildOffset, splitIndex)
    -              val rightChildStats = 
binAggregates.getParentImpurityCalculator()
    -                .subtract(leftChildStats)
    +          val numBins = binAggregates.metadata.numBins(featureIndex)
    +          val featureOffset = 
binAggregates.getFeatureOffset(featureIndexIdx)
    +
    +          val binStatsArray = Array.tabulate(numBins) { binIndex =>
    --- End diff --
    
    Could you please add a comment explaining what this is? E.g.:
    `// Each element of binStatsArray stores pre-computed label statistics for 
a single bin of the current future`


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