Github user sethah commented on the issue:
https://github.com/apache/spark/pull/18832
I don't agree the comment is _misleading_. It might be confusing, but
that's something different.
The reason that the `DTStatsAggregator` needs to keep track of
`parentStats` is so that we can get an `ImpurityCalculator` for the parent node
when we are finding best splits. However, an `ImpurityCalculator` for the
parent node already exists via `node.stats.impurityCalculator` on every
iteration except the first. It is precisely this reason that we had to add
`parentStats` at all. It's unnecessary otherwise.
If you want to change it, then something like "Parent stats need to be
explicitly tracked in the `DTStatsAggregator` because the parent [[Node]]
object does not have `ImpurityStats` on the first iteration."
I doubt that's much clearer. Just to note, this comment was intended for
developers anyway, since it's all private APIs.
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