Github user asolimando commented on a diff in the pull request:
https://github.com/apache/spark/pull/20632#discussion_r169742383
--- Diff: mllib/src/main/scala/org/apache/spark/ml/tree/Node.scala ---
@@ -287,6 +291,34 @@ private[tree] class LearningNode(
}
}
+ /**
+ * @return true iff the node is a leaf.
+ */
+ private def isLeafNode(): Boolean = leftChild.isEmpty &&
rightChild.isEmpty
+
+ // the set of (leaf) predictions appearing in the subtree rooted at the
given node.
+ private lazy val leafPredictions: Set[Double] = {
--- End diff --
The variant proposed by @sethah looks complete to me, and it was indeed not
necessary to store the predictions.
If I got right the counterexample of @srowen, the new code would behave on
it in this way:
(input tree)
1: InnerNode (not built yet)
--1.1: InnerNode (not built yet)
----1.1.1 (label x): LeafNode (not built yet)
----1.1.2 (label x): LeafNode (not built yet)
--1.2: InnerNode (not built yet)
----1.2.1 (label x): LeafNode (not built yet)
----1.2.2 (label x): LeafNode (not built yet)
toNode on 1.1.1 and 1.1.2 builds two LeafNode, so when recursion come back
to the context of 1.1, we have the following situation:
1: InnerNode (not built yet)
--1.1: InnerNode (not built yet)
----1.1.1 (label x): LeafNode (not built yet)
----1.1.2 (label x): LeafNode (not built yet)
--1.2: InnerNode (not built yet)
----1.2.1 (label x): LeafNode (not built yet)
----1.2.2 (label x): LeafNode (not built yet)
Therefore, the first case is triggered, and 1.1 becomes a LeafNode, giving
us this situation:
1: InnerNode (not built yet)
--1.1 (label x): LeafNode
--1.2: InnerNode (not built yet)
----1.2.1 (label x): LeafNode (not built yet)
----1.2.2 (label x): LeafNode (not built yet)
Then, toNode is called on 1.2, and similarly to what happened to subtree
rooted at 1.1 we have a match on the first case, resulting in this situation:
1: InnerNode (not built yet)
--1.1 (label x): LeafNode
--1.2 (label x): LeafNode
Now toNode on 1.1 and 1.2 has built two LeafNode, and the first case
matches also in the context of node 1, that is finally translated into a single
LeafNode, as expected.
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