Github user hirakendu commented on a diff in the pull request:
https://github.com/apache/spark/pull/79#discussion_r11229365
--- Diff:
mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Impurity.scala ---
@@ -0,0 +1,42 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.mllib.tree.impurity
+
+/**
+ * Trait for calculating information gain.
+ */
+trait Impurity extends Serializable {
--- End diff --
`Impurity` should be renamed to `Error` or something more technical and
familiar. Also see the comments earlier for the necessity and example design of
a generic `Error` interface.
The `calculate` method can be renamed to something verbose like `error`.
For a generic interface, an additional `ErrorStats` trait and
`error(errorStats: ErrorStats)` method can be added. For example, `Variance` or
more aptly, `SquareError`, would implement `case class SquareErrorStats(count:
Long, mean: Double, meanSquare: Double)` and `error(errorStats) =
errorStats.meanSquare - errorStats.mean * errorStats.mean / count`. Note that
`ErrorStats` should have aggregation methods, e.g., it's easy to see the
implementation for `SquareErrorStats`.
The `Variance` class should be renamed to `SquareError`, `Entropy` to
`EntropyError` or `KLDivergence`, `Gini` to `GiniError`.
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