Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/13440#discussion_r218209381
--- Diff:
mllib/src/main/scala/org/apache/spark/mllib/tree/impurity/Impurity.scala ---
@@ -52,6 +52,49 @@ trait Impurity extends Serializable {
@Since("1.0.0")
@DeveloperApi
def calculate(count: Double, sum: Double, sumSquares: Double): Double
+
+ /**
+ * :: DeveloperApi ::
+ * Compute a test-statistic p-value quality measure from left and right
split populations
+ * @param calcL impurity calculator for the left split population
+ * @param calcR impurity calculator for the right split population
+ * @return The p-value for the null hypothesis; that left and right
split populations
+ * represent the same distribution
+ * @note Unless overridden this method will fail with an exception, for
backward compatability
+ */
+ @Since("2.2.0")
+ @DeveloperApi
+ def calculate(calcL: ImpurityCalculator, calcR: ImpurityCalculator):
Double
+
+ /**
+ * :: DeveloperApi ::
+ * Determine if this impurity measure is a test-statistic measure
+ * @return True if this is a split quality measure based on a test
statistic (i.e. returns a
+ * p-value) or false otherwise.
+ * @note Unless overridden this method returns false by default, for
backward compatability
+ */
+ @Since("2.2.0")
+ @DeveloperApi
+ def isTestStatistic: Boolean
+}
+
+/**
+ * :: DeveloperApi ::
+ * Utility functions for Impurity measures
+ */
+@Since("2.0.0")
+@DeveloperApi
+object Impurity {
+ /**
+ * :: DeveloperApi ::
+ * Convert a test-statistic p-value into a "larger-is-better" gain value.
+ * @param pval The test statistic p-value
+ * @return The negative logarithm of the p-value. Any p-values smaller
than 10^-20 are clipped
+ * to 10^-20 to prevent arithmetic errors
+ */
+ @Since("2.0.0")
+ @DeveloperApi
+ def pValToGain(pval: Double): Double = -math.log(math.max(1e-20, pval))
--- End diff --
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