Github user sryza commented on a diff in the pull request:
https://github.com/apache/spark/pull/7075#discussion_r36125839
--- Diff:
mllib/src/main/scala/org/apache/spark/mllib/stat/test/KolmogorovSmirnovTest.scala
---
@@ -190,5 +191,104 @@ private[stat] object KolmogorovSmirnovTest extends
Logging {
val pval = 1 - new CommonMathKolmogorovSmirnovTest().cdf(ksStat,
n.toInt)
new KolmogorovSmirnovTestResult(pval, ksStat,
NullHypothesis.OneSampleTwoSided.toString)
}
+
+ /**
+ * Implements a two-sample, two-sided Kolmogorov-Smirnov test, which
tests if the 2 samples
+ * come from the same distribution
+ * @param data1 `RDD[Double]` first sample of data
+ * @param data2 `RDD[Double]` second sample of data
+ * @return
[[org.apache.spark.mllib.stat.test.KolmogorovSmirnovTestResult]] with the test
+ * statistic, p-value, and appropriate null hypothesis
+ */
+ def testTwoSamples(data1: RDD[Double], data2: RDD[Double]):
KolmogorovSmirnovTestResult = {
+ val n1 = data1.count().toDouble
+ val n2 = data2.count().toDouble
+ // identifier for sample 1, needed after co-sort
+ val isSample1 = true
+ // combine identified samples
+ val unionedData = data1.map((_, isSample1)).union(data2.map((_,
!isSample1)))
+ // co-sort and operate on each partition, returning local extrema to
the driver
+ val localData = unionedData.sortByKey().mapPartitions(
+ searchTwoSampleCandidates(_, n1, n2)
+ ).collect()
+ // result: global extreme
+ val ksStat = searchTwoSampleStatistic(localData, n1 * n2)
+ evalTwoSampleP(ksStat, n1.toInt, n2.toInt)
+ }
+
+ /**
+ * Calculates maximum distance candidates and counts of elements from
each sample within one
+ * partition for the two-sample, two-sided Kolmogorov-Smirnov test
implementation
+ * @param partData `Iterator[(Double, Boolean)]` the data in 1 partition
of the co-sorted RDDs,
+ * each element is additionally tagged with a boolean
flag for sample 1 membership
+ * @param n1 `Double` sample 1 size
+ * @param n2 `Double` sample 2 size
+ * @return `Iterator[(Double, Double, Double)]` where the first element
is an unadjusted minimum
+ * distance, the second is an unadjusted maximum distance (both
of which will later
+ * be adjusted by a constant to account for elements in prior
partitions), and the third is
+ * a count corresponding to the numerator of the adjustment
constant coming from this
+ * partition. This last value, the numerator of the adjustment
constant, is calculated as
+ * |sample 2| * |sample 1 in partition| - |sample 1| * |sample 2
in partition|. This comes
+ * from the fact that when we adjust for prior partitions, what
we are doing is
+ * adding the difference of the fractions (|prior elements in
sample 1| / |sample 1| -
+ * |prior elements in sample 2| / |sample 2|). We simply keep
track of the numerator
+ * portion that is attributable to each partition so that
following partitions can
+ * use it to cumulatively adjust their values.
+ */
+ private def searchTwoSampleCandidates(
+ partData: Iterator[(Double, Boolean)],
+ n1: Double,
+ n2: Double): Iterator[(Double, Double, Double)] = {
+ // fold accumulator: local minimum, local maximum, index for sample 1,
index for sample2
+ case class ExtremaAndIndices(min: Double, max: Double, ix1: Int, ix2:
Int)
+ val initAcc = ExtremaAndIndices(Double.MaxValue, Double.MinValue, 0, 0)
+ // traverse the data in the partition and calculate distances and
counts
+ val pResults = partData.foldLeft(initAcc) { case (acc, (v, isSample1))
=>
+ val (add1, add2) = if (isSample1) (1, 0) else (0, 1)
+ val cdf1 = (acc.ix1 + add1) / n1
+ val cdf2 = (acc.ix2 + add2) / n2
+ val dist = cdf1 - cdf2
+ ExtremaAndIndices(
+ math.min(acc.min, dist),
+ math.max(acc.max, dist),
+ acc.ix1 + add1, acc.ix2 + add2)
+ }
+ val results = if (pResults == initAcc) {
--- End diff --
Can you provide a comment on what situations we would expect to hit this
case in?
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