Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/1478#discussion_r15099390
--- Diff:
core/src/main/scala/org/apache/spark/util/random/SamplingUtils.scala ---
@@ -17,9 +17,49 @@
package org.apache.spark.util.random
+import scala.reflect.ClassTag
+
private[spark] object SamplingUtils {
/**
+ * Reservoir sampling implementation that also returns the input size.
+ *
+ * @param input input size
+ * @param k reservoir size
+ * @return (samples, input size)
+ */
+ def reservoirSampleAndCount[T: ClassTag](input: Iterator[T], k: Int):
(Array[T], Int) = {
+ val reservoir = new Array[T](k)
+ // Put the first k elements in the reservoir.
+ var i = 0
+ while (i < k && input.hasNext) {
+ val item = input.next()
+ reservoir(i) = item
+ i += 1
+ }
+
+ // If we have consumed all the elements, return them. Otherwise do the
replacement.
+ if (i < k) {
+ // If input size < k, trim the array to return only an array of
input size.
+ val trimReservoir = new Array[T](i)
+ System.arraycopy(reservoir, 0, trimReservoir, 0, i)
+ (trimReservoir, i)
+ } else {
+ // If input size > k, continue the sampling process.
+ val rand = new XORShiftRandom
--- End diff --
Please use a deterministic seed passed as an argument.
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