Github user squito commented on a diff in the pull request:
https://github.com/apache/spark/pull/16867#discussion_r106833737
--- Diff:
core/src/main/scala/org/apache/spark/util/collection/MedianHeap.scala ---
@@ -0,0 +1,95 @@
+/*
+ * 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.util.collection
+
+import scala.collection.mutable.PriorityQueue
+
+/**
+ * MedianHeap is designed to be used to quickly track the median of a
group of numbers
+ * that may contain duplicates. Inserting a new number has O(log n) time
complexity and
+ * determining the median has O(1) time complexity.
+ * The basic idea is to maintain two heaps: a smallerHalf and a
largerHalf. The smallerHalf
+ * stores the smaller half of all numbers while the largerHalf stores the
larger half.
+ * The sizes of two heaps need to be balanced each time when a new number
is inserted so
+ * that their sizes will not be different by more than 1. Therefore each
time when
+ * findMedian() is called we check if two heaps have the same size. If
they do, we should
+ * return the average of the two top values of heaps. Otherwise we return
the top of the
+ * heap which has one more element.
+ */
+
+private[spark] class MedianHeap(implicit val ord: Ordering[Double]) {
+
+ // Stores all the numbers less than the current median in a smallerHalf,
+ // i.e median is the maximum, at the root
+ private[this] var smallerHalf = PriorityQueue.empty[Double](ord)
--- End diff --
very minor -- could you make this comment a doc with `/**`? Even though
its private, I find that helpful as that is useful in IDEs where they'll show
this text w/ a hover on a reference
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