Github user squito commented on a diff in the pull request:
https://github.com/apache/spark/pull/16867#discussion_r105751547
--- Diff:
core/src/main/scala/org/apache/spark/util/collection/MedianHeap.scala ---
@@ -0,0 +1,130 @@
+/*
+ * 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
+
+/**
+ * MedianHeap stores numbers and returns the median by O(1) time
complexity.
+ * The basic idea is to maintain two heaps: a maxHeap and a minHeap. The
maxHeap stores
+ * the smaller half of all numbers while the minHeap 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 maxHeap,
+ // i.e median is the maximum, at the root
+ private[this] var maxHeap =
mutable.PriorityQueue.empty[Double](implicitly[Ordering[Double]])
+
+ // Stores all the numbers greater than the current median in a minHeap,
+ // i.e median is the minimum, at the root
+ private[this] var minHeap = mutable.PriorityQueue.empty[Double](
+ implicitly[Ordering[Double]].reverse)
--- End diff --
same here, `ord.reverse`.
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