Github user pwendell commented on a diff in the pull request:
https://github.com/apache/spark/pull/4450#discussion_r29112286
--- Diff:
core/src/main/scala/org/apache/spark/util/collection/WritablePartitionedPairCollection.scala
---
@@ -0,0 +1,117 @@
+/*
+ * 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 java.util.Comparator
+
+import org.apache.spark.storage.BlockObjectWriter
+
+/**
+ * A common interface for size-tracking collections of key-value pairs that
+ * - Have an associated partition for each key-value pair.
+ * - Support a memory-efficient sorted iterator
+ * - Support a WritablePartitionedIterator for writing the contents
directly as bytes.
+ */
+private[spark] trait WritablePartitionedPairCollection[K, V] extends
SizeTracker {
+ /**
+ * Insert a key-value pair with a partition into the collection
+ */
+ def insert(partition: Int, key: K, value: V): Unit
+
+ /**
+ * Estimate the collection's current memory usage in bytes.
+ */
+ def estimateSize(): Long
+
+ /**
+ * Iterate through the data in order of partition ID and then the given
comparator. This may
+ * destroy the underlying collection.
+ */
+ def partitionedDestructiveSortedIterator(keyComparator: Comparator[K]):
Iterator[((Int, K), V)]
--- End diff --
Can you use `Option[Comparator[K]]` to more explicitly model the fact that
this key comparator is optional? Right now it looks like you pass nulls around.
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