Github user tedyu commented on a diff in the pull request:
https://github.com/apache/spark/pull/21700#discussion_r200242174
--- Diff:
sql/core/src/main/java/org/apache/spark/sql/streaming/state/BoundedSortedMap.java
---
@@ -0,0 +1,79 @@
+/*
+ * 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.sql.streaming.state;
+
+import java.util.Comparator;
+import java.util.Map;
+import java.util.TreeMap;
+
+/**
+ * This class implements bounded {@link java.util.SortedMap} based on
{@link java.util.TreeMap}.
+ *
+ * As TreeMap does, this implementation sorts elements in natural order,
and cuts off
+ * smaller elements to retain at most bigger N elements.
+ *
+ * You can provide reversed order of comparator to retain smaller elements
instead.
+ *
+ * This class is not thread-safe, so synchronization would be needed to
use this concurrently.
+ *
+ * @param <K> key type
+ * @param <V> value type
+ */
+public class BoundedSortedMap<K, V> extends TreeMap<K, V> {
+
+ private final int limit;
+
+ /**
+ * Constructor
+ *
+ * @param comparator comparator instance to compare between keys
+ * @param limit bounded size
+ */
+ public BoundedSortedMap(Comparator<K> comparator, int limit) {
+ super(comparator);
+ this.limit = limit;
+ }
+
+ @Override
+ public void putAll(Map<? extends K, ? extends V> map) {
--- End diff --
Should the map parameter be of type SortedMap ?
With ordinary Map, the traversal order is not fixed. It may produce
non-deterministic result if the map's size is bigger than this
BoundedSortedMap's size
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