Github user hanm commented on a diff in the pull request:
https://github.com/apache/zookeeper/pull/590#discussion_r219659934
--- Diff: src/java/main/org/apache/zookeeper/server/watch/BitHashSet.java
---
@@ -0,0 +1,156 @@
+/**
+ * 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.zookeeper.server.watch;
+
+import java.util.BitSet;
+import java.util.Set;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.lang.Iterable;
+
+import org.apache.zookeeper.server.util.BitMap;
+
+/**
+ * Using BitSet to store all the elements, and use HashSet to cache limited
+ * number of elements to find a balance between memory and time complexity.
+ *
+ * Without HashSet, we need to to use O(N) time to get the elements, N is
+ * the bit numbers in elementBits. But we need to keep the size small to
make
+ * sure it doesn't cost too much in memory, there is a tradeoff between
--- End diff --
nit `trade off`
---
