Github user jparkie commented on a diff in the pull request:
https://github.com/apache/flink/pull/4652#discussion_r140832701
--- Diff:
flink-libraries/flink-table/src/main/java/org/apache/flink/table/runtime/functions/aggfunctions/cardinality/HyperLogLog.java
---
@@ -0,0 +1,333 @@
+/*
+ * 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.flink.table.runtime.functions.aggfunctions.cardinality;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.DataInput;
+import java.io.DataInputStream;
+import java.io.DataOutput;
+import java.io.DataOutputStream;
+import java.io.Externalizable;
+import java.io.IOException;
+import java.io.ObjectInput;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutput;
+import java.io.Serializable;
+
+/**
+ * Java implementation of HyperLogLog (HLL) algorithm from this paper:
+ * <p/>
+ * http://algo.inria.fr/flajolet/Publications/FlFuGaMe07.pdf
+ * <p/>
+ * HLL is an improved version of LogLog that is capable of estimating
+ * the cardinality of a set with accuracy = 1.04/sqrt(m) where
+ * m = 2^b. So we can control accuracy vs space usage by increasing
+ * or decreasing b.
+ * <p/>
+ * The main benefit of using HLL over LL is that it only requires 64%
+ * of the space that LL does to get the same accuracy.
+ * <p/>
+ * <p>
+ * Note that this implementation does not include the long range
correction function
+ * defined in the original paper. Empirical evidence shows that the
correction
+ * function causes more harm than good.
+ * </p>
+ */
+public class HyperLogLog implements ICardinality, Serializable {
--- End diff --
I see this class is adapted from https://github.com/addthis/stream-lib. I
think you should comment that this class was adapted from the link, so people
can track differences.
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