Github user greghogan commented on a diff in the pull request:
https://github.com/apache/flink/pull/1956#discussion_r62493996
--- Diff:
flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/library/HITSAlgorithm.java
---
@@ -0,0 +1,183 @@
+/*
+ * 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.graph.library;
+
+import org.apache.flink.api.common.aggregators.DoubleSumAggregator;
+import org.apache.flink.api.common.functions.MapFunction;
+import org.apache.flink.api.java.DataSet;
+
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.graph.Edge;
+import org.apache.flink.graph.EdgeDirection;
+import org.apache.flink.graph.Graph;
+import org.apache.flink.graph.GraphAlgorithm;
+import org.apache.flink.graph.Vertex;
+import org.apache.flink.graph.spargel.MessageIterator;
+import org.apache.flink.graph.spargel.MessagingFunction;
+import org.apache.flink.graph.spargel.ScatterGatherConfiguration;
+import org.apache.flink.graph.spargel.VertexUpdateFunction;
+import org.apache.flink.graph.utils.NullValueEdgeMapper;
+import org.apache.flink.types.DoubleValue;
+import org.apache.flink.types.NullValue;
+import org.apache.flink.util.Preconditions;
+
+/**
+ * This is an implementation of HITS algorithm, using a scatter-gather
iteration.
+ * The user can define the maximum number of iterations. HITS algorithm is
determined by two parameters,
+ * hubs and authorities. A good hub represented a page that pointed to
many other pages, and a good authority
+ * represented a page that was linked by many different hubs. The
implementation assumes that the two value on
+ * every vertex are the same at the beginning.
+ * <p>
+ * If the number of vertices of the input graph is known, it should be
provided as a parameter
+ * to speed up computation. Otherwise, the algorithm will first execute a
job to count the vertices.
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/HITS_algorithm";>HITS
Algorithm</a>
+ */
+public class HITSAlgorithm<K, VV, EV> implements GraphAlgorithm<K, VV, EV,
DataSet<Vertex<K, Tuple2<Double, Double>>>> {
--- End diff --
As a mutable type `DoubleValue` results in many fewer object instantiations
compared with Java's immutable `Double`. Normally garbage collection isn't a
concern but for simple algorithms this can have a noticeable impact.
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