Github user fhueske commented on a diff in the pull request:
https://github.com/apache/flink/pull/1105#discussion_r39277765
--- Diff:
flink-staging/flink-gelly/src/main/java/org/apache/flink/graph/library/TriangleCount.java
---
@@ -0,0 +1,208 @@
+/*
+ * 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.functions.FlatJoinFunction;
+import org.apache.flink.api.common.functions.GroupReduceFunction;
+import org.apache.flink.api.common.functions.MapFunction;
+import org.apache.flink.api.common.functions.ReduceFunction;
+import org.apache.flink.api.java.DataSet;
+import org.apache.flink.api.java.tuple.Tuple1;
+import org.apache.flink.api.java.tuple.Tuple2;
+import org.apache.flink.graph.GraphAlgorithm;
+import org.apache.flink.graph.Graph;
+import org.apache.flink.graph.Vertex;
+import org.apache.flink.graph.Edge;
+import org.apache.flink.graph.EdgeDirection;
+import org.apache.flink.graph.NeighborsFunctionWithVertexValue;
+import org.apache.flink.graph.utils.VertexToTuple2Map;
+import org.apache.flink.types.NullValue;
+import org.apache.flink.util.Collector;
+
+import java.util.Iterator;
+import java.util.TreeMap;
+
+/**
+ * Triangle Count Algorithm.
+ *
+ * This algorithm operates in three phases. First, vertices select
neighbors with id greater than theirs
+ * and send messages to them. Each received message is then propagated to
neighbors with higher id.
+ * Finally, if a node encounters the target id in the list of received
messages, it increments the number
+ * of triangles found.
+ *
+ * For skewed graphs, we recommend calling the GSATriangleCount library
method as it uses the more restrictive
+ * `reduceOnNeighbors` function which internally makes use of combiners to
speed up computation.
+ *
+ * This implementation is non - iterative.
+ *
+ * The algorithm takes an undirected, unweighted graph as input and
outputs a DataSet of
+ * Tuple1 which contains a single integer representing the number of
triangles.
+ */
+public class TriangleCount implements
+ GraphAlgorithm<Long, NullValue, NullValue,
DataSet<Tuple1<Integer>>> {
--- End diff --
The key must implement the `org.apache.flink.types.Key` interface.
Otherwise, you could not group, join, sort on this type. `Key` implements
`Comparable`, so you can simply do `key1.compareTo(key2)`.
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