Github user vasia commented on a diff in the pull request:
https://github.com/apache/flink/pull/1250#discussion_r42113501
--- Diff:
flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/library/TriangleEnumerator.java
---
@@ -0,0 +1,347 @@
+/*
+ * 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.MapFunction;
+import org.apache.flink.api.common.functions.ReduceFunction;
+import org.apache.flink.api.common.functions.FlatMapFunction;
+import org.apache.flink.api.common.functions.JoinFunction;
+import org.apache.flink.api.common.functions.GroupReduceFunction;
+import org.apache.flink.api.common.operators.Order;
+import org.apache.flink.api.java.DataSet;
+import org.apache.flink.api.java.ExecutionEnvironment;
+import org.apache.flink.api.java.functions.FunctionAnnotation;
+import org.apache.flink.api.java.tuple.Tuple3;
+import org.apache.flink.api.java.tuple.Tuple4;
+import org.apache.flink.graph.Edge;
+import org.apache.flink.graph.Graph;
+import org.apache.flink.graph.GraphAlgorithm;
+import org.apache.flink.graph.example.utils.TriangleCountData;
+import org.apache.flink.types.NullValue;
+import org.apache.flink.util.Collector;
+
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+
+
+/**
+ * This function returns number of triangles present in the input graph.
+ * A triangle consists of three edges that connect three vertices with
each other.
+ * <p>
+ * <p>
+ * The basic algorithm works as follows:
+ * It groups all edges that share a common vertex and builds triads, i.e.,
triples of vertices
+ * that are connected by two edges. Finally, all triads are filtered for
which no third edge exists
+ * that closes the triangle.
+ * <p>
+ * <p>
+ * For a group of <i>n</i> edges that share a common vertex, the number of
built triads is quadratic <i>((n*(n-1))/2)</i>.
+ * Therefore, an optimization of the algorithm is to group edges on the
vertex with the smaller output degree to
+ * reduce the number of triads.
+ * This implementation extends the basic algorithm by computing output
degrees of edge vertices and
+ * grouping on edges on the vertex with the smaller degree.
+ */
+
+public class TriangleEnumerator<K extends Comparable<K>, VV, EV>
implements GraphAlgorithm<K, VV, EV, DataSet<Tuple3<K,K,K>>> {
+ @Override
+ public DataSet<Tuple3<K,K,K>> run(Graph<K, VV, EV> input) throws
Exception {
+
+ DataSet<Edge<K, EV>> edges = input.getEdges();
+
+ // annotate edges with degrees
+ DataSet<EdgeWithDegrees<K>> edgesWithDegrees =
edges.flatMap(new EdgeDuplicator<K, EV>())
+ .groupBy(0).sortGroup(1,
Order.ASCENDING).reduceGroup(new DegreeCounter<K, EV>())
+ .groupBy(EdgeWithDegrees.V1,
EdgeWithDegrees.V2).reduce(new DegreeJoiner<K>());
+
+ // project edges by degrees
+ DataSet<Edge<K, NullValue>> edgesByDegree =
edgesWithDegrees.map(new EdgeByDegreeProjector<K>());
+ // project edges by vertex id
+ DataSet<Edge<K, NullValue>> edgesById = edgesByDegree.map(new
EdgeByIdProjector<K>());
+
+ DataSet<Tuple3<K,K,K>> triangles = edgesByDegree
+ // build triads
+
.groupBy(EdgeWithDegrees.V1).sortGroup(EdgeWithDegrees.V2, Order.ASCENDING)
+ .reduceGroup(new TriadBuilder())
+ // filter triads
+ .join(edgesById).where(Triad.V2,
Triad.V3).equalTo(0, 1).with(new TriadFilter<K>());
+
+ return triangles;
+ }
+
+ /**
+ * Emits for an edge the original edge and its switched version.
+ */
+ private static final class EdgeDuplicator<K, EV> implements
FlatMapFunction<Edge<K, EV>, Edge<K, EV>> {
+
+ @Override
+ public void flatMap(Edge<K, EV> edge, Collector<Edge<K, EV>>
out) throws Exception {
+ out.collect(edge);
+ Edge<K, EV> reversed = edge.reverse();
+ out.collect(reversed);
+ }
+ }
+
+ /**
+ * Counts the number of edges that share a common vertex.
+ * Emits one edge for each input edge with a degree annotation for the
shared vertex.
+ * For each emitted edge, the first vertex is the vertex with the
smaller id.
+ */
+ private static final class DegreeCounter<K extends Comparable<K>, EV>
--- End diff --
Hey, I was actually referring to the `TriadBuilder`.
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