Github user greghogan commented on a diff in the pull request:
https://github.com/apache/flink/pull/3802#discussion_r114324999
--- Diff:
flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/generator/CirculantGraph.java
---
@@ -0,0 +1,161 @@
+/*
+ * 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.generator;
+
+import org.apache.flink.api.common.functions.FlatMapFunction;
+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.Tuple2;
+import org.apache.flink.graph.Edge;
+import org.apache.flink.graph.Graph;
+import org.apache.flink.graph.Vertex;
+import org.apache.flink.types.LongValue;
+import org.apache.flink.types.NullValue;
+import org.apache.flink.util.Collector;
+import org.apache.flink.util.LongValueSequenceIterator;
+import org.apache.flink.util.Preconditions;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/*
+ * @see <a
href="http://mathworld.wolfram.com/CirculantGraph.html";>Circulant Graph at
Wolfram MathWorld</a>
+ */
+public class CirculantGraph
+extends AbstractGraphGenerator<LongValue, NullValue, NullValue> {
+
+ public static final int MINIMUM_VERTEX_COUNT = 1;
+
+ public static final int MINIMUM_OFFSET = 1;
+
+ // Required to create the DataSource
+ private final ExecutionEnvironment env;
+
+ // Required configuration
+ private long vertexCount;
+
+ private List<Tuple2<Long, Long>> startOffsetPairs = new ArrayList<>();
+
+ /**
+ * An undirected {@link Graph} whose {@link Vertex} connects to targets
appointed by an offset list.
+ *
+ * @param env the Flink execution environment
+ * @param vertexCount number of vertices
+ */
+ public CirculantGraph(ExecutionEnvironment env, long vertexCount) {
+ Preconditions.checkArgument(vertexCount >= MINIMUM_VERTEX_COUNT,
+ "Vertex count must be at least " +
MINIMUM_VERTEX_COUNT);
+
+ this.env = env;
+ this.vertexCount = vertexCount;
+ }
+
+ /**
+ * Required configuration for each offset of the graph.
+ *
+ * @param startOffset first offset appointing the vertices' position
should be linked by any vertex
+ * @param length offset in [startOffset, startOffset + length) will be
added
+ * @return this
+ */
+ public CirculantGraph addOffsets(long startOffset, long length) {
+ long maxOffset = vertexCount / 2;
+ for (int i = 0; i < length; i++) {
+ long offset = startOffset + i;
+ Preconditions.checkArgument(offset >= MINIMUM_OFFSET,
+ "Offset must be at least " +
MINIMUM_OFFSET);
+ Preconditions.checkArgument(offset <= maxOffset,
+ "Offset must be at most " + maxOffset);
+ }
+
+ // save startOffset and length pair
+ startOffsetPairs.add(new Tuple2<>(startOffset, length));
+
+ return this;
+ }
+
+ @Override
+ public Graph<LongValue, NullValue, NullValue> generate() {
+ // Vertices
+ DataSet<Vertex<LongValue, NullValue>> vertices =
GraphGeneratorUtils.vertexSequence(env, parallelism, vertexCount);
+
+ // Edges
+ LongValueSequenceIterator iterator = new
LongValueSequenceIterator(0, this.vertexCount - 1);
+
+ DataSet<Edge<LongValue, NullValue>> edges = env
+ .fromParallelCollection(iterator,
LongValue.class)
+ .setParallelism(parallelism)
+ .name("Edge iterators")
+ .flatMap(new LinkVertexToOffsets(vertexCount,
startOffsetPairs))
+ .setParallelism(parallelism)
+ .name("Circulant graph edges");
+
+ // Graph
+ return Graph.fromDataSet(vertices, edges, env);
+ }
+
+ @FunctionAnnotation.ForwardedFields("*->f0")
+ private static class LinkVertexToOffsets
+ implements FlatMapFunction<LongValue, Edge<LongValue,
NullValue>> {
+
+ private final long vertexCount;
+
+ private final List<Tuple2<Long, Long>> startOffsetPairs;
+
+ private LongValue target = new LongValue();
+
+ private Edge<LongValue, NullValue> edge = new Edge<>(null,
target, NullValue.getInstance());
+
+ public LinkVertexToOffsets(long vertexCount, List<Tuple2<Long,
Long>> startOffsetPairs) {
+ this.vertexCount = vertexCount;
+ this.startOffsetPairs = startOffsetPairs;
+ }
+
+ @Override
+ public void flatMap(LongValue source, Collector<Edge<LongValue,
NullValue>> out)
+ throws Exception {
+ edge.f0 = source;
+
+ // parse startOffsetPairs to offsets
+ List<Long> offsets = new ArrayList<>();
+ long maxOffset = vertexCount / 2;
+ for (Tuple2<Long, Long> offsetPair : startOffsetPairs) {
--- End diff --
There is no need to create an array of offsets, certainly not in every
`flatMap`. Can simply iterate over the list of `Tuple2` and collect new edges.
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