三角形计数算法用于计算通过每个顶点的三角形个数。
算法实现的流程如下：
- 每个顶点将其ID发送给所有出边邻居。
- 存储入边和出边邻居并发送给出边邻居。
- 对每条边计算其终点的交集数量，并求和，结果输出到表。
- 将表中的输出结果求和并除以三，即得到三角形个数。
代码示例
三角形计数算法的代码，如下所示。
import java.io.IOException;
import com.aliyun.odps.data.TableInfo;
import com.aliyun.odps.graph.ComputeContext;
import com.aliyun.odps.graph.Edge;
import com.aliyun.odps.graph.GraphJob;
import com.aliyun.odps.graph.GraphLoader;
import com.aliyun.odps.graph.MutationContext;
import com.aliyun.odps.graph.Vertex;
import com.aliyun.odps.graph.WorkerContext;
import com.aliyun.odps.io.LongWritable;
import com.aliyun.odps.io.NullWritable;
import com.aliyun.odps.io.Tuple;
import com.aliyun.odps.io.Writable;
import com.aliyun.odps.io.WritableRecord;
/**
* Compute the number of triangles passing through each vertex.
*
* The algorithm can be computed in three supersteps:
* I. Each vertex sends a message with its ID to all its outgoing
* neighbors.
* II. The incoming neighbors and outgoing neighbors are stored and
* send to outgoing neighbors.
* III. For each edge compute the intersection of the sets at destination
* vertex and sum them, then output to table.
*
* The triangle count is the sum of output table and divide by three since
* each triangle is counted three times.
*
**/
public class TriangleCount {
public static class TCVertex extends
Vertex<LongWritable, Tuple, NullWritable, Tuple> {
@Override
public void setup(
WorkerContext<LongWritable, Tuple, NullWritable, Tuple> context)
throws IOException {
// collect the outgoing neighbors
Tuple t = new Tuple();
if (this.hasEdges()) {
for (Edge<LongWritable, NullWritable> edge : this.getEdges()) {
t.append(edge.getDestVertexId());
}
}
this.setValue(t);
}
@Override
public void compute(
ComputeContext<LongWritable, Tuple, NullWritable, Tuple> context,
Iterable<Tuple> msgs) throws IOException {
if (context.getSuperstep() == 0L) {
// sends a message with its ID to all its outgoing neighbors
Tuple t = new Tuple();
t.append(getId());
context.sendMessageToNeighbors(this, t);
} else if (context.getSuperstep() == 1L) {
// store the incoming neighbors
for (Tuple msg : msgs) {
for (Writable item : msg.getAll()) {
if (!this.getValue().getAll().contains((LongWritable)item)) {
this.getValue().append((LongWritable)item);
}
}
}
// send both incoming and outgoing neighbors to all outgoing neighbors
context.sendMessageToNeighbors(this, getValue());
} else if (context.getSuperstep() == 2L) {
// count the sum of intersection at each edge
long count = 0;
for (Tuple msg : msgs) {
for (Writable id : msg.getAll()) {
if (getValue().getAll().contains(id)) {
count ++;
}
}
}
// output to table
context.write(getId(), new LongWritable(count));
this.voteToHalt();
}
}
}
public static class TCVertexReader extends
GraphLoader<LongWritable, Tuple, NullWritable, Tuple> {
@Override
public void load(
LongWritable recordNum,
WritableRecord record,
MutationContext<LongWritable, Tuple, NullWritable, Tuple> context)
throws IOException {
TCVertex vertex = new TCVertex();
vertex.setId((LongWritable) record.get(0));
String[] edges = record.get(1).toString().split(",");
for (int i = 0; i < edges.length; i++) {
try {
long destID = Long.parseLong(edges[i]);
vertex.addEdge(new LongWritable(destID), NullWritable.get());
} catch(NumberFormatException nfe) {
System.err.println("Ignore " + nfe);
}
}
context.addVertexRequest(vertex);
}
}
public static void main(String[] args) throws IOException {
if (args.length < 2) {
System.out.println("Usage: <input> <output>");
System.exit(-1);
}
GraphJob job = new GraphJob();
job.setGraphLoaderClass(TCVertexReader.class);
job.setVertexClass(TCVertex.class);
job.addInput(TableInfo.builder().tableName(args[0]).build());
job.addOutput(TableInfo.builder().tableName(args[1]).build());
long startTime = System.currentTimeMillis();
job.run();
System.out.println("Job Finished in "
+ (System.currentTimeMillis() - startTime) / 1000.0 + " seconds");
}
}