[Neo4j] GraphAlgoFactory#aStar from a node to itself throws Exception

[Jatin Puri]

This is a consistent behavior since a loong time. I think its a bug.

When you try finding a path from a node to itself using `astar` it throws 
exception when it should just return null. To verify:


import org.neo4j.graphalgo.*;
import org.neo4j.graphdb.*;
import org.neo4j.graphdb.factory.GraphDatabaseFactory;
import org.neo4j.kernel.impl.util.FileUtils;

import java.io.File;
import java.io.IOException;

public class Main {

    private static final String DB_PATH = "target/neo4j-hello-db";

    GraphDatabaseService graphDb;
    Node firstNode;
    Relationship relationship;

    private static enum RelTypes implements RelationshipType {

        KNOWS, LIKE
    }

    public static void main(final String[] args) {

        Main hello = new Main();
        hello.clearDb();
        hello.createDb();
        hello.removeData();
        hello.shutDown();


    }

    void createDb() {
        clearDb();
        // START SNIPPET: startDb
        graphDb = new GraphDatabaseFactory().newEmbeddedDatabase(DB_PATH);
        registerShutdownHook(graphDb);
            long l = System.currentTimeMillis();
            Transaction tx = graphDb.beginTx();
            try {
                firstNode = graphDb.createNode();

                PathFinder<WeightedPath> finder2 = GraphAlgoFactory.aStar(
                        PathExpanders.forTypeAndDirection(RelTypes.KNOWS, 
Direction.BOTH),
                        new CostEvaluator<Double>() {
                            @Override
                            public Double getCost(Relationship relationship, 
Direction direction) {
                                return 1d;
                            }
                        },
                        new EstimateEvaluator<Double>() {
                            @Override
                            public Double getCost(Node node, Node goal) {
                                return 1d;
                            }
                        });

                WeightedPath path = finder2.findSinglePath(firstNode, 
firstNode);
                tx.success();
            } finally {
                tx.finish();
                System.out.println(System.currentTimeMillis() - l);
            }
        }



    private void clearDb() {
        try {
            FileUtils.deleteRecursively(new File(DB_PATH));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    void removeData() {
        Transaction tx = graphDb.beginTx();
        try {
            firstNode.getSingleRelationship(RelTypes.KNOWS, 
Direction.OUTGOING).delete();
            firstNode.delete();
            tx.success();
        } finally {
            tx.finish();
        }
    }

    void shutDown() {
        System.out.println();
        System.out.println("Shutting down database ...");
        graphDb.shutdown();
    }


    private static void registerShutdownHook(final GraphDatabaseService 
graphDb) {
        Runtime.getRuntime().addShutdownHook(new Thread() {

            @Override
            public void run() {
                graphDb.shutdown();
            }
        });
    }
}





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