class Traverser extends AbstractCentAlgorithm.Traverser {

        public Traverser(UserJob<Object> job) {
            super(job);
        }

        public Object betweennessCentrality(Directions direction,
                                            String label,
                                            int depth,
                                            long degree,
                                            long sample,
                                            String sourceLabel,
                                            long sourceSample,
                                            String sourceCLabel,
                                            long topN) {
            assert depth > 0;
            assert degree > 0L || degree == NO_LIMIT;
            assert topN >= 0L || topN == NO_LIMIT;

            GraphTraversal<Vertex, Vertex> t = constructSource(sourceLabel,
                                                               sourceSample,
                                                               sourceCLabel);
            t = constructPath(t, direction, label, degree, sample,
                              sourceLabel, sourceCLabel);
            t = t.emit().until(__.loops().is(P.gte(depth)));
            t = filterNonShortestPath(t, false);

            GraphTraversal<Vertex, ?> tg = this.groupPathByEndpoints(t);
            tg = this.computeBetweenness(tg);
            GraphTraversal<Vertex, ?> tLimit = topN(tg, topN);

            return this.execute(tLimit, tLimit::next);
        }

        protected GraphTraversal<Vertex, ?> groupPathByEndpoints(
                GraphTraversal<Vertex, Vertex> t) {
            return t.map(it -> {
                // t.select(Pop.all, "v").unfold().id()
                List<HugeElement> path = it.path(Pop.all, "v");
                List<Id> pathById = new ArrayList<>(path.size());
                for (HugeElement v : path) {
                    pathById.add(v.id());
                }
                return pathById;
            }).group().by(it -> {
                // group by the first and last vertex
                @SuppressWarnings("unchecked")
                List<Id> path = (List<Id>) it;
                assert path.size() >= 2;
                String first = path.get(0).toString();
                String last = path.get(path.size() - 1).toString();
                return SplicingIdGenerator.concat(first, last);
            }).unfold();
        }

        protected GraphTraversal<Vertex, ?> computeBetweenness(
                GraphTraversal<Vertex, ?> t) {<FILL_FUNCTION_BODY>}
    }

            return t.fold(new HashMap<Id, MutableFloat>(), (results, it) -> {
                @SuppressWarnings("unchecked")
                Map.Entry<Id, List<?>> entry = (Map.Entry<Id, List<?>>) it;
                @SuppressWarnings("unchecked")
                List<List<Id>> paths = (List<List<Id>>) entry.getValue();
                for (List<Id> path : paths) {
                    int len = path.size();
                    if (len <= 2) {
                        // only two vertex, no betweenness vertex
                        continue;
                    }
                    // skip the first and last vertex
                    for (int i = 1; i < len - 1; i++) {
                        Id vertex = path.get(i);
                        MutableFloat value = results.get(vertex);
                        if (value == null) {
                            value = new MutableFloat();
                            results.put(vertex, value);
                        }
                        value.add(1.0f / paths.size());
                    }
                }
                return results;
            });
        
<methods>public non-sealed void <init>() ,public java.lang.String category() ,public void checkParameters(Map<java.lang.String,java.lang.Object>) <variables>private static final Logger LOG