* MutableGraph<Integer> graph = GraphBuilder.undirected().build();
 * }
 *
 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype
 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not
 * need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph),

 * MutableGraph<Integer> graph = GraphBuilder.undirected().build();
 * }
 *
 * <p>{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype
 * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not
 * need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph),

        assertThat(graph.degree(node)).isEqualTo(graph.adjacentNodes(node).size() + selfLoopCount);
        assertThat(graph.predecessors(node)).isEqualTo(graph.adjacentNodes(node));
        assertThat(graph.successors(node)).isEqualTo(graph.adjacentNodes(node));
        assertThat(graph.inDegree(node)).isEqualTo(graph.degree(node));
        assertThat(graph.outDegree(node)).isEqualTo(graph.degree(node));

    private static void addPhase(
            Graph graph, Graph.Vertex before, Graph.Vertex after, org.apache.maven.api.Lifecycle.Phase phase) {
        Graph.Vertex ep0 = graph.addVertex(BEFORE + phase.name());
        Graph.Vertex ep1 = graph.addVertex("$$" + phase.name());
        Graph.Vertex ep2 = graph.addVertex(phase.name());
        Graph.Vertex ep3 = graph.addVertex(AFTER + phase.name());
        graph.addEdge(ep0, ep1);

        ProjectDependencyGraph graph =
                new DefaultProjectDependencyGraph(Arrays.asList(aProject, bProject, cProject, dProject, eProject));
        graph = new FilteredProjectDependencyGraph(graph, Arrays.asList(aProject, dProject, eProject));
        final List<MavenProject> downstreamProjects = graph.getDownstreamProjects(aProject, false);
        assertEquals(2, downstreamProjects.size());

import org.apache.maven.api.model.Parent;
import org.apache.maven.api.model.Plugin;
import org.apache.maven.artifact.ArtifactUtils;
import org.apache.maven.project.Graph.Vertex;

/**
 * ProjectSorter
 */
public class ProjectSorter {
    private Graph graph;

    private List<MavenProject> sortedProjects;

    private Map<String, MavenProject> projectMap;

    /**
     * Sort a list of projects.
     * <ul>

    /**
     *  null in dirty graph, actual
     *  scope for conflict-resolved graph
     */
    ArtifactScopeEnum scope;

    // ------------------------------------------------------------------------
    /**
     * init graph
     */
    public MetadataGraph(int nVertices) {
        init(nVertices, 2 * nVertices);
    }

    public MetadataGraph(int nVertices, int nEdges) {

  // ValueGraph-level accessors
  //

  /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */
  @Override
  Set<N> nodes();

  /** Returns all edges in this graph. */
  @Override
  Set<EndpointPair<N>> edges();

  /**
   * Returns a live view of this graph as a {@link Graph}. The resulting {@link Graph} will have an

            ProjectDependencyGraph graph = new DefaultProjectDependencyGraph(session.getAllProjects());
            if (session.getProjects() != null) {
                graph = new FilteredProjectDependencyGraph(graph, session.getProjects());
            }

            result = Result.success(graph);
        }

        return result;
    }

    graph.putEdge("a", "b");

    Traverser.forTree(graph); // Does not throw
  }

  @Test
  public void forTree_withUndirectedGraph_throws() throws Exception {
    MutableGraph<String> graph = GraphBuilder.undirected().build();
    graph.putEdge("a", "b");

    assertThrows(IllegalArgumentException.class, () -> Traverser.forTree(graph));
  }

  @Test
  @SuppressWarnings("CheckReturnValue")