content: /*
   * 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.calcite.util.graph;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.Collection;
  import java.util.List;
  import java.util.Set;
  
  /**
   * Directed graph.
   *
   * @param <V> Vertex type
   * @param <E> Edge type
   */
  public interface DirectedGraph<V, E> {
    /** Adds a vertex to this graph.
     *
     * @param vertex Vertex
     * @return Whether vertex was added
     */
    boolean addVertex(V vertex);
  
    /** Adds an edge to this graph.
     *
     * @param vertex Source vertex
     * @param targetVertex Target vertex
     * @return New edge, if added, otherwise null
     * @throws IllegalArgumentException if either vertex is not already in graph
     */
    @Nullable E addEdge(V vertex, V targetVertex);
  
    @Nullable E getEdge(V source, V target);
  
    boolean removeEdge(V vertex, V targetVertex);
  
    Set<? extends V> vertexSet();
  
    /** Removes from this graph all vertices that are in {@code collection},
     * and the edges into and out of those vertices. */
    void removeAllVertices(Collection<V> collection);
  
    List<E> getOutwardEdges(V source);
  
    List<E> getInwardEdges(V vertex);
  
    Set<E> edgeSet();
  
    /** Factory for edges.
     *
     * @param <V> Vertex type
     * @param <E> Edge type
     */
    interface EdgeFactory<V, E> {
      E createEdge(V v0, V v1);
    }
  }
  