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/*
 * 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 com.hazelcast.org.apache.calcite.util.graph;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

/**
 * Iterates over the vertices in a directed graph in depth-first order.
 *
 * @param  Vertex type
 * @param  Edge type
 */
public class DepthFirstIterator
    implements Iterator {
  private final Iterator iterator;

  public DepthFirstIterator(DirectedGraph graph, V start) {
    // Dumb implementation that builds the list first.
    iterator = buildList(graph, start).iterator();
  }

  private static  List buildList(
      DirectedGraph graph, V start) {
    final List list = new ArrayList<>();
    buildListRecurse(list, new HashSet<>(), graph, start);
    return list;
  }

  /** Creates an iterable over the vertices in the given graph in a depth-first
   * iteration order. */
  public static  Iterable of(
      DirectedGraph graph, V start) {
    // Doesn't actually return a DepthFirstIterator, but a list with the same
    // contents, which is more efficient.
    return buildList(graph, start);
  }

  /** Populates a collection with the nodes reachable from a given node. */
  public static  void reachable(Collection list,
      final DirectedGraph graph, final V start) {
    buildListRecurse(list, new HashSet<>(), graph, start);
  }

  private static  void buildListRecurse(
      Collection list, Set activeVertices, DirectedGraph graph,
      V start) {
    if (!activeVertices.add(start)) {
      return;
    }
    list.add(start);
    List edges = graph.getOutwardEdges(start);
    for (E edge : edges) {
      //noinspection unchecked
      buildListRecurse(list, activeVertices, graph, (V) edge.target);
    }
    activeVertices.remove(start);
  }

  @Override public boolean hasNext() {
    return iterator.hasNext();
  }

  @Override public V next() {
    return iterator.next();
  }

  @Override public void remove() {
    throw new UnsupportedOperationException();
  }
}