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.apache.calcite.plan.hep.HepMatchOrder;
  
  import org.checkerframework.checker.initialization.qual.UnderInitialization;
  import org.checkerframework.checker.nullness.qual.RequiresNonNull;
  
  import java.util.ArrayList;
  import java.util.HashMap;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Iterates over the edges of a graph in topological order.
   *
   * @param <V> Vertex type
   * @param <E> Edge type
   */
  public class TopologicalOrderIterator<V, E extends DefaultEdge>
      implements Iterator<V> {
    final Map<V, int[]> countMap = new HashMap<>();
    final List<V> empties = new ArrayList<>();
    final HepMatchOrder hepMatchOrder;
    private final DefaultDirectedGraph<V, E> graph;
  
    public TopologicalOrderIterator(DirectedGraph<V, E> graph) {
      this(graph, HepMatchOrder.TOP_DOWN);
    }
  
    public TopologicalOrderIterator(DirectedGraph<V, E> graph, HepMatchOrder hepMatchOrder) {
      assert hepMatchOrder == HepMatchOrder.TOP_DOWN || hepMatchOrder == HepMatchOrder.BOTTOM_UP;
      this.graph = (DefaultDirectedGraph<V, E>) graph;
      this.hepMatchOrder = hepMatchOrder;
      populate(countMap, empties);
    }
  
    public static <V, E extends DefaultEdge> Iterable<V> of(
        final DirectedGraph<V, E> graph) {
      return () -> new TopologicalOrderIterator<>(graph);
    }
  
    public static <V, E extends DefaultEdge> Iterable<V> of(
        final DirectedGraph<V, E> graph, HepMatchOrder hepMatchOrder) {
      return () -> new TopologicalOrderIterator<>(graph, hepMatchOrder);
    }
  
    @RequiresNonNull("graph")
    private void populate(
        @UnderInitialization TopologicalOrderIterator<V, E> this,
        Map<V, int[]> countMap, List<V> empties) {
      for (V v : graph.vertexMap.keySet()) {
        countMap.put(v, new int[] {0});
      }
      if (hepMatchOrder == HepMatchOrder.TOP_DOWN) {
        for (DefaultDirectedGraph.VertexInfo<V, E> info
            : graph.vertexMap.values()) {
          for (E edge : info.outEdges) {
            //noinspection SuspiciousMethodCalls
            final int[] ints =
                requireNonNull(countMap.get(edge.target),
                    () -> "no value for " + edge.target);
            ++ints[0];
          }
        }
      }
      if (hepMatchOrder == HepMatchOrder.BOTTOM_UP) {
        for (DefaultDirectedGraph.VertexInfo<V, E> info
            : graph.vertexMap.values()) {
          for (E edge : info.outEdges) {
            //noinspection SuspiciousMethodCalls
            final int[] ints =
                requireNonNull(countMap.get(edge.source),
                    () -> "no value for " + edge.source);
            ++ints[0];
          }
        }
      }
      for (Map.Entry<V, int[]> entry : countMap.entrySet()) {
        if (entry.getValue()[0] == 0) {
          empties.add(entry.getKey());
        }
      }
      countMap.keySet().removeAll(empties);
    }
  
    @Override public boolean hasNext() {
      return !empties.isEmpty();
    }
  
    @Override public V next() {
      V v = empties.remove(0);
      DefaultDirectedGraph.VertexInfo<V, E> vertexInfo =
          requireNonNull(graph.vertexMap.get(v),
              () -> "no vertex " + v);
      if (hepMatchOrder == HepMatchOrder.TOP_DOWN) {
        for (E o : vertexInfo.outEdges) {
          //noinspection unchecked
          final V target = (V) o.target;
          int[] ints =
              requireNonNull(countMap.get(target),
                  () -> "no counts found for target " + target);
          if (--ints[0] == 0) {
            countMap.remove(target);
            empties.add(target);
          }
        }
      }
      if (hepMatchOrder == HepMatchOrder.BOTTOM_UP) {
        for (E o : vertexInfo.inEdges) {
          //noinspection unchecked
          final V source = (V) o.source;
          int[] ints =
              requireNonNull(countMap.get(source),
                  () -> "no counts found for source " + source);
          if (--ints[0] == 0) {
            countMap.remove(source);
            empties.add(source);
          }
        }
      }
      return v;
    }
  
    @Override public void remove() {
      throw new UnsupportedOperationException();
    }
  
    Set<V> findCycles() {
      while (hasNext()) {
        next();
      }
      //noinspection RedundantCast
      return (Set<V>) countMap.keySet();
    }
  }
  