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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 com.hazelcast.com.liance with
 * the License.  You may obtain a copy of the License at
 *
 * http://www.apache.com.hazelcast.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.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;

/**
 * Iterates over the edges of a graph in topological order.
 *
 * @param  Vertex type
 * @param  Edge type
 */
public class TopologicalOrderIterator
    implements Iterator {
  final Map countMap = new HashMap<>();
  final List empties = new ArrayList<>();
  private final DefaultDirectedGraph graph;

  public TopologicalOrderIterator(DirectedGraph graph) {
    this.graph = (DefaultDirectedGraph) graph;
    populate(countMap, empties);
  }

  public static  Iterable of(
      final DirectedGraph graph) {
    return () -> new TopologicalOrderIterator<>(graph);
  }

  private void populate(Map countMap, List empties) {
    for (V v : graph.vertexMap.keySet()) {
      countMap.put(v, new int[] {0});
    }
    for (DefaultDirectedGraph.VertexInfo info
        : graph.vertexMap.values()) {
      for (E edge : info.outEdges) {
        //noinspection SuspiciousMethodCalls
        final int[] ints = countMap.get(edge.target);
        ++ints[0];
      }
    }
    for (Map.Entry entry : countMap.entrySet()) {
      if (entry.getValue()[0] == 0) {
        empties.add(entry.getKey());
      }
    }
    countMap.keySet().removeAll(empties);
  }

  public boolean hasNext() {
    return !empties.isEmpty();
  }

  public V next() {
    V v = empties.remove(0);
    for (E o : graph.vertexMap.get(v).outEdges) {
      //noinspection unchecked
      final V target = (V) o.target;
      if (--countMap.get(target)[0] == 0) {
        countMap.remove(target);
        empties.add(target);
      }
    }
    return v;
  }

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

  Set findCycles() {
    while (hasNext()) {
      next();
    }
    return countMap.keySet();
  }
}