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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.ArrayDeque;
import java.util.Deque;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

/**
 * Iterates over the vertices in a directed graph in breadth-first order.
 *
 * @param  Vertex type
 * @param  Edge type
 */
public class BreadthFirstIterator
    implements Iterator {
  private final DirectedGraph graph;
  private final Deque deque = new ArrayDeque<>();
  private final Set set = new HashSet<>();

  public BreadthFirstIterator(DirectedGraph graph, V root) {
    this.graph = graph;
    this.deque.add(root);
  }

  public static  Iterable of(
      final DirectedGraph graph, final V root) {
    return () -> new BreadthFirstIterator<>(graph, root);
  }

  /** Populates a set with the nodes reachable from a given node. */
  public static  void reachable(Set set,
      final DirectedGraph graph, final V root) {
    final Deque deque = new ArrayDeque<>();
    deque.add(root);
    set.add(root);
    while (!deque.isEmpty()) {
      V v = deque.removeFirst();
      for (E e : graph.getOutwardEdges(v)) {
        @SuppressWarnings("unchecked") V target = (V) e.target;
        if (set.add(target)) {
          deque.addLast(target);
        }
      }
    }
  }

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

  public V next() {
    V v = deque.removeFirst();
    for (E e : graph.getOutwardEdges(v)) {
      @SuppressWarnings("unchecked") V target = (V) e.target;
      if (set.add(target)) {
        deque.addLast(target);
      }
    }
    return v;
  }

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