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/*
 * Copyright (c) 2002 - 2006 IBM Corporation.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 */
package com.ibm.wala.util.graph.traverse;

import com.ibm.wala.util.collections.HashSetFactory;
import com.ibm.wala.util.collections.ReverseIterator;
import com.ibm.wala.util.graph.Graph;
import com.ibm.wala.util.graph.impl.GraphInverter;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;
import org.jspecify.annotations.Nullable;

/**
 * This class computes strongly connected components for a Graph (or a subset of it). It does not
 * store the SCCs in any lookaside structure, but rather simply generates an enumeration of them.
 * See Cormen, Leiserson, Rivest Ch. 23 Sec. 5
 */
public class SCCIterator implements Iterator> {
  /** The second DFS (the reverse one) needed while computing SCCs */
  private final DFSFinishTimeIterator rev;

  /**
   * Construct an enumeration across the SCCs of a given graph.
   *
   * @param G The graph over which to construct SCCs
   * @throws NullPointerException if G is null
   */
  public SCCIterator(Graph G) throws NullPointerException {
    this(G, G == null ? null : G.iterator());
  }

  /**
   * Construct an enumeration of the SCCs of the subset of a given graph determined by starting at a
   * given set of nodes.
   */
  public SCCIterator(Graph G, @Nullable Iterator nodes) {
    if (G == null) {
      throw new IllegalArgumentException("G cannot be null");
    }
    Iterator reverseFinishTime = ReverseIterator.reverse(DFS.iterateFinishTime(G, nodes));

    rev = DFS.iterateFinishTime(GraphInverter.invert(G), reverseFinishTime);
  }

  /** Determine whether there are any more SCCs remaining in this enumeration. */
  @Override
  public boolean hasNext() {
    return rev.hasNext();
  }

  /** Find the next SCC in this enumeration */
  @Override
  public Set next() throws NoSuchElementException {
    Set currentSCC = HashSetFactory.make();

    T v = rev.next();
    currentSCC.add(v);

    while (rev.hasNext() && !rev.isEmpty()) {
      currentSCC.add(rev.next());
    }

    return currentSCC;
  }

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