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Carrot2 Text Clustering Library
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/*
 * Carrot2 project.
 *
 * Copyright (C) 2002-2020, Dawid Weiss, Stanisław Osiński.
 * All rights reserved.
 *
 * Refer to the full license file "carrot2.LICENSE"
 * in the root folder of the repository checkout or at:
 * https://www.carrot2.org/carrot2.LICENSE
 */
package org.carrot2.util;

import com.carrotsearch.hppc.IntArrayDeque;
import com.carrotsearch.hppc.IntArrayList;
import java.util.ArrayList;
import java.util.List;

/** Various utilities for processing graphs. */
public class GraphUtils {
  /**
   * Finds coherent subgraphs of an undirected graph.
   *
   * @param vertexCount the number of vertices in the graph
   * @param arcPredicate a predicate that determines which vertices are connected by an arc
   * @param pruneOneNodeSubrgaphs if true, one-node subgraphs will not be included in
   *     the result
   * @return a list of {@link IntArrayList}s containing vertices of the coherent subgraphs
   */
  public static List findCoherentSubgraphs(
      int vertexCount, IArcPredicate arcPredicate, boolean pruneOneNodeSubrgaphs) {
    // Find coherent sub-graphs using breadth-first search
    final boolean[] nodesChecked = new boolean[vertexCount];
    final List clusterGroups = new ArrayList<>();
    final IntArrayDeque nodeQueue = new IntArrayDeque();

    for (int i = 0; i < vertexCount; i++) {
      if (!nodesChecked[i]) {
        nodeQueue.clear();
        nodeQueue.addLast(i);
        nodesChecked[i] = true;
        IntArrayList clusterGroup = new IntArrayList();

        while (!nodeQueue.isEmpty()) {
          // Get a node from the queue
          int node = nodeQueue.removeFirst();

          // Add to the current sub-graph (cluster group)
          clusterGroup.add(node);

          // Add all its non-checked neighbors to the queue
          for (int j = i + 1; j < vertexCount; j++) {
            if (!nodesChecked[j]) {
              if (arcPredicate.isArcPresent(node, j)) {
                nodeQueue.addLast(j);
                nodesChecked[j] = true;
              }
            }
          }
        }

        if (clusterGroup.size() > 1 || !pruneOneNodeSubrgaphs) {
          clusterGroups.add(clusterGroup);
        }
      }
    }

    return clusterGroups;
  }

  /** A predicate defining arcs of an undirected graph. */
  public static interface IArcPredicate {
    /**
     * Returns true if there is an arc connecting vertexA and 
     * vertexB.
     */
    public boolean isArcPresent(int vertexA, int vertexB);
  }
}