Package it.unimi.dsi.webgraph.algo

Class StronglyConnectedComponents

java.lang.Object
it.unimi.dsi.webgraph.algo.StronglyConnectedComponents
public class StronglyConnectedComponents
extends Object
Computes the strongly connected components (and optionally the buckets) of an immutable graph.

The compute(ImmutableGraph, boolean, ProgressLogger) method of this class will return an instance that contains the data computed by running a variant of Tarjan's algorithm on an immutable graph. The implementation is iterative, rather than recursive, to work around known limitations on the size of the stack in current JVMs. Besides the usually strongly connected components, it is possible to compute the buckets of the graph, that is, nodes belonging to components that are terminal, but not dangling, in the component DAG.

After getting an instance, it is possible to run the computeSizes() and sortBySize(int[]) methods to obtain further information. This scheme has been devised to exploit the available memory as much as possible—after the components have been computed, the returned instance keeps no track of the graph, and the related memory can be freed by the garbage collector.

  • Field Details

    • numberOfComponents

      public final int numberOfComponents
      The number of strongly connected components.

    • component

      public final int[] component
      The component of each node.

    • buckets

      public final LongArrayBitVector buckets
      The bit vector for buckets, or null, in which case buckets have not been computed.

  • Constructor Details

    • StronglyConnectedComponents

      protected StronglyConnectedComponents​(int numberOfComponents, int[] status, LongArrayBitVector buckets)

  • Method Details

    • compute

      public static StronglyConnectedComponents compute​(ImmutableGraph graph, boolean computeBuckets, ProgressLogger pl)
      Computes the strongly connected components of a given graph.
      Parameters:
      graph - the graph whose strongly connected components are to be computed.
      computeBuckets - if true, buckets will be computed.
      pl - a progress logger, or null.
      Returns:
      an instance of this class containing the computed components.

    • compute

      public static StronglyConnectedComponents compute​(ArcLabelledImmutableGraph graph, Transform.LabelledArcFilter filter, boolean computeBuckets, ProgressLogger pl)
      Computes the strongly connected components of a given arc-labelled graph, filtering its arcs.
      Parameters:
      graph - the arc-labelled graph whose strongly connected components are to be computed.
      filter - a filter selecting the arcs that must be taken into consideration.
      computeBuckets - if true, buckets will be computed.
      pl - a progress logger, or null.
      Returns:
      an instance of this class containing the computed components.

    • computeSizes

      public int[] computeSizes()
      Returns the size array for this set of strongly connected components.
      Returns:
      the size array for this set of strongly connected components.

    • sortBySize

      public void sortBySize​(int[] size)
      Renumbers by decreasing size the components of this set.

      After a call to this method, both the internal status of this class and the argument array are permuted so that the sizes of strongly connected components are decreasing in the component index.

      Parameters:
      size - the components sizes, as returned by computeSizes().

    • main

      public static void main​(String[] arg) throws IOException, JSAPException
      Throws:
      IOException
      JSAPException