Class: MOSAIK::Algorithms::Louvain

Inherits:
MOSAIK::Algorithm show all
Defined in:
lib/mosaik/algorithms/louvain.rb

Overview

Louvain method for community detection (Blondel et al., 2008)

Constant Summary collapse

EPSILON =

Threshold of modularity improvement

1e-6

Instance Attribute Summary

Attributes inherited from MOSAIK::Algorithm

#graph, #options

Instance Method Summary collapse

Methods inherited from MOSAIK::Algorithm

#initialize, #validate

Constructor Details

This class inherits a constructor from MOSAIK::Algorithm

Instance Method Details

#call ⇒ Object 



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# File 'lib/mosaik/algorithms/louvain.rb', line 14

def call
  # Store the original graph
  original_graph = graph

  # Initialize modularity
  modularity = nil

  # Final mapping of vertices to communities
  mapping = graph
    .vertices
    .keys
    .index_with { |vertex_id| vertex_id }

  # Iterate until no further improvement in modularity
  1.step do |i|
    # Assign initial set of communities (each vertex in its own community)
    graph.vertices.each_value do |vertex|
      graph
        .add_cluster(vertex.id)
        .add_vertex(vertex)
    end

    # Calculate and print initial modularity on first iteration
    if modularity.nil?
      modularity = modularity_for(graph)

      info "Initial modularity: #{modularity}"
    end

    debug "Iteration #{i}: start modularity=#{modularity}, vertices=#{graph.vertices.count}, communities=#{graph.clusters.count}"

    # Phase 1: reassign vertices to optimize modularity
    progress = ProgressBar.create(
      title: "Reassigning vertices",
      total: graph.vertices.size,
      length: 80,
      format: "%t%e [%b>%i] %p%%".cyan,
      output: options[:debug] ? $stderr : File.open(File::NULL, "w"),
    )

    graph.vertices.each_value do |vertex|
      progress.increment

      reassign_vertex(graph, vertex)
    end

    # Phase 2: reduce communities to a single node
    debug "Reducing graph"
    g, reduced_mapping = reduce_graph(graph)

    debug "Reduced #{graph.vertices.size} vertices to #{g.vertices.size} vertices"

    if options[:visualize]
      MOSAIK::Graph::Visualizer
        .new(options, g)
        .to_svg("louvain_#{i}")
    end

    # Merge the reduced mapping with the original mapping
    mapping = mapping.transform_values { |v| reduced_mapping[v] }

    # Calculate final modularity
    final_modularity = modularity_for(graph)

    debug "Iteration #{i}: end modularity=#{final_modularity}, vertices=#{graph.vertices.count}, communities=#{graph.clusters.count}"

    # Stop iterating if no further improvement in modularity
    break if final_modularity - modularity <= EPSILON

    # Update modularity
    modularity = final_modularity

    # Update the reduced graph
    @graph = g
  end

  info "Final modularity: #{modularity}"

  # Copy the final communities to the original graph
  original_graph.clusters.clear

  mapping.each do |vertex_id, community_id|
    original_graph
      .find_or_add_cluster(community_id)
      .add_vertex(original_graph.find_vertex(vertex_id))
  end
end