Files

RGL::GraphVisitor

Module GraphVisitor defines the BGL BFS Visitor Concept).

Visitors provide a mechanism for extending an algorithm (i.e., for customizing what is done at each step of the algorithm). They allow users to insert their own operations at various steps within a graph algorithm.

Graph algorithms typically have multiple event points where one may want to insert a call-back. Therefore, visitors have several methods that correspond to the various event points. Each algorithm has a different set of event points. The following are common to both DFS and BFS search.

* examine_vertex
* finish_vertex
* examine_edge
* tree_edge
* back_edge
* forward_edge

These methods are all called handle_* and can be set to appropriate blocks, using the methods set_*_event_handler, which are defined for each event mentioned above.

As an alternative, you can also override the handle_* methods in a subclass, to configure the algorithm (as an example, see TarjanSccVisitor).

During a graph traversal, vertices are colored using the colors :GRAY (when waiting) and :BLACK when finished. All other vertices are :WHITE. The color_map is also maintained in the visitor.

Attributes

color_map[R]

Public Class Methods

def_event_handler(m) click to toggle source

Visitor Event Points

# File lib/rgl/traversal.rb, line 127
def self.def_event_handler (m)
  params = m =~ /edge/ ? "u,v" : "u"
  self.class_eval %{
    def handle_#{m} (#{params})
      @#{m}_event_handler.call(#{params}) if defined? @#{m}_event_handler
    end

    def set_#{m}_event_handler (&b)
      @#{m}_event_handler = b
    end
  }
end
distance_to_root(v) click to toggle source

Answer the distance to the start vertex.

# File lib/rgl/traversal.rb, line 112
def distance_to_root (v)
  @dist_map[v]
end
handle_tree_edge(u, v) click to toggle source
# File lib/rgl/traversal.rb, line 105
def handle_tree_edge (u, v)
  super
  @dist_map[v] = @dist_map[u] + 1
end
new(graph) click to toggle source

Create a new GraphVisitor on graph.

# File lib/rgl/traversal.rb, line 74
def initialize (graph)
  super graph
  reset
end

Public Instance Methods

attach_distance_map(map = Hash.new(0)) click to toggle source

Attach a map to the visitor which records the distance of a visited vertex to the start vertex.

This is similar to BGLs distance_recorder.

After the distance_map is attached, the visitor has a new method distance_to_root, which answers the distance to the start vertex.

# File lib/rgl/traversal.rb, line 100
def attach_distance_map (map = Hash.new(0))
  @dist_map = map

  class << self

    def handle_tree_edge (u, v)
      super
      @dist_map[v] = @dist_map[u] + 1
    end

    # Answer the distance to the start vertex.

    def distance_to_root (v)
      @dist_map[v]
    end

  end       # class
end
finished_vertex?(v) click to toggle source

Returns true if vertex v is colored :BLACK (i.e. finished).

# File lib/rgl/traversal.rb, line 87
def finished_vertex? (v)
  @color_map[v] == :BLACK
end
reset() click to toggle source

Mark each vertex unvisited (i.e. :WHITE)

# File lib/rgl/traversal.rb, line 81
def reset
  @color_map = Hash.new(:WHITE)
end

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