module 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
* examine_edge
* tree_edge
* back_edge
* forward_edge
* finish_vertex

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

included(base) click to toggle source
    # File lib/rgl/graph_visitor.rb
125 def self.included(base)
126   base.extend ClassMethods # when GraphVisitor is included into a class/module, add class methods as well
127 end
new(graph) click to toggle source

Create a new GraphVisitor on graph.

Calls superclass method
   # File lib/rgl/graph_visitor.rb
43 def initialize(graph)
44   super(graph)
45   reset
46 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/graph_visitor.rb
75 def attach_distance_map(map = Hash.new(0))
76   @distance_map = map
77 
78   # add distance map support to the current visitor instance
79   extend(DistanceMapSupport)
80 end
finished_vertex?(v) click to toggle source

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

   # File lib/rgl/graph_visitor.rb
56 def finished_vertex?(v)
57   @color_map[v] == :BLACK
58 end
reset() click to toggle source

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

   # File lib/rgl/graph_visitor.rb
50 def reset
51   @color_map = Hash.new(:WHITE)
52 end