Module: GraphMatching::Graph::Weighted

Included in:
WeightedBigraph, WeightedGraph
Defined in:
lib/graph_matching/graph/weighted.rb

Overview

The ‘Weighted` module is mixed into undirected graphs to support edge weights. Directed graphs are not supported.

Data Structure

Weights are stored in a 2D array. The weight of an edge i,j is stored twice, at ‘[i]` and `[j]`.

Storing the weight twice wastes memory. A symmetrical matrix can be stored in a 1D array (bit.ly/1DMfLM3) However, translating the 2D coordinates into a 1D index marginally increases the cost of access, and this is a read-heavy structure, so maybe the extra memory is an acceptable trade-off. It’s also conceptually simpler, for what that’s worth.

If directed graphs were supported (they are not) this 2D array would be an obvious choice.

Algorithms which operate on weighted graphs are tightly coupled to this data structure due to optimizations.

Defined Under Namespace

Modules: ClassMethods

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.included(base) ⇒ Object 



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# File 'lib/graph_matching/graph/weighted.rb', line 28

def self.included(base)
  base.extend ClassMethods
  base.class_eval do
    attr_accessor :weight
  end
end

Instance Method Details

#init_weights ⇒ Object 



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# File 'lib/graph_matching/graph/weighted.rb', line 73

def init_weights
  @weight = Array.new(num_vertices) { |_| Array.new(num_vertices) }
end

#max_w ⇒ Object 



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# File 'lib/graph_matching/graph/weighted.rb', line 77

def max_w
  edges.map { |edge| w(edge.to_a) }.max
end

#set_w(edge, weight) ⇒ Object

‘set_w` sets a single weight. It not efficient, and is only provided for situations where constructing the entire graph with `.[]` is not convenient.



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# File 'lib/graph_matching/graph/weighted.rb', line 95

def set_w(edge, weight)
  if edge[0].nil? || edge[1].nil?
    fail ArgumentError, "Invalid edge: #{edge}"
  end
  unless weight.is_a?(Integer)
    fail TypeError, "Edge weight must be integer"
  end
  init_weights if @weight.nil?
  i, j = edge[0] - 1, edge[1] - 1
  fail "Edge not found: #{edge}" unless has_edge?(*edge)
  @weight[i] ||= []
  @weight[j] ||= []
  @weight[i][j] = weight
  @weight[j][i] = weight
end

#w(edge) ⇒ Object

Returns the weight of an edge. Accessing ‘#weight` is much faster, so this method should only be used where clarity outweighs performance.



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# File 'lib/graph_matching/graph/weighted.rb', line 84

def w(edge)
  i, j = edge
  fail ArgumentError, "Invalid edge: #{edge}" if i.nil? || j.nil?
  fail "Edge not found: #{edge}" unless has_edge?(*edge)
  init_weights if @weight.nil?
  @weight[i - 1][j - 1]
end