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  • tsort.rb

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TSort

TSort implements topological sorting using Tarjan's algorithm for strongly connected components.

TSort is designed to be able to be used with any object which can be interpreted as a directed graph.

TSort requires two methods to interpret an object as a graph, #tsort_each_node and tsort_each_child.

The equality of nodes are defined by eql? and hash since TSort uses Hash internally.

A Simple Example

The following example demonstrates how to mix the TSort module into an existing class (in this case, Hash). Here, we’re treating each key in the hash as a node in the graph, and so we simply alias the required tsort_each_node method to Hash’s each_key method. For each key in the hash, the associated value is an array of the node’s child nodes. This choice in turn leads to our implementation of the required tsort_each_child method, which fetches the array of child nodes and then iterates over that array using the user-supplied block.

require 'tsort'

class Hash
  include TSort
  alias tsort_each_node each_key
  def tsort_each_child(node, &block)
    fetch(node).each(&block)
  end
end

{1=>[2, 3], 2=>[3], 3=>[], 4=>[]}.tsort
#=> [3, 2, 1, 4]

{1=>[2], 2=>[3, 4], 3=>[2], 4=>[]}.strongly_connected_components
#=> [[4], [2, 3], [1]]

A More Realistic Example

A very simple `make’ like tool can be implemented as follows:

require 'tsort'

class Make
  def initialize
    @dep = {}
    @dep.default = []
  end

  def rule(outputs, inputs=[], &block)
    triple = [outputs, inputs, block]
    outputs.each {|f| @dep[f] = [triple]}
    @dep[triple] = inputs
  end

  def build(target)
    each_strongly_connected_component_from(target) {|ns|
      if ns.length != 1
        fs = ns.delete_if {|n| Array === n}
        raise TSort::Cyclic.new("cyclic dependencies: #{fs.join ', '}")
      end
      n = ns.first
      if Array === n
        outputs, inputs, block = n
        inputs_time = inputs.map {|f| File.mtime f}.max
        begin
          outputs_time = outputs.map {|f| File.mtime f}.min
        rescue Errno::ENOENT
          outputs_time = nil
        end
        if outputs_time == nil ||
           inputs_time != nil && outputs_time <= inputs_time
          sleep 1 if inputs_time != nil && inputs_time.to_i == Time.now.to_i
          block.call
        end
      end
    }
  end

  def tsort_each_child(node, &block)
    @dep[node].each(&block)
  end
  include TSort
end

def command(arg)
  print arg, "\n"
  system arg
end

m = Make.new
m.rule(%w[t1]) { command 'date > t1' }
m.rule(%w[t2]) { command 'date > t2' }
m.rule(%w[t3]) { command 'date > t3' }
m.rule(%w[t4], %w[t1 t3]) { command 'cat t1 t3 > t4' }
m.rule(%w[t5], %w[t4 t2]) { command 'cat t4 t2 > t5' }
m.build('t5')

Bugs

  • ‘tsort.rb’ is wrong name because this library uses Tarjan’s algorithm for strongly connected components. Although ‘strongly_connected_components.rb’ is correct but too long.

References

    1. Tarjan, “Depth First Search and Linear Graph Algorithms”,

SIAM Journal on Computing, Vol. 1, No. 2, pp. 146-160, June 1972.

Public Instance Methods

each_strongly_connected_component() click to toggle source

The iterator version of the strongly_connected_components method. obj.each_strongly_connected_component is similar to obj.strongly_connected_components.each, but modification of obj during the iteration may lead to unexpected results.

each_strongly_connected_component returns nil.

 
               # File tsort.rb, line 178
def each_strongly_connected_component # :yields: nodes
  id_map = {}
  stack = []
  tsort_each_node {|node|
    unless id_map.include? node
      each_strongly_connected_component_from(node, id_map, stack) {|c|
        yield c
      }
    end
  }
  nil
end
            
each_strongly_connected_component_from(node, id_map={}, stack=[]) click to toggle source

Iterates over strongly connected component in the subgraph reachable from node.

Return value is unspecified.

each_strongly_connected_component_from doesn't call tsort_each_node.

 
               # File tsort.rb, line 199
def each_strongly_connected_component_from(node, id_map={}, stack=[]) # :yields: nodes
  minimum_id = node_id = id_map[node] = id_map.size
  stack_length = stack.length
  stack << node

  tsort_each_child(node) {|child|
    if id_map.include? child
      child_id = id_map[child]
      minimum_id = child_id if child_id && child_id < minimum_id
    else
      sub_minimum_id =
        each_strongly_connected_component_from(child, id_map, stack) {|c|
          yield c
        }
      minimum_id = sub_minimum_id if sub_minimum_id < minimum_id
    end
  }

  if node_id == minimum_id
    component = stack.slice!(stack_length .. -1)
    component.each {|n| id_map[n] = nil}
    yield component
  end

  minimum_id
end
            
strongly_connected_components() click to toggle source

Returns strongly connected components as an array of arrays of nodes. The array is sorted from children to parents. Each elements of the array represents a strongly connected component.

 
               # File tsort.rb, line 163
def strongly_connected_components
  result = []
  each_strongly_connected_component {|component| result << component}
  result
end
            
tsort() click to toggle source

Returns a topologically sorted array of nodes. The array is sorted from children to parents, i.e. the first element has no child and the last node has no parent.

If there is a cycle, TSort::Cyclic is raised.

 
               # File tsort.rb, line 134
def tsort
  result = []
  tsort_each {|element| result << element}
  result
end
            
tsort_each() click to toggle source

The iterator version of the tsort method. obj.tsort_each is similar to obj.tsort.each, but modification of obj during the iteration may lead to unexpected results.

tsort_each returns nil. If there is a cycle, TSort::Cyclic is raised.

 
               # File tsort.rb, line 148
def tsort_each # :yields: node
  each_strongly_connected_component {|component|
    if component.size == 1
      yield component.first
    else
      raise Cyclic.new("topological sort failed: #{component.inspect}")
    end
  }
end
            
tsort_each_child(node) click to toggle source

Should be implemented by a extended class.

tsort_each_child is used to iterate for child nodes of node.

 
               # File tsort.rb, line 240
def tsort_each_child(node) # :yields: child
  raise NotImplementedError.new
end
            
tsort_each_node() click to toggle source

Should be implemented by a extended class.

tsort_each_node is used to iterate for all nodes over a graph.

 
               # File tsort.rb, line 231
def tsort_each_node # :yields: node
  raise NotImplementedError.new
end
            

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