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.
tsort_each_node is used to iterate for all nodes over a graph.
tsort_each_child is used to iterate for child nodes of a given node.
The equality of nodes are defined by eql? and hash since TSort uses Hash internally.
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 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')
‘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.
Tarjan, “Depth First Search and Linear Graph Algorithms”,
SIAM Journal on Computing, Vol. 1, No. 2, pp. 146-160, June 1972.
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 177
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
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 198
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
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 162
def strongly_connected_components
result = []
each_strongly_connected_component {|component| result << component}
result
end
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 133
def tsort
result = []
tsort_each {|element| result << element}
result
end
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 147
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
Should be implemented by a extended class.
tsort_each_child is used to iterate for child nodes of node.
# File tsort.rb, line 239
def tsort_each_child(node) # :yields: child
raise NotImplementedError.new
end
Should be implemented by a extended class.
tsort_each_node is used to iterate for all nodes over a graph.
# File tsort.rb, line 230
def tsort_each_node # :yields: node
raise NotImplementedError.new
end