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.


Add this line to your application's Gemfile:

gem 'tsort'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install tsort


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)

{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]]


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.


Bug reports and pull requests are welcome on GitHub at https://github.com/ruby/tsort.