http://codekata.pragprog.com/2007/01/kata_eighteen_t.html

dependencies.rb

require 'set'

module Dependencies
  class Dependencies
    def initialize
      @graph = Hash.new([])
    end

    def add_direct(dependent, dependencies)
      @graph[dependent] += dependencies
    end

    def dependencies_for(dependent)
      resolve(Set.new([dependent]),
              Set.new(@graph[dependent])).to_a.sort
    end

    private

    def resolve(resolved_set, to_be_resolved_set)
      result_set = to_be_resolved_set.dup

      to_be_resolved_set.each do |dependency|
        next if resolved_set.include? dependency

        result_set.merge(@graph[dependency])
        resolved_set.add(dependency)
        result_set = resolve(resolved_set, result_set)
      end

      result_set
    end
  end
end

dependencies_spec.rb

require './dependencies'

describe Dependencies::Dependencies do
  describe 'add_direct' do
    it 'sets dependencies' do
      subject.add_direct('A', %w{ B C } )
      subject.dependencies_for('A').should == %w{ B C }
    end

    it 'adds to existing dependencies' do
      subject.add_direct('A', %w{ B } )
      subject.add_direct('A', %w{ C } )
      subject.dependencies_for('A').should == %w{ B C }
    end
  end

  describe 'dependencies_for' do
    it 'returns a list of dependencies' do
      subject.add_direct('B', %w{ C E } )
      subject.dependencies_for('B').should == %w{ C E }
    end

    it 'returns a list of transitive dependencies' do
      subject.add_direct('A', %w{ B C } )
      subject.add_direct('B', %w{ C E } )
      subject.dependencies_for('A').should == %w{ B C E }
    end

    it 'returns an exhaustive list of transitive dependencies' do
      subject.add_direct('A', %w{ B C } )
      subject.add_direct('B', %w{ C E } )
      subject.add_direct('E', %w{ G } )
      subject.dependencies_for('A').should == %w{ B C E G }
    end

    it 'returns a list of circular dependencies' do
      subject.add_direct('A', %w{ B } )
      subject.add_direct('B', %w{ C } )
      subject.add_direct('C', %w{ A } )
      subject.dependencies_for('A').should == %w{ A B C }
      subject.dependencies_for('B').should == %w{ A B C }
      subject.dependencies_for('C').should == %w{ A B C }
    end
  end

  describe 'integration' do
    it 'works like http://codekata.pragprog.com/2007/01/kata_eighteen_t.html' do
      subject.add_direct('A', %w{ B C } )
      subject.add_direct('B', %w{ C E } )
      subject.add_direct('C', %w{ G   } )
      subject.add_direct('D', %w{ A F } )
      subject.add_direct('E', %w{ F   } )
      subject.add_direct('F', %w{ H   } )

      subject.dependencies_for('A').should == %w{ B C E F G H }
      subject.dependencies_for('B').should == %w{ C E F G H }
      subject.dependencies_for('C').should == %w{ G }
      subject.dependencies_for('D').should == %w{ A B C E F G H }
      subject.dependencies_for('E').should == %w{ F H }
      subject.dependencies_for('F').should == %w{ H }
    end
  end
end