Enumerable#combinations - returns an array of arrays with all permutations of enumerable (order doesn't matter)

combinations.rb

module Enumerable
  def combinations
    case self.size
    when 0 then []
    when 1 then self.to_a
    else 
      (1..self.size).reduce([]) { |combined, choose| combined + self.combination(choose).to_a }
    end
  end
end

irb

ree-1.8.7-2011.03 :001 > ["A", "B", "C"].combinations
 => [["A"], ["B"], ["C"], ["A", "B"], ["A", "C"], ["B", "C"], ["A", "B", "C"]]