Array#permute in ruby

ruby1.8.6-permutations.rb

class Array
  def permute
    return Array(first).map {|e| [e] } if length == 1
    Array(first).inject([]) {|a,e|
      Array(self[1..-1]).permute.each {|p|
        a << [e] + p
      }
      a
    }
  end
end


a = []
a.permute                                          # => []
a.permute == []                                    # => true
a = [[1, 2]]
a.permute                                          # => [[1], [2]]
a.permute == [[1], [2]]                            # => true
a = [[4, 5]]
a.permute                                          # => [[4], [5]]
a.permute == [[4], [5]]                            # => true
a = [[1], [2]]
a.permute                                          # => [[1, 2]]
a.permute == [[1,2]]                               # => true
a = [[1, 2], [4]]
a.permute                                          # => [[1, 4], [2, 4]]
a.permute == [[1,4], [2,4],]                       # => true
a = [[1, 2], [4]]
a.permute                                          # => [[1, 4], [2, 4]]
a.permute == [[1,4], [2,4],]                       # => true
a = [[1, 2], [4, 5]]
a.permute                                          # => [[1, 4], [1, 5], [2, 4], [2, 5]]
a.permute == [[1,4], [1,5], [2,4], [2,5]]          # => true
a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
a.permute                                          # => [[1, 4, 7], [1, 4, 8], [1, 4, 9], [1, 5, 7], [1, 5, 8], [1, 5, 9], [1, 6, 7], [1, 6, 8], [1, 6, 9], [2, 4, 7], [2, 4, 8], [2, 4, 9], [2, 5, 7], [2, 5, 8], [2, 5, 9], [2, 6, 7], [2, 6, 8], [2, 6, 9], [3, 4, 7], [3, 4, 8], [3, 4, 9], [3, 5, 7], [3, 5, 8], [3, 5, 9], [3, 6, 7], [3, 6, 8], [3, 6, 9]]