# File lib/facets/core/enumerable/permutation.rb, line 29 def permutation_number(original_array=self.to_a.sort) arr = to_a m = 1 v = 0 last_indicies = Hash.new(0) original_array.each_with_index {|x,i| next if i==0 m *= i done = original_array[0..i] v += m * arr.select {|y| done.include?(y)}.rindex(x) } v end