fibonnaci.rb

class Numeric
  def fibonacci()
    if self < 0 
      raise "Cannot Fibonacci a negative number"
    elsif [0,1].include? self 
      self
    else
      ar = [0,1]
      2.upto(self) do |n|
        ar << ar[0] + ar[1]
        ar = ar[1,2]
      end
      ar[1]
    end
  end
end