Memoising Integer#factorial in Ruby

factorial.rb

# Memoising factorial method.
class Integer
  class << self
    def factorial_results(number)
      raise "number has to be greater than 0" if number < 0

      @factorial_results ||= begin
        results = Hash.new do |h, k|
          h[k] = k * @factorial_results[k - 1] 
        end

        results[0] = 1 # Need a base-line.

        results
      end

      @factorial_results[number]
    end
  end

  def factorial
    Integer.factorial_results self
  end
end

p 0.factorial #=> 1
p 1.factorial #=> 1
p 5.factorial #=> 120
p 35.factorial #=> 10333147966386144929666651337523200000000

# Perfectly acceptable way to do it!
p 35.downto(1).reduce {|m, i| m * i } #=> 10333147966386144929666651337523200000000