factorial lookup table

factorial.rb

##
# Factorial[positive_integer] computes and memoizes the 
# factorial of a given positive integer.
module Factorial
  class << self
    def [](n)
      @factorial_table ||= Hash.new { |hash, key| 
        hash[key] = (1..key).reduce(:*) || 1
      }
      @factorial_table[n]
    end
  end
end

require 'benchmark'

a = 13
b = 37
r = 100000

Benchmark.bm do |x|
  x.report do 
    r.times do
      (a..b).each {|i| (1..i).reduce(:*) }
    end
  end
  x.report do 
    r.times do
      (a..b).each {|i| Factorial[i] }
    end
  end
end

# My Results
# ==========
#
#     user     system      total        real
# 9.180000   0.020000   9.200000 ( 10.834001)
# 0.710000   0.010000   0.720000 (  0.706927)
#
# You should probably run the benchmark yourself to get relevant data.