From: Florian Frank <flori@...>
Date: 2007-04-17T19:32:26+09:00
Subject: Re: factorial in ruby

ara.t.howard@noaa.gov wrote:
> not the integer wrap from the c version - this is a case where c gets 
> you crap
> answers real quick.  you need more that just c, but also an arbitrary 
> precision
> arithmitic library to do factorial fast.

Caching can avoid unnecessary multiplications (while using Bignums), if 
one wants to compute a lot of factorials:

module Factorial
   module_function

   @@cache = [ 1 ]

   def fact(n)
     raise ArgumentError, "n has to be >= 0" if n < 0
     @@cache.size.upto(n) { |i| @@cache[i] =  i * @@cache[i - 1] }
     @@cache[n]
   end
end

if $0 == __FILE__
   require 'test/unit'
   class TestFactorial < Test::Unit::TestCase
     include Factorial
     def test_fact
       assert_raises(ArgumentError) { fact(-1) }
       assert_equal 1, fact(0)
       assert_equal 1, fact(1)
       assert_equal 2, fact(2)
       assert_equal 6, fact(3)
       assert_equal 24, fact(4)
       assert_equal 120, fact(5)
       assert_equal 3628800, fact(10)
     end
   end
end

This should get faster, the more factorials you want to compute.

-- 
Florian Frank