From: "Michael W. Ryder" <_mwryder@...>
Date: 2007-04-17T17:20:09+09:00
Subject: Re: factorial in ruby

ara.t.howard@noaa.gov wrote:
> On Tue, 17 Apr 2007, Trans wrote:
> 
>>
>>
>> On Apr 16, 1:56 pm, "Jason Roelofs" <jameskil...@gmail.com> wrote:
>>> No and most likely not.
>>
>> Why not? Seems to me factorial is a pretty basic/common math function.
>> It would be nice if written in C for speed.
> 
> perhaps.  perhaps not:
> 
> 
> fortytwo: ~> cat a.rb
> #
> # gem install inline @ http://rubyforge.org/projects/rubyinline/
> #
>   require 'benchmark'
>   require 'rubygems'
>   require 'inline'
> #
> # setup two factorial methods, one in ruby and in using inline.  the inline
> # version is compiled on the fly using ruby2c and cc
> #
>   module Math
>     class << self
>       def factorial_ruby n
>         f = 1
>         n.downto(2){|x| f *= x }
>         f
>       end
> 
>       inline do |builder|
>         builder.c <<-'c'
>           int
>           factorial_inline (int max)
>           {
>             int i = max, result = 1;
>             while (i >= 2)
>               {
>                 result *= i--;
>               }
>             return result;
>           }
>         c
>       end
>     end
>   end
> #
> # check how fast they each are. we run many times to show the differences.
> #
>   n = 4242
>   Benchmark.bm do |x|
>     x.report 'factorial_ruby' do
>       n.times{ Math.factorial_ruby 42 }
>     end
> 
>     x.report 'factorial_inline' do
>       n.times{ Math.factorial_inline 42 }
>     end
>   end
> #
> # now show accuracy.  how many bits is a signed int again?
> #
>   42.times do |i|
>     a, b = Math.factorial_ruby(i), Math.factorial_inline(i)
>     p [i, a, b]
>     break unless a == b
>   end
> #
> # check this out.  automatic bigint boxing.
> #
>   p Math.factorial_ruby(42)
>   p Math.factorial_ruby(42).class
> 
> 
> 
> 
> fortytwo: ~> ruby a.rb
>       user     system      total        real
> factorial_ruby  0.550000   0.010000   0.560000 (  0.767810)
> factorial_inline  0.010000   0.000000   0.010000 (  0.003574)
> [0, 1, 1]
> [1, 1, 1]
> [2, 2, 2]
> [3, 6, 6]
> [4, 24, 24]
> [5, 120, 120]
> [6, 720, 720]
> [7, 5040, 5040]
> [8, 40320, 40320]
> [9, 362880, 362880]
> [10, 3628800, 3628800]
> [11, 39916800, 39916800]
> [12, 479001600, 479001600]
> [13, 6227020800, -215430144]
> 1405006117752879898543142606244511569936384000000000
> Bignum
> 
> 
> 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.
> 
> kind regards.
> 
> 
> -a

Is it possible to use long integers with rubyinline and would it make a 
difference with the results?
Also where would I find more documentation on rubyinline?  I tried to 
use your example program and got the gem installed but it then 
complained about an environment variable.  I don't know if it is looking 
for a specific C compiler or what, I have Borland C installed and in the 
path.