From: Michael Morin <uzimonkey@...>
Date: 2008-08-18T07:13:15+09:00
Subject: Re: [QUIZ] Not So Random (#173)

brabuhr@gmail.com wrote:
> On Fri, Aug 15, 2008 at 11:33 AM, Matthew Moss <matthew.moss@gmail.com> wrote:
>> ## Not So Random (#173)
>>
>> The first part is to create a wrapper around the random number
>> generator `rand`, supporting the appropriate parameter (i.e. the
>> parameter intrinsic to `rand`). Your wrapper should implement two
>> additional functions: `next` which returns the next random number, and
>> `reset` which restarts the sequence.
>>
>> The second part is a little trickier: creating multiple, concurrent,
>> _reproducible_ sequences of pseudorandom numbers isn't so easy. As
>> convenient as the built-in generator is, there is only one seed.
> 
> Since a single Ruby process only has one PRNG, slave off an extra
> process for each Random class:
> 
> #!/usr/bin/env ruby
> 
> require 'rubygems'
> require 'slave'
> 
> class Random
>   def initialize(ceiling, seed = nil)
>     @ceiling = ceiling
>     @seed = seed || rand(2112)
>     @slave = Slave::new{
>       lambda {|reset|
>         if reset
>           srand(@seed)
>         else
>           rand(@ceiling)
>         end
>       }
>     }
>     self.reset
>   end
> 
>   def next
>     @slave.object.call(false)
>   end
> 
>   def reset
>     @slave.object.call(true)
>   end
> end
> 
> require 'test/unit'
> 
> class RandomTest < Test::Unit::TestCase
>   def test_001
>     x = Random.new(100, 2112)
>     assert_equal( [38,  1,  5, 57, 11], Array.new(5) { x.next })
>     assert_equal( [ 1, 31,  3, 56, 14], Array.new(5) { x.next })
>     x.reset
>     assert_equal( [38,  1,  5, 57, 11], Array.new(5) { x.next })
>   end
> 
>   def test_002
>     x = Random.new(100, 2112)
>     assert_equal( [38,  1,  5, 57, 11], Array.new(5) { x.next })
>     y = Random.new(100, 1221)
>     assert_equal( [ 5, 99, 88, 48, 86], Array.new(5) { y.next })
>     x.reset
>     assert_equal( [38,  1,  5, 57, 11], Array.new(5) { x.next })
>     assert_equal( [34, 28, 72, 77, 87], Array.new(5) { y.next })
>     assert_equal( [ 1, 31,  3, 56, 14], Array.new(5) { x.next })
>   end
> end
> 
> 
> Loaded suite foo
> Started
> ..
> Finished in 0.251163 seconds.
> 
> 2 tests, 8 assertions, 0 failures, 0 errors
> 

This is probably the cleanest solution you can get without implementing 
your own PRNG or mucking around in C code to copy the internal state of 
the PRNG.  As long as it uses fork, it shouldn't eat up RAM or anything. 
  The popen solution probably would if a large number of Random objects 
were created.

-- 
Michael Morin
Guide to Ruby
http://ruby.about.com/
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