From: Mark Hubbart <discord@...>
Date: 2004-09-16T02:49:47+09:00
Subject: Re: Bignum Troubles


On Sep 15, 2004, at 9:52 AM, James Edward Gray II wrote:

> On Sep 15, 2004, at 10:39 AM, Robert Klemme wrote:
>
>> Err, can't you just compare them?  If it only *seems* that values 
>> differ
>> then there could be any number of other reasons for the differences 
>> you
>> observe.
>
> My project is a simulation.  I have a "dump" from a correct simulator. 
>  I can compare my dump with theirs.
>
> Part of the simulation includes random number generation.  To solve 
> this, the spec gives you their random number generation system to use 
> for the purposes of testing.  Dumps do not include random numbers that 
> were generated.
>
> My simulation agrees with theirs for 938 turns.  At 939, we diverge.  
> The step we part on is 90% about random number generation and I've 
> tried to verify everything else to the best of my ability.
>
> I am "guessing", but I think it's a good guess.
>
>> Bignum ist just integer numbers.  AFAIK there is no such thing as a
>> precision for this - all digits are correct.
>
> Well, when you divide two Integers you can get a decimal, right?  My 
> understanding was that "big math" libraries cut off the division at 
> some point, to keep from choking on repeating decimal results.  Please 
> correct me, if I misunderstand.
>
> Here's a look at my math:
>
> 	def initialize()
> 		# non-related setup work here
>
> 		@random_seeds = [ 12345 ]
> 	end
>
> 	# and later
>
> 	def test_random(limit)
> 		until @random_seeds.size == 5
> 			@random_seeds.push @random_seeds[-1] * 22695477 + 1
> 		end
> 		@random_seeds.shift
> 		
> 		return ((@random_seeds[-1] / 65536) % 16384) % limit
> 	end
>
> test_random() is written to the provided specification, returning an 
> integer between 0 and limit.
>
> The spec include the first 100 answers, which I agree with.  In fact, 
> we agree much farther than that.  As I said it takes 939 turns for us 
> to disagree and the random number generator can be used many times in 
> a turn.
>
> I believe we see above that @random_seeds should always contain 
> Bignums (after the initial few Fixnums, of course).
>
> What you've made me wonder is, what stages does the last line go 
> through?  Does that first division produce a Float?  If so, that's 
> probably my error right there.

That first division should always produce an Integer (either Fixnum or 
Bignum). Are you expecting to get a decimal or a float somewhere? I 
don't see anywhere in this code where you will get one... It all looks 
like integer math to me; they should be precise, but with any decimal 
part truncated.

HTH,
Mark

>
> Thanks for your help.
>
> James Edward Gray II
>
>