[Newbies] LargeIntegers atRandom more analysis

Jerome Peace peace_the_dreamer at yahoo.com
Sat Aug 9 02:34:09 UTC 2008

LargeIntegers atRandom more analysis

Hi Tim, Hi Randal

Nothing like a little coding to go

>From confident Ignorance:
To humble Understanding:

Playing and trying to debug my notion of large integer randoms,
 show that what I had proposed as a decomposition for the problem
 had bigger problems than I first thought. 
I did not get far enough to blame the generator. 
But ran into problems with convincing myself
 that with the decomposition I could get an evenly distributed probility
 for numbers from 1 to large n.

I still believe that LargeInteger has the responsibility for giving back a good random.

Along the way, or actually while I slept,
 I realized that the limitation is not necessarily in the generator
 but in the combining of the floating point and large integer in nextInt: .

 This is a problem.

24 factorial + 1000000 = (24 factorial * 1.0) 
will answer true. 

( 24 factorial - (24 factorial * 1.0 ) asInteger ) = 0
will answer false.

There is a truncation difficulty with floats. 

I say difficulty rather than bug because it is not spec'ed to work otherwise.

With this knowledge some relief can be had by gening up a large random number
and treating it as a fraction.

So a step towards the solution would be to define:
   atRandom: aGenerator
	"Answer a random integer from 1 to self picked from aGenerator.
	To handle Large integers the arithmetic must use integer arithmetic.
	This will work if the generator is up to multiple uses."

| nHigits numerator radix maxSigBits nextLargeInteger  denominator |

maxSigBits := 53 . " aGenerator class maxSignificantBits. "
radix := radix :=  ( 2 raisedTo:  maxSigBits ) .
nHigits :=
 (self  highBit // maxSigBits ) + 1 .
numerator := 0 .

nextLargeInteger := [ ( aGenerator next * radix ) asInteger ] .

nHigits timesRepeat: [ numerator := numerator * radix + nextLargeInteger value ] .

denominator := radix raisedTo: nHigits .

^ self * numerator // denominator + 1 .

This leaves the problem of the usable/reusalbe generator to be solved. 
I would also want a way for the generator classes to return maxSigBits.

The pleasing thing about the above code is that with the method in place
I did not have to change anything else to get atRandom to work better. 
So #atRandom: proves itself a useful hook.

This solution passes the oddity test. That is a very low threshold test.
This is also a fragile solution. 
There are probably some places that it fails. 
It would be interesting to find a test that it couldn't pass.
Then use that to vet the next iteration of the fix.

Yours in curiosity and service, --Jerome Peace


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