From Guy:
You are right that the comment in the JEP was a little loose, and that the
implementation(s) of the split/splits methods could in principle draw random
values from a RandomGenerator that is not itself splittable. There might even
be applications for such functionality.
However, we chose not to support that more general functionality for a fairly
subtle reason: there are concerns that if a PNRG is less than perfect, using it
as a source of entropy for seeding a PRNG that uses a different algorithm might
result in unexpected correlations that could drastically reduce the quality of
the output of the new PRNG instance. Restricting the implementation of the
split method so that it always constructs a new generator of the same class has
allowed us the opportunity to test each class separately and independently for
quality of output when the split method is used.
Programmers who know what they are doing can get the effect of the more general
split functionality by calling constructors explicitly. This forces them to
think about the specific algorithms. For example:
RandomGenerator r = new SplittableRandom();
RandomGenerator g = new L64X256MixRandom(r.nextLong(), r.nextLong(),
r.nextLong(), r.nextLong(), r.nextLong(), r.nextLong());
The programmer may then reason that, in this specific case, the fact that the
period of r is only 2^64 implies that the last four values generated cannot all
be zero, which is one desideratum for g to have good quality. (The programmer
would also have the responsibility to decide more generally whether this
particular combination of entropy source and generated instance will in fact
have good quality.)
—Guy Steele
> On Jul 13, 2021, at 11:52 AM, Michael Bien <[email protected]> wrote:
>
> just wanted to add that the JEP says: "SplittableRandomGenerator extends
> RandomGenerator and also provides
> methods named split and splits. Splittability allows the user to spawn a new
> RandomGenerator from an existing RandomGenerator that will generally produce
> statistically independent results."
>
> which adds to my suspicion that this might be a api bug.
>
> -michael
>
> On 13.07.21 14:45, Michael Bien wrote:
>> Hello,
>>
>> i was wondering if SplittableGenerator.split(SplittableGenerator source) is
>> missing out on a potential usecase when the source itself is not splittable.
>> The implementation looks like it only requires basic calls which could be
>> also provided by the RandomGenerator interface as source.
>>
>> best regards,
>>
>> michael
>>
>
