Hi.
Le jeu. 9 mai 2019 à 13:31, Alex Herbert <alex.d.herb...@gmail.com> a écrit :
>
> The Middle Square Weyl Sequence (MSWS) generator uses an internal Weyl
> sequence [1] to create randomness. This is basically a linear increment added
> to a sum that will eventually wrap (due to overflow) to restart at the
> beginning. The MSWS paper recommends an increment with a high number of
> different bits set in a random pattern across the 64-bit of the long. The
> paper recommends using a permutation of 8 from the 16 hex digits for the
> upper and lower 32-bits.
>
> The source code for the MSWS provides a routine that generates a permutation.
> Unfortunately:
>
> - The code is GPL 3 so restricting it from use under the Apache licence
> (without jumping through some hoops)
> - The algorithm is a simple rejection method that suffers from high rejection
> probability when approaching 8 digits already chosen
>
> I have created an alternative faster implementation for use when seeding the
> MSWS generator. However it may be a function to be reused in other places.
>
> The question is where to put this utility function. It requires a source of
> randomness to create the permutation. It has the following signature:
>
> /**
> * Creates an {@code int} containing a permutation of 8 hex digits chosen
> from 16.
> *
> * @param rng Source of randomness.
> * @return Hex digit permutation.
> */
> public static int createIntHexPermutation(UniformRandomProvider rng);
>
> Likewise:
>
> /**
> * Creates a {@code long} containing a permutation of 8 hex digits chosen
> from 16 in
> * the upper and lower 32-bits.
> *
> * @param rng Source of randomness.
> * @return Hex digit permutation.
> */
> public static long createLongHexPermutation(UniformRandomProvider rng);
>
> Options:
>
> - Put it as a package private function inside the MSWS generator to be used
> only when creating this generator. Package private allows unit testing the
> algorithm does provides the random permutation 16-choose-8
> - Put it as a helper function in org.apache.commons.rng.core.util
- In "SeedFactory" (?).
For MSWS ("core" module), the increment would be an argument to the constructor
(allowing the user to shoot himself in the foot, like when passing a
bad seed), and
"RandomSource" ("simple" module) would offer to provide an instance
for which the
increment was computed according to the recommendation.
Regards,
Gilles
>
> Note that the function is an alternative to that used by the SplittableRandom
> to create an increment for its own Weyl sequence. That uses a fast method
> that is prone to weak randomness in potential output.
>
> If other methods will potentially be added to the helper class a more generic
> name should be used. Possibilities are:
>
> PermutationUtils
> SequenceUtils
> IncrementUtils
> SeedUtils
>
> Given that the method is for seeding Weyl sequences then I am favouring
> SeedUtils.
>
>
> [1] https://en.wikipedia.org/wiki/Weyl_sequence
> <https://en.wikipedia.org/wiki/Weyl_sequence>
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