[jira] [Resolved] (RNG-154) Avoid infinite samples in the ZigguratNormalisedGaussianSampler

["Alex Herbert (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/RNG-154?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Alex Herbert resolved RNG-154.
------------------------------
    Fix Version/s: 1.4
       Resolution: Implemented

Added in commit:

482e4b5e371152f1341e64bbbf952392268e7993​_

> Avoid infinite samples in the ZigguratNormalisedGaussianSampler
> ---------------------------------------------------------------
>
>                 Key: RNG-154
>                 URL: https://issues.apache.org/jira/browse/RNG-154
>             Project: Commons RNG
>          Issue Type: Improvement
>          Components: sampling
>    Affects Versions: 1.3
>            Reporter: Alex Herbert
>            Priority: Minor
>             Fix For: 1.4
>
>
> The ZigguratNormalisedGaussianSampler uses the following method to sample the tail of the Gaussian:
> {code:java}
> // Note
> // R = 3.442619855899
> // 1 / R = 0.2905...
> double y;
> double x;
> do {
>     y = -Math.log(rng.nextDouble());
>     x = -Math.log(rng.nextDouble()) * ONE_OVER_R;
> } while (y + y < x * x);
> final double out = R + x;
> return hz > 0 ? out : -out;
> {code}
> In the unlikely event the RNG produces 0 then Math.log(0) is infinity. Two zeros in a row can result in an infinite sample for the tail. This is very unlikely but would be unexpected for a user of the library since a sample should be roughly within +/-3.
> Note that if the RNG is sampling from the 2^53 dyadic rationals in [0, 1) then the next value is:
> {code:java}
> Math.log(0x1.0p-53) == -36.7368005696771
> {code}
> Here the returned tail would be 3.44 + 36.7368005696771 / 3.44 = 14.11. This is very far from the extreme of infinity.
> To avoid infinity this can be fixed by:
>  1. Assuming the RNG is returning a value in [0, 1) and using Math.log(1.0 - rng.nextDouble())
>  2. Generating the double u from a long to ensure the value is in [0, 1) and using 1.0 - u.
>  



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