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From "Dennis Hendriks (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MATH-720) RandomDataImpl uses both JDKRandomGenerator and Well19937c, while contract/javadoc only specifies Well19937c.
Date Wed, 07 Dec 2011 14:06:40 GMT

     [ https://issues.apache.org/jira/browse/MATH-720?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Dennis Hendriks updated MATH-720:
---------------------------------

    Attachment: TestRandom.java

TestRandom.java: shows the problem, as described in the issue description.
                
> RandomDataImpl uses both JDKRandomGenerator and Well19937c, while contract/javadoc only
specifies Well19937c.
> -------------------------------------------------------------------------------------------------------------
>
>                 Key: MATH-720
>                 URL: https://issues.apache.org/jira/browse/MATH-720
>             Project: Commons Math
>          Issue Type: Bug
>    Affects Versions: 3.0
>            Reporter: Dennis Hendriks
>              Labels: random, seed
>         Attachments: TestRandom.java
>
>
> See attached unit test. I create a distribution, sample it (not printed), set the seed
to 0, and then print the next sample. I also create the same distribution again, set the seed
to 0, and then print the next sample. I expect the same sample, as in both cases the seed
was set to 0, just before sampling. I however get this output:
> {code}
> 5
> 4
> {code}
> The problem is in the org.apache.commons.math.random.RandomDataImpl class:
>  - The RandomDataImpl(RandomGenerator rand) constructor states in javadoc: "@param rand
the source of (non-secure) random data (may be null, resulting in default JDK-supplied generator)"
>  - reSeed(long seed) method does: if (rand == null) rand = new JDKRandomGenerator();
>  - reSeed() method does: if (rand == null) rand = new JDKRandomGenerator();
>  - class javadoc states: "If no <code>RandomGenerator</code> is provided
in the constructor, the default is to use a Well19937c generator."
>  - getRan() does: if (rand == null) rand = new Well19937c(System.currentTimeMillis()
+ System.identityHashCode(this));
>  - getRan() states in javadoc: "Creates and initializes a default generator if null.
Uses a Well19937c generator with System.currentTimeMillis() + System.identityHashCode(this))
as the default seed."
> It seems that only Well19937c should be used, but the constructor javadoc, and the implementation
of the reSeed methods was not updated to match this. I think the partial changes were done
in MATH-701, more specifically, in commit [1197626] (and related commit [1197716]).

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