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