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From "Gilles (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (RNG-79) Benchmark methods for producing nextDouble
Date Mon, 04 Mar 2019 06:57:00 GMT

    [ https://issues.apache.org/jira/browse/RNG-79?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16783036#comment-16783036
] 

Gilles commented on RNG-79:
---------------------------

bq. half of all possible double values

It's reason enough to apply the fix.
Moreover, the operators intervening in the conversion being the same, it would be surprising
to see a difference.

bq. This is the method employed in JDK 1.7 ThreadLocalRandom.

Yet another reason.
It is [not documented|https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadLocalRandom.html#nextDouble--].
 Does it mean that the implementation is different in {{java.util.Random}}?


> Benchmark methods for producing nextDouble
> ------------------------------------------
>
>                 Key: RNG-79
>                 URL: https://issues.apache.org/jira/browse/RNG-79
>             Project: Commons RNG
>          Issue Type: Task
>          Components: core
>    Affects Versions: 1.3
>            Reporter: Alex D Herbert
>            Assignee: Alex D Herbert
>            Priority: Minor
>
> Benchmark the speed of methods for producing a {{double}} from a {{long}}:
> {code:java}
> long v;
> double d1 = Double.longBitsToDouble(0x3FFL << 52 | v >>> 12) - 1.0;
> double d2 = (v >>> 12) * 0x1.0p-52d; // 1.0 / (1L << 52);
> double d3 = (v >>> 11) * 0x1.0p-53d; // 1.0 / (1L << 53);
> {code}
> Method d1 and d2 are both currently employed in the {{NumberFactory}} (makeDouble(int,int)
and makeDouble(long)). However they suffer from producing a double whose least significant
bit is always 0, i.e. they produce half of all possible double values from 0-1. This is discussed
in the reference for the [XorShiRo generators|http://xoshiro.di.unimi.it/].
> This task will benchmark the methods using JMH. A switch to method d3 may be appropriate
as it generates more values. This is the method employed in JDK 1.7 ThreadLocalRandom.
> A similar analysis can be made for producing a float:
> {code:java}
> int v;
> double f1 = ...; // TBC if a float equivalent is valid?
> double f2 = (v >>> 9) * 0x1.0p-23f; // 1.0f / (1 << 23);
> double f3 = (v >>> 8) * 0x1.0p-24f; // 1.0f / (1 << 24)
> {code}
> Method f2 is currently used in the {{NumberFactory}}.



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