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From "Alex D Herbert (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (RNG-79) Benchmark methods for producing nextDouble
Date Tue, 05 Mar 2019 12:32:00 GMT

    [ https://issues.apache.org/jira/browse/RNG-79?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16784374#comment-16784374
] 

Alex D Herbert commented on RNG-79:
-----------------------------------

The original benchmark tested methods with a linear sequence generator. This was bad.

I have updated this to use a uniform random number generator.

I find the {{nextDouble}} methods are still all similar. The method to convert to a float
using bits is faster on this test machine:
||Method||Score||Error||Median||
|nextDoubleBaseline|313.53|0.32|313.54|
|nextDoubleUsingBitsToDouble|248.65|2.56|248.84|
|nextDoubleUsingMultiply52bits|249.69|2.14|249.97|
|nextDoubleUsingMultiply53bits|249.80|2.14|250.01|
|nextFloatBaseline|314.11|0.46|314.15|
|nextFloatUsingBitsToFloat|283.66|3.81|284.29|
|nextFloatUsingMultiply23bits|250.58|0.29|250.61|
|nextFloatUsingMultiply24bits|249.01|2.91|249.20|

Tested on:
 * Ubuntu Linux 16.04 LTS using OpenJDK Runtime Environment (build 1.8.0_191-8u191-b12-2ubuntu0.16.04.1-b12).
 * Intel(R) Xeon(R) CPU E5-1680 v3 @ 3.20GHz

> Benchmark methods for producing nextDouble
> ------------------------------------------
>
>                 Key: RNG-79
>                 URL: https://issues.apache.org/jira/browse/RNG-79
>             Project: Commons RNG
>          Issue Type: New Feature
>          Components: core
>    Affects Versions: 1.3
>            Reporter: Alex D Herbert
>            Assignee: Alex D Herbert
>            Priority: Minor
>          Time Spent: 50m
>  Remaining Estimate: 0h
>
> 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 = Float.intBitsToFloat(0x7f << 23 | v >>> 9) - 1.0f;
> 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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