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From "Aleksei Dievskii (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (MATH-1293) Tabulating the logarithmic factorial
Date Wed, 18 Nov 2015 13:47:11 GMT

     [ https://issues.apache.org/jira/browse/MATH-1293?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Aleksei Dievskii updated MATH-1293:
-----------------------------------
    Attachment:     (was: factlog.patch)

> Tabulating the logarithmic factorial
> ------------------------------------
>
>                 Key: MATH-1293
>                 URL: https://issues.apache.org/jira/browse/MATH-1293
>             Project: Commons Math
>          Issue Type: Improvement
>    Affects Versions: 3.5
>            Reporter: Aleksei Dievskii
>            Priority: Minor
>              Labels: feature, patch
>
> Presently, CombinatoricsUtils#factorialLog is calculated via a tabulated value for the
first 21 entries, and for the rest it employs linear-complexity direct summing. For the factorial-heavy
computations this can be rather painful. I propose a patch which allocates additional 16KB
of tabulated log-factorial values, allowing very fast (constant complexity) computation for
arguments up to 17000, and delegating to the log-gamma for the rest. Benchmarks show a speed-up
of up to 200x.



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