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From "Aleksei Dievskii (JIRA)" <j...@apache.org>
Subject [jira] [Created] (MATH-1293) Tabulating the logarithmic factorial
Date Wed, 18 Nov 2015 10:40:11 GMT
Aleksei Dievskii created MATH-1293:
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             Summary: 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


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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