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Date: Wed, 18 Nov 2015 10:40:11 +0000 (UTC)
From: "Aleksei Dievskii (JIRA)" <jira@apache.org>
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Subject: [jira] [Created] (MATH-1293) Tabulating the logarithmic factorial
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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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