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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