Sean Owen created MATH-1056:
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Summary: Small error in PoissonDistribution.nextPoisson() algorithm
Key: MATH-1056
URL: https://issues.apache.org/jira/browse/MATH-1056
Project: Commons Math
Issue Type: Bug
Affects Versions: 3.2
Reporter: Sean Owen
Priority: Minor
Attachments: MATH-1056.patch
Here's a tiny bug I noticed via static inspection, since it flagged the integer
division. PoissonDistribution.java:325 says:
{code:java}
final double a1 = FastMath.sqrt(FastMath.PI * twolpd) * FastMath.exp(1 / 8 *
lambda);
{code}
The "1 / 8 * lambda)" is evidently incorrect, since this will always evaluate
to 0. I rechecked the original algorithm
(http://luc.devroye.org/devroye-poisson.pdf) and it should instead be:
{code:java}
final double a1 = FastMath.sqrt(FastMath.PI * twolpd) * FastMath.exp(1 / (8 *
lambda));
{code}
(lambda is a double so there is no int division issue.) This matches a later
expression.
I'm not sure how to evaluate the effect of the bug. Better to be correct of
course; it may never have made much practical difference.
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