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https://issues.apache.org/jira/browse/STATISTICS-83?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17801268#comment-17801268
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Alex Herbert commented on STATISTICS-83:
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The correct non-finite value can be computed using a sum of the encountered
non-finite values. This involves an if statement when processing each input
value to detect if it is non-finite. It also involves a divide per value which
is a relatively slow operation. The result is that the branch is efficiently
avoided by branch prediction and it does not significantly impact performance
on finite input data:
Benchmarked using JMH on JDK 11.0.21, OpenJDK 64-Bit Server VM, 11.0.21+9
||Length||UpdatingMean||UpdatingMean+||Sum||Sum+||
|1|4.118|4.349|5.156|5.393|
|10|15.63|20.46|9.495|11.877|
|1000|4473|4535|983.6|1041|
|10000|46089|46120|9814|10416|
+ = with non-finite computation
Note:
This issue also effects the Sum. In that case the extra computation to track
the correct non-finite result is more noticeable (approximately 6% at large
input sizes).
> Mean returns a non-finite value that can be incorrect
> -----------------------------------------------------
>
> Key: STATISTICS-83
> URL: https://issues.apache.org/jira/browse/STATISTICS-83
> Project: Commons Statistics
> Issue Type: Bug
> Components: descriptive
> Reporter: Alex Herbert
> Priority: Trivial
>
> The Mean maintains a non-finite value to return in the event that the input
> contains non-finite values and the result is not finite.
> The non-finite value is a simple sum of the input. This returns the correct
> result for most cases of nan/infinte values. However if the values contain
> only one sign of infinity and the other finite values overflow to an opposite
> infinity before the infinity is encountered the result is incorrect (due to
> +inf + -inf == nan), e.g. the following test fails:
> {code:java}
> double[] values = {Double.MAX_VALUE, Double.MAX_VALUE,
> Double.NEGATIVE_INFINITY};
> // Computed value = NaN
> Assertions.assertEquals(Double.NEGATIVE_INFINITY,
> Mean.of(values).getAsDouble());
> {code}
> Investigate alternative options for non-finite results and either: correct
> the support; or drop support if performance is significantly impacted in
> order to handle for these rare cases.
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