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https://issues.apache.org/jira/browse/MATH-562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13023337#comment-13023337
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Phil Steitz commented on MATH-562:
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Indeed, why not?  I am not following exactly what is being proposed, though.  
Is the idea that the data are assumed to be coming from a periodic function 
with known period?  Is there any constraint on the number of periods that the 
sample data is supposed to represent, or are we assuming that it represents 
just one period with some points extending before and after?  I guess the 
application is to extend the domain of the interpolating function outside of 
the domain determined by the data?  Don't you maybe need two parameters for 
"extend" (number before beginning of function period and number after) and 
maybe even two "offset" parameters (distance between beginning of the period 
and the first "interior" domain value and between last "interior" domain value 
and end of the period)?  Could well be I am misunderstanding the setup here.  
Another thing that I don't quite get is it would seem some kind of fitness 
measure would be needed to ensure that the designation of the "interior" points 
is consistent with periodicity (i.e., behavior in a neighborhood of period 
start and end points is similar).

> "UnivariateRealPeriodicInterpolator"
> ------------------------------------
>
>                 Key: MATH-562
>                 URL: https://issues.apache.org/jira/browse/MATH-562
>             Project: Commons Math
>          Issue Type: New Feature
>            Reporter: Gilles
>            Assignee: Gilles
>            Priority: Minor
>             Fix For: 3.0
>
>
> It would be an interpolator implementation that assumes that the data to be 
> interpolated is periodic. Thus values that are outside of the range can be 
> passed to the interpolation function:
> They will be wrapped into the initial range before being passed to the class 
> that actually computes the interpolation.
> The constructor prototype would be:
> {code}
> /**
>  * Builds an interpolator.
>  *
>  * @param interpolator Interpolator.
>  * @param period Period.
>  * @param offset Offset.
>  * @param extend Number of points to be appended at the beginning and
>  * end of the sample arrays in order to avoid interpolation failure at
>  * the (periodic) boundaries of the orginal interval. The value is the
>  * number of sample points which the original {@code interpolator} needs
>  * on each side of the interpolation point.
>  */
> public UnivariateRealPeriodicInterpolator(UnivariateRealInterpolator 
> interpolator,
>                                           double period,
>                                           double offset,
>                                           int extend) {
>     // ...
> }
> {code}
> Would it be useful to have that in CM?

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