[Rd] stats::fft produces inconsistent results

[Martin Maechler]

>>>>> Martin Maechler 
>>>>>     on Wed, 20 Oct 2021 11:26:21 +0200 writes:

[............]

    > Thank you, André , that's very good.

    > Just to state the obvious conclusion:

    > If Ben's suggestion is correct (and André has explained *how*
    > that could happen) this would mean  a
    > SEVERE BUG in package ravetools's  mvfftw() function.

    > and it would have been (yet another) case of gaining speed by
    > killing correctness...

    > ... but then ravetools  is not even a CRAN package, so why
    > should you dare to use it for anything serious ?

    > ... yes, being grouchy ..

which I should rather not be.

Dipterix Wang *did* say initially that he is currently
developing ravetools so it's very reasonabl this is not yet a
CRAN package..

Best,
Martin

    >> -----Message d'origine-----
    >> De : R-devel <r-devel-bounces using r-project.org> De la part de Ben Bolker
    >> Envoyé : mercredi 20 octobre 2021 03:27
    >> À : r-devel using r-project.org
    >> Objet : Re: [Rd] stats::fft produces inconsistent results


    >> This is a long shot, but here's a plausible scenario:

    >> as part of its pipeline, ravetools::mvfftw computes the mean of the
    >> input vector **and then centers it to a mean of zero** (intentionally or
    >> accidentally?)

    >> because variables are passed to compiled code by reference (someone
    >> can feel free to correct my terminology), this means that the original
    >> vector in R now has a mean of zero

    >> the first element of fft() is mean(x)*length(x), so if mean(x) has
    >> been forced to zero, that would explain your issue.

    >> I don't know about the non-reproducibility part.

    >> On 10/19/21 7:06 PM, Dipterix Wang wrote:
    >>> Dear R-devel Team,
    >>> 
    >>> I'm developing a neuroscience signal pipeline package in R (https://github.com/dipterix/ravetools) and I noticed a weird issue that failed my unit test.
    >>> 
    >>> Basically I was trying to use `fftw3` library to implement fast multivariate fft function in C++. When I tried to compare my results with stats::fft, the test result showed the first element of **expected** (which was produced by stats::fft) was zero, which, I am pretty sure, is wrong, and I can confirm that my function produces correct results.
    >>> 
    >>> However, somehow I couldn’t reproduce this issue on my personal computer (osx, M1, R4.1.1), the error simply went away.
    >>> 
    >>> The catch is my function produced consistent and correct results but stats::fft was not. This does not mean `stats::fft` has bugs. Instead, I suspect there could be some weird interactions between my code and stats::fft at C/C++ level, but I couldn’t figure it out why.
    >>> 
    >>> +++ Details:
    >>> 
    >>> Here’s the code I used for the test:
    >>> 
    >>> https://github.com/dipterix/ravetools/blob/4dc35d64763304aff869d92dddad38a7f2b30637/tests/testthat/test-fftw.R#L33-L41
    >>> 
    >>> ————————Test code————————
    >>> set.seed(1)
    >>> x <- rnorm(1000)
    >>> dim(x) <- c(100,10)
    >>> a <- ravetools:::mvfftw_r2c(x, 0)
    >>> c <- apply(x, 2, stats::fft)[1:51,]
    >>> expect_equal(a, c)
    >>> ————————————————————————
    >>> 
    >>> Here are the tests that gave me the errors:
    >>> 
    >>> The test logs on win-builder
    >>> https://win-builder.r-project.org/07586ios8AbL/00check.log
    >>> 
    >>> Test logs on GitHub
    >>> https://github.com/dipterix/ravetools/runs/3944874310?check_suite_focus=true
    >>> 
    >>> 
    >>> —————————————— Failed tests ——————————————
    >>> -- Failure (test-fftw.R:41:3): mvfftw_r2c --------------------------------------
    >>> `a` (`actual`) not equal to `c` (`expected`).
    >>> 
    >>> actual vs expected
    >>> [,1]                    [,2]                  [,3]                  [,4]                    ...
    >>> - actual[1, ]     10.8887367+ 0.0000000i  -3.7808077+ 0.0000000i   2.967354+ 0.000000i   5.160186+ 0.000000i ...
    >>> + expected[1, ]    0.0000000+ 0.0000000i  -3.7808077+ 0.0000000i   2.967354+ 0.000000i   5.160186+ 0.000000i...
    >>> 
    >>> ————————————————————————
    >>> 
    >>> The first columns are different, `actual` is the results I produced via `ravetools:::mvfftw_r2c`, and `expected` was produced by `stats::fft`
    >>> 
    >>> 
    >>> Any help or attention is very much appreciated.
    >>> Thanks,
    >>> - Zhengjia

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