Move conv to Convolutions.jl or ConvolutionsBase.jl?

[dlfivefifty]

Downstream I have a DSP extension just to overload conv:

https://github.com/JuliaArrays/InfiniteArrays.jl/blob/master/ext/InfiniteArraysDSPExt.jl

I wonder if moving conv out would be better. This could take two approaches:

1. Move out all convolutions code to Convolutions.jl. Potentially, FFTW could be a weak dependency but that may be hard.
2. Make a package that just defines conv with no implementation called ConvolutionsBase.jl. This has the benefit that there is no FFTW.jl dependency.

[martinholters]

Potentially, FFTW could be a weak dependency but that may be hard.

Even if we came up with a dispatch where an extension could hook in to do frequency-domain convolution, I wouldn't be happy to see the result of a convolution change depending on whether FFTW is loaded, even if it is within the numerical noise floor.

Make a package that just defines conv with no implementation called ConvolutionsBase.jl.

Sounds easy, but might not be, as it should then also define (by documentation) the exact API. Probably including any dispatch hooks like conv_axis_with_offset; would be too bad if one had to depend on DSP because one doesn't want to add a method to conv, but rather conv_axis_with_offset or conv_output_axes or whatever might become public API there.

So maybe it's better to start with a Convolutions.jl package that tries to fulfill the need of any downstream users and provide useful hooks. Once that has stabilized, one can then think about moving the definition of the API to a ConvolutionsBase.jl package.

Maybe we should try to bring in more users of convolution to the discussion. E.g. multiplication of polynomials is basically convolution, but Polynomials.jl can't depend on DSP.jl because DSP.jl depends on Polynomials.jl. Would a Convolutions.jl package be helpful for Polynomials.jl, @jverzani? If so, what would you expect of the API?

[wheeheee]

I think (among others) ApproxFunBase.jl and Korg.jl also use only conv. Don't know if the maintainers are ok with being pinged?

[dlfivefifty]

I can speak for ApproxFunBase. It's fine to ping me 😂

[jverzani]

Polynomials would likely benefit from this and could work around any API chose, I would assume as the requirements are pretty minimal (convolution of two vectors or ntuples basically).

[dlfivefifty]

I think convolution of two Float64 vectors is the case where FFTW.jl is needed but I think the current proposal is we leave FFTW as a hard dependency.

[wheeheee]

I can speak for ApproxFunBase. It's fine to ping me 😂

oh man, I saw jishnub's icon and thought that was it 😅

Polynomials would likely benefit from this and could work around any API chose, I would assume as the requirements are pretty minimal (convolution of two vectors or ntuples basically).

Hm, currently we don't support convolution of ntuples. @martinholters would it be appropriate to add that?

[martinholters]

Hm, currently we don't support convolution of ntuples. @martinholters would it be appropriate to add that?

Sure, why not? Maybe not in DSP.jl, but in Convolutions.jl, that sounds reasonable. For future reference: for short homogenous tuples, this works quite well:

conv(a::Tuple, b::Tuple) = ntuple(n -> sum(a[k]*b[n-k+1] for k in max(1, n-length(b)+1):min(length(a), n)), length(a) + length(b) - 1)

We'd probably want to do some @generated magic, but the above might still be useful for an else part of an if @generated.

[dlfivefifty]

I think there needs to be an obvious use case for Tuple. When would one want it instead of an SVector? Does a convolution make sense when the types are different?

[jverzani]

It comes up in Polynomials for tuple-backed polynomials. But that might be quite niche, so needn't be in a more general package. I was just answering what was used, not necessarily, what was expected in an external package.

[dlfivefifty]

When do polynomials have coefficients of different types? Or are you using it instead of an SVector to avoid a dependency?

[jverzani]

The different types might occur, but primarily we have an option for using an ntuple which speeds up some operations (it was originally borrowed from https://github.com/tkoolen/StaticUnivariatePolynomials.jl). As surmised, an SVector is avoided just because of the dependency. Perhaps we can revisit that decision.

[dlfivefifty]

I haven’t looked at how Polynomials.jl is implemented but supporting tuples seems like it must require a bunch of special cases, eg addition. I suspect it would be simpler to only allow AbstractVector coefficients. It probably wouldn’t need to know anything about StaticArrays.jl,, eg, the special implementation of conv mentioned would just be in a ConvulationsStaticArraysExt.jl

[jverzani]

Fine with me. Thanks for the consideration.