[Numpy-discussion] BSD C port of FFTPACK incl. bluestein algorithm

[Dag Sverre Seljebotn]

On 11/18/2011 01:18 PM, Dag Sverre Seljebotn wrote:
> On 11/18/2011 12:58 PM, David Cournapeau wrote:
>> On Fri, Nov 18, 2011 at 11:42 AM, Robert Kern<robert.kern at gmail.com>   wrote:
>>> On Fri, Nov 18, 2011 at 11:19, Dag Sverre Seljebotn
>>> <d.s.seljebotn at astro.uio.no>   wrote:
>>>> I've been in touch with Martin Reinecke, author of the libpsht code for
>>>> spherical harmonic transforms, about licensing issues.
>>>>
>>>> libpsht itself will remain under the GPL, but he is likely to release
>>>> his C port of FFTPACK under BSD in the near future, as it is based on
>>>> the public domain FFTPACK.
>>>>
>>>> I'm grateful for this change for my own purposes (allows releasing my
>>>> own competing SHT library under the BSD) -- but it could perhaps be
>>>> useful for NumPy or SciPy as well, depending on how complete the port
>>>> is? E.g., perhaps make numpy.fft more complete (is the
>>>> numpy.fft/scipy.fftpack split simply because of the Fortran dependency?).
>>>
>>> It used to be the case that scipy.fftpack allowed one to build against
>>> multiple different, usually faster, FFT libraries like FFTW. I think
>>> we have backed away from that since the cost of maintaining the build
>>> configuration for all of those different backends was so high. It's
>>> worth noting that numpy.fft is already using a C translation of
>>> FFTPACK. I'm not sure what the differences are between this
>>> translation and Martin's.
>
> Here's some more info forwarded from Martin:
>
> """
> - only FFTs are supported (no DCTs/DSTs)
> - only double precision is supported (extension to single precision might
>     not be much work, though)
> - both complex and real FFTs are supported
> - real FFTs allow various storage schemes for the (half)complex frequency
>     domain data (classic FFTPACK scheme, FFTW or halfcomplex scheme,
>     uncompressed complex storage)
> - precision of transforms involving large prime factors should be slightly
>     better than with original FFTPACK
> - Bluestein's algorithm is automatically selected if considered profitable
> - small accuracy self-testing code is provided.
>
> Fairly complete interface documentation is available in Doxygen format.
> I'll prepare a source package later in the afternoon and send it around.

OK, Martin sent me a BSD-ed version of his libfftpack, and I stuck it on 
github:

https://github.com/dagss/libfftpack

Note that ls_fft.h contains Martin's API for it (plan construction and 
execution etc.)

Dag Sverre