[FFmpeg-devel] [PATCH 2/2] imdct15: replace the FFT with a faster PFA FFT algorithm

[Rostislav Pehlivanov]

On 4 January 2017 at 14:14, Peter Barfuss <bofh.h4x at gmail.com> wrote:

> First off, many thanks.
>
> > +    const int inv_1 = l_ptwo << ((4 - b_ptwo) & 3);
> > +    const int inv_2 = 0xeeeeeeef & ((1U << b_ptwo) - 1);
>
> It would be nice to add a comment here that the expression for inv_1
> is (2^b_ptwo)^-1 mod 15 and inv_2 is 15^-1 mod 2^b_ptwo. (A general
> PFA FFT would need to use extended Euclidean algorithm here, but
> because both cases are fixed, it simplifies to these expressions. I
> have a sketch of a proof (basically solving the relevant diophantine
> equation you get) in case anyone is nervous, though it's easy to
> verify by hand for 1 < b_ptwo < 18, which are all the cases that
> ffmpeg's power-of-two FFT currently supports).
>
> Rest of patch seems good.
>
> -Peter
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>

Done

Also I didn't like how s->exptab[20].re needed a negative sign so I removed
it
and moved the subtraction to the 5-point FFT, just so you know.


I'll push both patches tomorrow evening if no one else has anything to say.