[FFmpeg-devel] [RFC] support encrypted asf

[Michael Niedermayer]

Hi

On Tue, Oct 09, 2007 at 01:31:54PM +0200, Michael Niedermayer wrote:
> Hi
> 
> On Tue, Oct 09, 2007 at 01:03:57AM +0300, Uoti Urpala wrote:
> > 
> > On Mon, 2007-10-08 at 21:29 +0200, Michael Niedermayer wrote:
> > > or a LUT based variant
> > > 
> > > static uint32_t inverse2(uint32_t v) {
> > >     uint32_t a,b;
> > >     a= inv[v&255];
> > > 
> > >     b= (a*v)>>8;
> > >     b*= a;
> > >     a-= b<<8;
> > >     b= (a*v)>>16;
> > >     b*= a;
> > > 
> > >     return a - (b<<16);
> > > }
> > > 
> > > for(i=1; i<256; i+=2)
> > >     inv[i]= inverse(i);
> > 
> > In my test the following was about twice as fast (though minimizing the
> > multiply sizes through shifts could help on some architecture):
> > 
> > static uint32_t inverse2(uint32_t v) {
> >     uint32_t i = inv[v & 255];
> >     i *= 2 - v * i;
> >     i *= 2 - v * i;
> >     return i;
> > }
> > 
> > Which also gives the following tableless variant:
> > 
> > static uint32_t inverse3(uint32_t v) {
> >     uint32_t i;
> >     i =  2 - v;
> >     i *= 2 - v * i;
> >     i *= 2 - v * i;
> >     i *= 2 - v * i;
> >     i *= 2 - v * i;
> >     return i;
> > }
> 
> or even faster:
> 
> static uint32_t inverse4(uint32_t v) {
>     int i = v * v * v;
>     i *= 2 - v * i;
>     i *= 2 - v * i;
>     i *= 2 - v * i;
>     return i;
> }
> 
> and i would not be surprised if that can be simplified further ...

actually the following would in theory be better

static uint_fast32_t inverse4(uint_fast32_t v) {
    uint_fast32_t i = v * v * v;
    i *= 2 - v * i;
    i *= 2 - v * i;
    i *= 2 - v * i;
    return i;
}

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

It is dangerous to be right in matters on which the established authorities
are wrong. -- Voltaire
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