[FFmpeg-devel] [PATCH] Demuxer for Leitch/Harris' VR native stream format (LXF)

[Måns Rullgård]

Daniel Verkamp <daniel at drv.nu> writes:

> 2010/9/13 M?ns Rullg?rd <mans at mansr.com>:
>> Tomas H?rdin <tomas.hardin at codemill.se> writes:
>>
>>>> > > > +//returns number of bits set in value
>>>> > > > +static int num_set_bits(uint32_t value) {
>>>> > > > + ? ?int ret;
>>>> > > > +
>>>> > > > + ? ?for(ret = 0; value; ret += (value & 1), value >>= 1);
>>>> > > > +
>>>> > > > + ? ?return ret;
>>>> > > > +}
>>>> > >
>>>> > > if we dont have a population count function yet, than one should be added
>>>> > > to some header in libavutil
>>>> >
>>>> > I couldn't find one. That probably belongs in its own thread though.
>>>> >
>>>> > Which files would such a function belong in - intmath.h/c, common.h or
>>>> > somewhere else? Also, which name would be best: ff_count_bits(),
>>>> > av_count_bits() or something else?
>>>>
>>>> av_popcount()
>>>> would be similar to gccs __builtin_popcount()
>>>
>>> OK. I attached popcount.patch which adds such a function to common.h.
>>> Also bumped minor of lavu. The implementation uses a 16-byte LUT and
>>> therefore counts four bits at a time. I suspect there are better
>>> solutions though. I did verify that it returns exactly the same number
>>> the other implementation does for all 2^32 possible input values.
>>
>> I can't think of a better generic solution off the top of my head.
>
> There is at least one algorithm to do this without loops or lookup
> tables using SWAR tricks, but I haven't benchmarked it:
> http://aggregate.org/MAGIC/#Population Count (Ones Count)

That method will be several times slower on any modern hardware.

-- 
M?ns Rullg?rd
mans at mansr.com