[FFmpeg-devel] [PATCH 1/3] lavc/h274: fix PRNG definition

[Michael Niedermayer]

On Wed, Sep 27, 2023 at 03:56:15PM +0200, Niklas Haas wrote:
> From: Niklas Haas <git at haasn.dev>
> 
> The spec specifies x^31 + x^3 + 1 as the polynomial, but the diagram in
> Figure 1-1 omits the +1 offset. The initial implementation was based on
> the diagram, but this is wrong (produces subtly incorrect results).

the +1 changes "nothing"
if you take the prng with and without the +1 the 2 will produce 2
sequences that are negations of each other

or said different if you start one from 1 and the other from ~1
they will produce the same sequence just 0 and 1 swaped

you can also compute 32 bits at once using this:
(this needs 64bits of the sequence as input though)
not sure how useful it is, but it produces more bits quicker

 static void prng_shift32(uint64_t *state)
 {
     uint64_t x = *state;
     uint64_t y = x ^ x>>3;
     y ^= y>>6;
     y ^= y>>12;
     uint32_t feedback = (x >> 1) ^ (y >> 5) ^ (x >> 29) ^ (x >> 30);
     *state = (x << 32) | feedback;
 }


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

When you are offended at any man's fault, turn to yourself and study your
own failings. Then you will forget your anger. -- Epictetus
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