[FFmpeg-devel] [PATCH 10/25] avcodec/magicyuv: Fix building Huffman table

[Paul B Mahol]

On Sat, Sep 26, 2020 at 12:27:49PM +0200, Andreas Rheinhardt wrote:
> The MagicYUV format stores Huffman tables in its bitstream by coding
> the length of a given symbol; it does not code the actual code directly,
> instead this is to be inferred by the rule that a symbol is to the left
> of every shorter symbol in the Huffman tree and that for symbols of the
> same length the symbol is ascending from left to right. With one
> exception, this is also what our decoder did.
> 
> The exception only matters when there are codes of length 32, because
> in this case the first symbol of this length did not get the code 0,
> but 1; e.g. if there were exactly two nodes of length 32, then they
> would get assigned the codes 1 and 2 and a node of length 31 will get
> the 31-bit code 1 which is a prefix of the 32 bit code 2, making the
> Huffman table invalid. On the other hand, if there were only one symbol
> with the length 32, the earlier code would accept this un-Huffman-tree.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
> ---
>  libavcodec/magicyuv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

should be ok with subject fixed, it is not fix, its just fixes single
case that never happens in reality.

> 
> diff --git a/libavcodec/magicyuv.c b/libavcodec/magicyuv.c
> index 1b3f4cfc6b..17dea69d76 100644
> --- a/libavcodec/magicyuv.c
> +++ b/libavcodec/magicyuv.c
> @@ -86,7 +86,7 @@ static int huff_build(HuffEntry he[], VLC *vlc, int nb_elems)
>  
>      AV_QSORT(he, nb_elems, HuffEntry, huff_cmp_len);
>  
> -    code = 1;
> +    code = 0;
>      for (unsigned i = 0; i < nb_elems; i++) {
>          he[i].code = code >> (32 - he[i].len);
>          code += 0x80000000u >> (he[i].len - 1);
> -- 
> 2.25.1
> 
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