[FFmpeg-devel] [PATCH 02/10] crc: add av_crc_bits() to compute CRC on block with bit boundary

[Aurelien Jacobs]

On Thu, Dec 21, 2017 at 11:39:02AM +0000, Rostislav Pehlivanov wrote:
> On 20 December 2017 at 19:58, Aurelien Jacobs <aurel at gnuage.org> wrote:
> 
> > ---
> >  libavutil/crc.c | 20 ++++++++++++++++++++
> >  libavutil/crc.h | 12 ++++++++++++
> >  2 files changed, 32 insertions(+)
> >
> > diff --git a/libavutil/crc.c b/libavutil/crc.c
> > index 8e44a76ec8..cb26a09a20 100644
> > --- a/libavutil/crc.c
> > +++ b/libavutil/crc.c
> > @@ -413,3 +413,23 @@ uint32_t av_crc(const AVCRC *ctx, uint32_t crc,
> >
> >      return crc;
> >  }
> > +
> > +uint32_t av_crc_bits(const AVCRC *ctx, uint32_t crc,
> > +                     const uint8_t *buffer, size_t length)
> > +{
> > +    size_t byte_length = length >> 3;
> > +    int bit_length = length & 7;
> > +
> > +    crc = av_crc(ctx, crc, buffer, byte_length);
> > +
> > +    if (bit_length) {
> > +        uint8_t bits = buffer[byte_length];
> > +        while (bit_length--) {
> > +            int8_t mask = (bits ^ crc);
> > +            crc = (crc << 1) ^ (ctx[1] & (mask >> 7));
> > +            bits <<= 1;
> > +        }
> > +    }
> > +
> > +    return crc;
> > +}
> > diff --git a/libavutil/crc.h b/libavutil/crc.h
> > index e57a1af903..bde31f858a 100644
> > --- a/libavutil/crc.h
> > +++ b/libavutil/crc.h
> > @@ -86,6 +86,7 @@ const AVCRC *av_crc_get_table(AVCRCId crc_id);
> >  /**
> >   * Calculate the CRC of a block.
> >   * @param crc CRC of previous blocks if any or initial value for CRC
> > + * @param length number of bytes in the given block
> >   * @return CRC updated with the data from the given block
> >   *
> >   * @see av_crc_init() "le" parameter
> > @@ -93,6 +94,17 @@ const AVCRC *av_crc_get_table(AVCRCId crc_id);
> >  uint32_t av_crc(const AVCRC *ctx, uint32_t crc,
> >                  const uint8_t *buffer, size_t length) av_pure;
> >
> > +/**
> > + * Calculate the CRC of a block with bits boundary.
> > + * @param crc CRC of previous blocks if any or initial value for CRC
> > + * @param length number of bits in the given block
> > + * @return CRC updated with the data from the given block
> > + *
> > + * @see av_crc_init() "le" parameter
> > + */
> > +uint32_t av_crc_bits(const AVCRC *ctx, uint32_t crc,
> > +                     const uint8_t *buffer, size_t length) av_pure;
> > +
> >  /**
> >   * @}
> >   */
> >
> > _______________________________________________
> > ffmpeg-devel mailing list
> > ffmpeg-devel at ffmpeg.org
> > http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
> >
> 
> Sorry, I didn't see that sbc does fucked up crc on 7 bits.

Well, it is not on 7 bits. That actual data length is a multiple of
4 bits.

> In that case could you submit a new version of just the sbc patch which
> adds this back in the decoder?

I moved the function back to sbc, but I keept its usage of av_crc() for
the whole buffer except for the last bits that don't form a full byte.