[FFmpeg-devel] [PATCH 3/4] avcodec/siren: add checksum calculation

[Lynne]

7 Sept 2021, 11:28 by pross at xvid.org:

> ---
>  libavcodec/siren.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/siren.c b/libavcodec/siren.c
> index ccc69aaf40..31fb9346da 100644
> --- a/libavcodec/siren.c
> +++ b/libavcodec/siren.c
> @@ -752,7 +752,34 @@ static int siren_decode(AVCodecContext *avctx, void *data,
>  frame_error = 1;
>  }
>  
> -    skip_bits(gb, s->checksum_bits);
> +    if (s->checksum_bits) {
> +        static const uint16_t ChecksumTable[4] = {0x7F80, 0x7878, 0x6666, 0x5555};
> +        int wpf, checksum, sum, calculated_checksum, temp1, temp2;
> +
> +        wpf = bits_per_frame / 16;
> +
> +        checksum = AV_RB16(avpkt->data + (wpf - 1) * 2) & ((1 << s->checksum_bits) - 1);
> +
> +        sum = 0;
> +        for (int i = 0; i < wpf - 1; i++)
> +            sum ^= AV_RB16(avpkt->data + i * 2) << (i % 15);
> +        sum ^= (AV_RB16(avpkt->data + (wpf - 1) * 2) & ~checksum) << ((wpf - 1) % 15);
> +        sum = (sum >> 15) ^ (sum & 0x7FFF);
> +
> +        calculated_checksum = 0;
> +        for (int i = 0; i < 4; i++) {
> +            temp1 = ChecksumTable[i] & sum;
> +            for (int j = 8; j > 0; j >>= 1) {
> +                temp2 = temp1 >> j;
> +                temp1 ^= temp2;
> +            }
> +            calculated_checksum <<= 1;
> +            calculated_checksum |= temp1 & 1;
> +        }
> +
> +        if (checksum != calculated_checksum)
> +            frame_error = 1;
> +    }
>  
>  if (frame_error) {
>  memcpy(s->imdct_in, s->backup_frame, number_of_valid_coefs * sizeof(float));
>

Good idea, but only check it if AV_EF_CRCCHECK is set in avctx->err_recognition
and only error on mismatches if AV_EF_EXPLODE is set in the same field.