Re: [FFmpeg-devel] [PATCH 1/5 v2] avcodec/ac3dec: split off code discarding garbage at the beginning of a packet

[James Almer <jamrial@gmail.com>]

On 10/22/2022 6:02 PM, James Almer wrote:
> Signed-off-by: James Almer <jamrial@gmail.com>
> ---
> No changes since v1.
> 
>   libavcodec/ac3_parser.c          | 19 +++++++++++++++++++
>   libavcodec/ac3_parser_internal.h |  2 ++
>   libavcodec/ac3dec.c              | 15 ++-------------
>   3 files changed, 23 insertions(+), 13 deletions(-)
> 
> diff --git a/libavcodec/ac3_parser.c b/libavcodec/ac3_parser.c
> index 4f154bb7c4..425e1b4742 100644
> --- a/libavcodec/ac3_parser.c
> +++ b/libavcodec/ac3_parser.c
> @@ -53,6 +53,25 @@ static const uint8_t center_levels[4] = { 4, 5, 6, 5 };
>    */
>   static const uint8_t surround_levels[4] = { 4, 6, 7, 6 };
>   
> +int ff_ac3_find_syncword(const uint8_t *buf, int buf_size)
> +{
> +    int i;
> +
> +    for (i = 1; i < buf_size; i += 2) {
> +        if (buf[i] == 0x77 || buf[i] == 0x0B) {
> +            if ((buf[i] ^ buf[i-1]) == (0x77 ^ 0x0B)) {
> +                i--;
> +                break;
> +            } else if ((buf[i] ^ buf[i+1]) == (0x77 ^ 0x0B)) {
> +                break;
> +            }
> +        }
> +    }
> +    if (i >= buf_size)
> +        return AVERROR_INVALIDDATA;
> +
> +    return i;
> +}
>   
>   int ff_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
>   {
> diff --git a/libavcodec/ac3_parser_internal.h b/libavcodec/ac3_parser_internal.h
> index bd4e1bbffb..2ac0e67ec2 100644
> --- a/libavcodec/ac3_parser_internal.h
> +++ b/libavcodec/ac3_parser_internal.h
> @@ -79,4 +79,6 @@ int ff_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr);
>   int avpriv_ac3_parse_header(AC3HeaderInfo **hdr, const uint8_t *buf,
>                               size_t size);
>   
> +int ff_ac3_find_syncword(const uint8_t *buf, int buf_size);
> +
>   #endif /* AVCODEC_AC3_PARSER_INTERNAL_H */
> diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
> index 340f6e1e37..8e40587ff1 100644
> --- a/libavcodec/ac3dec.c
> +++ b/libavcodec/ac3dec.c
> @@ -1508,19 +1508,8 @@ static int ac3_decode_frame(AVCodecContext *avctx, AVFrame *frame,
>       s->superframe_size = 0;
>   
>       buf_size = full_buf_size;
> -    for (i = 1; i < buf_size; i += 2) {
> -        if (buf[i] == 0x77 || buf[i] == 0x0B) {
> -            if ((buf[i] ^ buf[i-1]) == (0x77 ^ 0x0B)) {
> -                i--;
> -                break;
> -            } else if ((buf[i] ^ buf[i+1]) == (0x77 ^ 0x0B)) {
> -                break;
> -            }
> -        }
> -    }
> -    if (i >= buf_size)
> -        return AVERROR_INVALIDDATA;
> -    if (i > 10)
> +    i = ff_ac3_find_syncword(buf, buf_size);
> +    if (i < 0 || i > 10)
>           return i;
>       buf += i;
>       buf_size -= i;

Will apply set.
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