[FFmpeg-devel] CNG (consistent noise generation) patch for AC-3 decoder

[James Almer]

On 9/3/2016 7:50 AM, Jonathan Campbell wrote:
> +int av_lfg_init_from_data(AVLFG *c, const uint8_t *data, unsigned int length) {
> +    unsigned int beg, end, segm;
> +    const AVCRC *avcrc;
> +    uint32_t crc = 1;
> +
> +    c->index = 0;

The AVLFG struct should IMO remain untouched if init is guaranteed to fail,
as it would be the case with an out or range length value, so move this after
that check.

> +    avcrc = av_crc_get_table(AV_CRC_32_IEEE); /* This can't fail. It's a well-defined table in crc.c */

You could also move this one below.

> +
> +    /* avoid integer overflow in the loop below. */
> +    if (length > (UINT_MAX / 128U)) return AVERROR(EINVAL);
> +
> +    /* across 64 segments of the incoming data,
> +     * do a running crc of each segment and store the crc as the state for that slot.
> +     * this works even if the length of the segment is 0 bytes. */
> +    beg = 0;
> +    for (segm = 0;segm < 64;segm++) {
> +        end = (((segm + 1) * length) / 64);
> +        crc = av_crc(avcrc, crc, data + beg, end - beg);

The thing about speed made me wonder, wouldn't using adler32 be faster?
If so, this feature could maybe even be enabled by default with it.

> +        c->state[segm] = (unsigned int)crc;
> +        beg = end;
> +    }
> +
> +    return 0;
> +}
> diff --git a/libavutil/lfg.h b/libavutil/lfg.h
> index ec90562..72eb673 100644
> --- a/libavutil/lfg.h
> +++ b/libavutil/lfg.h
> @@ -22,6 +22,8 @@
>  #ifndef AVUTIL_LFG_H
>  #define AVUTIL_LFG_H
>  
> +#include <stdint.h> /* uint8_t type */
> +
>  typedef struct AVLFG {
>      unsigned int state[64];
>      int index;
> @@ -29,6 +31,13 @@ typedef struct AVLFG {
>  
>  void av_lfg_init(AVLFG *c, unsigned int seed);
>  
> +/*

This is meant for doxygen, so /** like in every other function.

> + * Seed the state of the ALFG using binary data.
> + *
> + * Return value: 0 on success, negative value (AVERROR) on failure.
> + */
> +int av_lfg_init_from_data(AVLFG *c, const uint8_t *data, unsigned int length);
> +
>  /**
>   * Get the next random unsigned 32-bit number using an ALFG.
>   *

No more comments for me.