Re: [FFmpeg-devel] [PATCH] avutil/eval: Use integer for random() state

[Stefano Sabatini <stefasab@gmail.com>]

On date Tuesday 2024-01-02 03:28:45 +0100, Michael Niedermayer wrote:
> rounding the 64bit integer state to double between each iteration
> causes a reduction in quality of the random number generator.
> For example its period drops from 2^64 to around 200 million
> 
> Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
> ---
>  libavutil/eval.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/libavutil/eval.c b/libavutil/eval.c
> index bad9e4ecb8d..89c61ba4bf5 100644
> --- a/libavutil/eval.c
> +++ b/libavutil/eval.c
> @@ -55,6 +55,7 @@ typedef struct Parser {
>      void *log_ctx;
>  #define VARS 10
>      double *var;
> +    uint64_t *var_uint64;
>  } Parser;
>  
>  static const AVClass eval_class = {
> @@ -173,6 +174,7 @@ struct AVExpr {
>      } a;
>      struct AVExpr *param[3];
>      double *var;
> +    uint64_t *var_uint64;
>  };
>  
>  static double etime(double v)
> @@ -230,9 +232,10 @@ static double eval_expr(Parser *p, AVExpr *e)
>          }
>          case e_random:{
>              int idx= av_clip(eval_expr(p, e->param[0]), 0, VARS-1);
> -            uint64_t r= isnan(p->var[idx]) ? 0 : p->var[idx];
> +            uint64_t r= p->var_uint64[idx] ? p->var_uint64[idx] : (isnan(p->var[idx]) ? 0 : p->var[idx]);
>              r= r*1664525+1013904223;
>              p->var[idx]= r;
> +            p->var_uint64[idx]= r;
>              return e->value * (r * (1.0/UINT64_MAX));
>          }
>          case e_while: {
> @@ -319,7 +322,11 @@ static double eval_expr(Parser *p, AVExpr *e)
>                  case e_div: return e->value * (d2 ? (d / d2) : d * INFINITY);
>                  case e_add: return e->value * (d + d2);
>                  case e_last:return e->value * d2;
> -                case e_st : return e->value * (p->var[av_clip(d, 0, VARS-1)]= d2);
> +                case e_st :  {
> +                    int index = av_clip(d, 0, VARS-1);

> +                    p->var_uint64[index] = 0;

can't you set this to d2 and then in case e_random:

uint64_t r = isnan(p->var[idx]) ? 0 : p->var_uint64[idx];

?

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