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

[Michael Niedermayer]

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 at 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;
+                    return e->value * (p->var[index]= d2);
+                }
                 case e_hypot:return e->value * hypot(d, d2);
                 case e_atan2:return e->value * atan2(d, d2);
                 case e_bitand: return isnan(d) || isnan(d2) ? NAN : e->value * ((long int)d & (long int)d2);
@@ -339,6 +346,7 @@ void av_expr_free(AVExpr *e)
     av_expr_free(e->param[1]);
     av_expr_free(e->param[2]);
     av_freep(&e->var);
+    av_freep(&e->var_uint64);
     av_freep(&e);
 }
 
@@ -724,7 +732,8 @@ int av_expr_parse(AVExpr **expr, const char *s,
         goto end;
     }
     e->var= av_mallocz(sizeof(double) *VARS);
-    if (!e->var) {
+    e->var_uint64= av_mallocz(sizeof(uint64_t) *VARS);
+    if (!e->var || !e->var_uint64) {
         ret = AVERROR(ENOMEM);
         goto end;
     }
@@ -766,6 +775,7 @@ double av_expr_eval(AVExpr *e, const double *const_values, void *opaque)
 {
     Parser p = { 0 };
     p.var= e->var;
+    p.var_uint64= e->var_uint64;
 
     p.const_values = const_values;
     p.opaque     = opaque;
-- 
2.17.1