[FFmpeg-devel] [PATCH] avutil/eval: Use even better PRNG

[Michael Niedermayer <michael@niedermayer.cc>]

This is the 64bit version of Chris Doty-Humphreys SFC64

Compared to the LCGs these produce much better quality numbers.
Compared to LFGs this needs less state. (our LFG has 224 byte
state for its 32bit version) this has 32byte state
Also the initialization for our LFG is slower.
This is also much faster than KISS or PCG.

This could be merged with the change to integer LCG
Also a few fate tests need an update. I will update fate if SFC64
is the chosen PRNG

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
---
 libavutil/eval.c    | 26 ++++++++++++--------
 libavutil/sfc64.h   | 59 +++++++++++++++++++++++++++++++++++++++++++++
 tests/ref/fate/eval |  2 +-
 3 files changed, 76 insertions(+), 11 deletions(-)
 create mode 100644 libavutil/sfc64.h

diff --git a/libavutil/eval.c b/libavutil/eval.c
index 9d41140056c..d15becf9cda 100644
--- a/libavutil/eval.c
+++ b/libavutil/eval.c
@@ -33,6 +33,7 @@
 #include "eval.h"
 #include "ffmath.h"
 #include "internal.h"
+#include "sfc64.h"
 #include "log.h"
 #include "mathematics.h"
 #include "time.h"
@@ -55,7 +56,7 @@ typedef struct Parser {
     void *log_ctx;
 #define VARS 10
     double *var;
-    uint64_t *var_uint64;
+    SFC64 *prng_state;
 } Parser;
 
 static const AVClass eval_class = {
@@ -174,7 +175,7 @@ struct AVExpr {
     } a;
     struct AVExpr *param[3];
     double *var;
-    uint64_t *var_uint64;
+    SFC64 *prng_state;
 };
 
 static double etime(double v)
@@ -233,10 +234,15 @@ static double eval_expr(Parser *p, AVExpr *e)
 
 #define COMPUTE_NEXT_RANDOM()                                        \
             int idx = av_clip(eval_expr(p, e->param[0]), 0, VARS-1); \
-            uint64_t r = p->var_uint64[idx] ? p->var_uint64[idx] : (isnan(p->var[idx]) ? 0 : p->var[idx]);\
-            r = r * 1664525 + 1013904223;                            \
+            SFC64 *s = p->prng_state + idx;                          \
+            uint64_t r;                                              \
+                                                                     \
+            if (!s->counter) {                                       \
+                r = isnan(p->var[idx]) ? 0 : p->var[idx];            \
+                sfc64_init(s, r, r, r, 12);                          \
+            }                                                        \
+            r = sfc64_get(s);                                        \
             p->var[idx] = r;                                         \
-            p->var_uint64[idx]= r;
 
         case e_random: {
             COMPUTE_NEXT_RANDOM();
@@ -334,7 +340,7 @@ static double eval_expr(Parser *p, AVExpr *e)
                 case e_last:return e->value * d2;
                 case e_st :  {
                     int index = av_clip(d, 0, VARS-1);
-                    p->var_uint64[index] = 0;
+                    p->prng_state[index].counter = 0;
                     return e->value * (p->var[index]= d2);
                 }
                 case e_hypot:return e->value * hypot(d, d2);
@@ -356,7 +362,7 @@ 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->prng_state);
     av_freep(&e);
 }
 
@@ -744,8 +750,8 @@ int av_expr_parse(AVExpr **expr, const char *s,
         goto end;
     }
     e->var= av_mallocz(sizeof(double) *VARS);
-    e->var_uint64= av_mallocz(sizeof(uint64_t) *VARS);
-    if (!e->var || !e->var_uint64) {
+    e->prng_state = av_mallocz(sizeof(*e->prng_state) *VARS);
+    if (!e->var || !e->prng_state) {
         ret = AVERROR(ENOMEM);
         goto end;
     }
@@ -787,7 +793,7 @@ 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.prng_state= e->prng_state;
 
     p.const_values = const_values;
     p.opaque     = opaque;
diff --git a/libavutil/sfc64.h b/libavutil/sfc64.h
new file mode 100644
index 00000000000..25bc43abef1
--- /dev/null
+++ b/libavutil/sfc64.h
@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2024 Michael Niedermayer <michael-ffmpeg@niedermayer.cc>
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ *
+ * This is a implementation of SFC64 a 64-bit PRNG by Chris Doty-Humphrey.
+ *
+ * This Generator is much faster (0m1.872s) than 64bit KISS (0m3.823s) and PCG-XSH-RR-64/32 (0m2.700s)
+ * And passes testu01 and practrand test suits.
+ *
+ */
+
+/**
+ * @file
+ * simple Pseudo Random Number Generator
+ *
+ */
+
+#ifndef AVUTIL_SFC64_H
+#define AVUTIL_SFC64_H
+
+#include <inttypes.h>
+
+typedef struct SFC64 {
+    uint64_t a,b,c,counter;
+} SFC64;
+
+static inline uint64_t sfc64_get(SFC64 *s) {
+    uint64_t tmp = s->a + s->b + s->counter++;
+    s->a = s->b ^ (s->b >> 11);
+    s->b = s->c + (s->c << 3); // This is a multiply by 9
+    s->c = ((s->c << 24) | (s->c >> 40)) + tmp;
+    return tmp;
+}
+
+static inline void sfc64_init(SFC64 *s, uint64_t seeda, uint64_t seedb, uint64_t seedc, int rounds) {
+    s->a       = seeda;
+    s->b       = seedb;
+    s->c       = seedc;
+    s->counter = 1;
+    while (rounds--)
+        sfc64_get(s);
+}
+
+#endif // AVUTIL_SFC64_H
diff --git a/tests/ref/fate/eval b/tests/ref/fate/eval
index 5b4d93f4274..441f9846c46 100644
--- a/tests/ref/fate/eval
+++ b/tests/ref/fate/eval
@@ -257,7 +257,7 @@ Evaluating 'root(sin(ld(0))+6+sin(ld(0)/12)-log(ld(0)), 100)'
 'root(sin(ld(0))+6+sin(ld(0)/12)-log(ld(0)), 100)' -> 60.965601
 
 Evaluating '7000000B*random(0)'
-'7000000B*random(0)' -> 0.003078
+'7000000B*random(0)' -> 12864914.486611
 
 Evaluating 'squish(2)'
 'squish(2)' -> 0.000335
-- 
2.17.1

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