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lfg.c
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1 /*
2  * Lagged Fibonacci PRNG
3  * Copyright (c) 2008 Michael Niedermayer
4  *
5  * This file is part of FFmpeg.
6  *
7  * FFmpeg is free software; you can redistribute it and/or
8  * modify it under the terms of the GNU Lesser General Public
9  * License as published by the Free Software Foundation; either
10  * version 2.1 of the License, or (at your option) any later version.
11  *
12  * FFmpeg is distributed in the hope that it will be useful,
13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15  * Lesser General Public License for more details.
16  *
17  * You should have received a copy of the GNU Lesser General Public
18  * License along with FFmpeg; if not, write to the Free Software
19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20  */
21 
22 #include <inttypes.h>
23 #include <limits.h>
24 #include <math.h>
25 #include "lfg.h"
26 #include "md5.h"
27 #include "intreadwrite.h"
28 #include "attributes.h"
29 
30 av_cold void av_lfg_init(AVLFG *c, unsigned int seed)
31 {
32  uint8_t tmp[16] = { 0 };
33  int i;
34 
35  for (i = 8; i < 64; i += 4) {
36  AV_WL32(tmp, seed);
37  tmp[4] = i;
38  av_md5_sum(tmp, tmp, 16);
39  c->state[i ] = AV_RL32(tmp);
40  c->state[i + 1] = AV_RL32(tmp + 4);
41  c->state[i + 2] = AV_RL32(tmp + 8);
42  c->state[i + 3] = AV_RL32(tmp + 12);
43  }
44  c->index = 0;
45 }
46 
47 void av_bmg_get(AVLFG *lfg, double out[2])
48 {
49  double x1, x2, w;
50 
51  do {
52  x1 = 2.0 / UINT_MAX * av_lfg_get(lfg) - 1.0;
53  x2 = 2.0 / UINT_MAX * av_lfg_get(lfg) - 1.0;
54  w = x1 * x1 + x2 * x2;
55  } while (w >= 1.0);
56 
57  w = sqrt((-2.0 * log(w)) / w);
58  out[0] = x1 * w;
59  out[1] = x2 * w;
60 }
61 
62 #ifdef TEST
63 #include "log.h"
64 #include "timer.h"
65 
66 int main(void)
67 {
68  int x = 0;
69  int i, j;
70  AVLFG state;
71 
72  av_lfg_init(&state, 0xdeadbeef);
73  for (j = 0; j < 10000; j++) {
75  for (i = 0; i < 624; i++) {
76  //av_log(NULL, AV_LOG_ERROR, "%X\n", av_lfg_get(&state));
77  x += av_lfg_get(&state);
78  }
79  STOP_TIMER("624 calls of av_lfg_get");
80  }
81  av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
82 
83  /* BMG usage example */
84  {
85  double mean = 1000;
86  double stddev = 53;
87 
88  av_lfg_init(&state, 42);
89 
90  for (i = 0; i < 1000; i += 2) {
91  double bmg_out[2];
92  av_bmg_get(&state, bmg_out);
94  "%f\n%f\n",
95  bmg_out[0] * stddev + mean,
96  bmg_out[1] * stddev + mean);
97  }
98  }
99 
100  return 0;
101 }
102 #endif