libavutil/lfg.c

   1 /*
   2  * Lagged Fibonacci PRNG
   3  * Copyright (c) 2008 Michael Niedermayer
   4  *
   5  * This file is part of Libav.
   6  *
   7  * Libav 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  * Libav 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 Libav; 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++) {
  74         START_TIMER
  75         for (i = 0; i < 624; i++)
  76             x += av_lfg_get(&state);
  77         STOP_TIMER("624 calls of av_lfg_get");
  78     }
  79     av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
  80
  81     /* BMG usage example */
  82     {
  83         double mean   = 1000;
  84         double stddev = 53;
  85
  86         av_lfg_init(&state, 42);
  87
  88         for (i = 0; i < 1000; i += 2) {
  89             double bmg_out[2];
  90             av_bmg_get(&state, bmg_out);
  91             av_log(NULL, AV_LOG_INFO,
  92                    "%f\n%f\n",
  93                    bmg_out[0] * stddev + mean,
  94                    bmg_out[1] * stddev + mean);
  95         }
  96     }
  97
  98     return 0;
  99 }
 100 #endif