libavutil/random.c

   1 /*
   2  * Mersenne Twister Random Algorithm
   3  * Copyright (c) 2006 Ryan Martell.
   4  * Based on A C-program for MT19937, with initialization improved 2002/1/26. Coded by
   5  * Takuji Nishimura and Makoto Matsumoto.
   6  *
   7  * This file is part of FFmpeg.
   8  *
   9  * FFmpeg is free software; you can redistribute it and/or
  10  * modify it under the terms of the GNU Lesser General Public
  11  * License as published by the Free Software Foundation; either
  12  * version 2.1 of the License, or (at your option) any later version.
  13  *
  14  * FFmpeg is distributed in the hope that it will be useful,
  15  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  16  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17  * Lesser General Public License for more details.
  18  *
  19  * You should have received a copy of the GNU Lesser General Public
  20  * License along with FFmpeg; if not, write to the Free Software
  21  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  22  */
  23
  24
  25 /**
  26 see http://en.wikipedia.org/wiki/Mersenne_twister for an explanation of this algorithm.
  27 */
  28 #include <stdio.h>
  29 #include "random.h"
  30
  31
  32 /* Period parameters */
  33 #define M 397
  34 #define A 0x9908b0df /* constant vector a */
  35 #define UPPER_MASK 0x80000000 /* most significant w-r bits */
  36 #define LOWER_MASK 0x7fffffff /* least significant r bits */
  37
  38 /** initializes mt[AV_RANDOM_N] with a seed */
  39 void av_init_random(unsigned int seed, AVRandomState *state)
  40 {
  41     int index;
  42
  43     /*
  44      This differs from the wikipedia article.  Source is from the
  45      Makoto Matsumoto and Takuji Nishimura code, with the following comment:
  46      */
  47      /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
  48      /* In the previous versions, MSBs of the seed affect   */
  49      /* only MSBs of the array mt[].                        */
  50     state->mt[0] = seed & 0xffffffff;
  51     for (index = 1; index < AV_RANDOM_N; index++) {
  52         unsigned int prev= state->mt[index - 1];
  53         state->mt[index] = (1812433253UL * (prev ^ (prev>>30)) + index) & 0xffffffff;
  54     }
  55     state->index= index; // will cause it to generate untempered numbers the first iteration
  56 }
  57
  58 /** generate AV_RANDOM_N words at one time (which will then be tempered later) (av_random calls this; you shouldn't) */
  59 void av_random_generate_untempered_numbers(AVRandomState *state)
  60 {
  61     int kk;
  62     unsigned int y;
  63
  64     for (kk = 0; kk < AV_RANDOM_N - M; kk++) {
  65         y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
  66         state->mt[kk] = state->mt[kk + M] ^ (y >> 1) ^ ((y&1)*A);
  67     }
  68     for (; kk < AV_RANDOM_N - 1; kk++) {
  69         y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
  70         state->mt[kk] = state->mt[kk + (M - AV_RANDOM_N)] ^ (y >> 1) ^ ((y&1)*A);
  71     }
  72     y = (state->mt[AV_RANDOM_N - 1] & UPPER_MASK) | (state->mt[0] & LOWER_MASK);
  73     state->mt[AV_RANDOM_N - 1] = state->mt[M - 1] ^ (y >> 1) ^ ((y&1)*A);
  74     state->index = 0;
  75 }
  76
  77 #ifdef TEST
  78 #include "common.h"
  79 #include "log.h"
  80 int main(void)
  81 {
  82     int x=0;
  83     int i, j;
  84     AVRandomState state;
  85
  86     av_init_random(0xdeadbeef, &state);
  87     for (j = 0; j < 10000; j++) {
  88         START_TIMER
  89         for (i = 0; i < 624; i++) {
  90             x+= av_random(&state);
  91         }
  92         STOP_TIMER("624 calls of av_random");
  93     }
  94     av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
  95     return 0;
  96 }
  97 #endif