libavutil/random.c

   1 /*
   2  * Mersenne Twister PRNG algorithm
   3  * Copyright (c) 2006 Ryan Martell
   4  * Based on a C program for MT19937, with initialization improved 2002/1/26.
   5  * Coded by 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
  27  * for an explanation of this algorithm.
  28  */
  29 #include <stdio.h>
  30 #include "random.h"
  31
  32
  33 /* period parameters */
  34 #define M 397
  35 #define A 0x9908b0df /* constant vector a */
  36 #define UPPER_MASK 0x80000000 /* most significant w-r bits */
  37 #define LOWER_MASK 0x7fffffff /* least significant r bits */
  38
  39 /** Initializes mt[AV_RANDOM_N] with a seed. */
  40 void av_random_init(AVRandomState *state, unsigned int seed)
  41 {
  42     int index;
  43
  44     /*
  45      This differs from the Wikipedia article.  Source is from the
  46      Makoto Matsumoto and Takuji Nishimura code, with the following comment:
  47      */
  48      /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
  49      /* In the previous versions, MSBs of the seed affect   */
  50      /* only MSBs of the array mt[].                        */
  51     state->mt[0] = seed & 0xffffffff;
  52     for (index = 1; index < AV_RANDOM_N; index++) {
  53         unsigned int prev= state->mt[index - 1];
  54         state->mt[index] = (1812433253UL * (prev ^ (prev>>30)) + index) & 0xffffffff;
  55     }
  56     state->index= index; // Will cause it to generate untempered numbers in the first iteration.
  57 }
  58
  59 /** Generates AV_RANDOM_N words at one time (which will then be tempered later).
  60  * av_random calls this; you shouldn't. */
  61 void av_random_generate_untempered_numbers(AVRandomState *state)
  62 {
  63     int kk;
  64     unsigned int y;
  65
  66     for (kk = 0; kk < AV_RANDOM_N - M; kk++) {
  67         y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
  68         state->mt[kk] = state->mt[kk + M] ^ (y >> 1) ^ ((y&1)*A);
  69     }
  70     for (; kk < AV_RANDOM_N - 1; kk++) {
  71         y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
  72         state->mt[kk] = state->mt[kk + (M - AV_RANDOM_N)] ^ (y >> 1) ^ ((y&1)*A);
  73     }
  74     y = (state->mt[AV_RANDOM_N - 1] & UPPER_MASK) | (state->mt[0] & LOWER_MASK);
  75     state->mt[AV_RANDOM_N - 1] = state->mt[M - 1] ^ (y >> 1) ^ ((y&1)*A);
  76     state->index = 0;
  77 }
  78
  79 #ifdef TEST
  80 #include "common.h"
  81 #include "log.h"
  82 int main(void)
  83 {
  84     int x=0;
  85     int i, j;
  86     AVRandomState state;
  87
  88     av_random_init(&state, 0xdeadbeef);
  89     for (j = 0; j < 10000; j++) {
  90         START_TIMER
  91         for (i = 0; i < 624; i++) {
  92             x+= av_random(&state);
  93         }
  94         STOP_TIMER("624 calls of av_random");
  95     }
  96     av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
  97     return 0;
  98 }
  99 #endif