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 //#define DEBUG
  32
  33 #ifdef DEBUG
  34 #include "common.h"
  35 #include "log.h"
  36 #endif
  37
  38
  39 /* Period parameters */
  40 #define M 397
  41 #define A 0x9908b0df /* constant vector a */
  42 #define UPPER_MASK 0x80000000 /* most significant w-r bits */
  43 #define LOWER_MASK 0x7fffffff /* least significant r bits */
  44
  45 /** initializes mt[AV_RANDOM_N] with a seed */
  46 void av_init_random(unsigned int seed, AVRandomState *state)
  47 {
  48     int index;
  49
  50     /*
  51      This differs from the wikipedia article.  Source is from the Makoto
  52      Makoto Matsumoto and Takuji Nishimura code, with the following comment:
  53      */
  54      /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
  55      /* In the previous versions, MSBs of the seed affect   */
  56      /* only MSBs of the array mt[].                        */
  57     state->mt[0] = seed & 0xffffffff;
  58     for (index = 1; index < AV_RANDOM_N; index++) {
  59         unsigned int prev= state->mt[index - 1];
  60         state->mt[index] = (1812433253UL * (prev ^ (prev>>30)) + index) & 0xffffffff;
  61     }
  62     state->index= index; // will cause it to generate untempered numbers the first iteration
  63 }
  64
  65 /** generate AV_RANDOM_N words at one time (which will then be tempered later) (av_random calls this; you shouldn't) */
  66 void av_random_generate_untempered_numbers(AVRandomState *state)
  67 {
  68     int kk;
  69     unsigned int y;
  70
  71     for (kk = 0; kk < AV_RANDOM_N - M; kk++) {
  72         y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
  73         state->mt[kk] = state->mt[kk + M] ^ (y >> 1) ^ ((y&1)*A);
  74     }
  75     for (; kk < AV_RANDOM_N - 1; kk++) {
  76         y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
  77         state->mt[kk] = state->mt[kk + (M - AV_RANDOM_N)] ^ (y >> 1) ^ ((y&1)*A);
  78     }
  79     y = (state->mt[AV_RANDOM_N - 1] & UPPER_MASK) | (state->mt[0] & LOWER_MASK);
  80     state->mt[AV_RANDOM_N - 1] = state->mt[M - 1] ^ (y >> 1) ^ ((y&1)*A);
  81     state->index = 0;
  82 }
  83
  84 #ifdef DEBUG
  85 void av_benchmark_random(void)
  86 {
  87     int x=0;
  88     int i, j;
  89     AVRandomState state;
  90
  91     av_init_random(0xdeadbeef, &state);
  92     for (j = 0; j < 100; j++) {
  93         START_TIMER;
  94         x+= av_random(&state);
  95         STOP_TIMER("first call to av_random");
  96         for (i = 1; i < AV_RANDOM_N; i++) {
  97             START_TIMER;
  98             x+= av_random(&state);
  99             STOP_TIMER("AV_RANDOM_N calls of av_random");
 100         }
 101     }
 102     av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
 103 }
 104 #endif