libavutil/random.h

   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 #ifndef AVUTIL_RANDOM_H
  25 #define AVUTIL_RANDOM_H
  26
  27 #define AV_RANDOM_N 624
  28
  29 typedef struct {
  30     unsigned int mt[AV_RANDOM_N]; ///< the array for the state vector
  31     int index; ///< Current untempered value we use as the base.
  32 } AVRandomState;
  33
  34
  35 void av_init_random(unsigned int seed, AVRandomState *state); ///< To be inlined, the struct must be visible. So it does not make sense to try and keep it opaque with malloc/free-like calls.
  36 void av_random_generate_untempered_numbers(AVRandomState *state); ///< Regenerate the untempered numbers (must be done every 624 iterations, or it will loop).
  37
  38 /**
  39  * Generates a random number from the interval [0,0xffffffff].
  40  *
  41  * Please do NOT use the Mersenne Twister, it is slow. Use the random generator
  42  * from lfg.c/h or a simple LCG like state= state*1664525+1013904223.
  43  * If you still choose to use MT, expect that you will have to provide
  44  * some evidence that it makes a difference for the case where you use it.
  45  */
  46 static inline unsigned int av_random(AVRandomState *state)
  47 {
  48     unsigned int y;
  49
  50     // Regenerate the untempered numbers if we should...
  51     if (state->index >= AV_RANDOM_N)
  52         av_random_generate_untempered_numbers(state);
  53
  54     // Grab one...
  55     y = state->mt[state->index++];
  56
  57     /* Now temper (Mersenne Twister coefficients). The coefficients for MT19937 are.. */
  58     y ^= (y >> 11);
  59     y ^= (y << 7) & 0x9d2c5680;
  60     y ^= (y << 15) & 0xefc60000;
  61     y ^= (y >> 18);
  62
  63     return y;
  64 }
  65
  66 /** Return random in range [0-1] as double. */
  67 static inline double av_random_real1(AVRandomState *state)
  68 {
  69     /* divided by 2^32-1 */
  70     return av_random(state) * (1.0 / 4294967296.0);
  71 }
  72
  73 #endif /* AVUTIL_RANDOM_H */