Statistics
| Branch: | Revision:

ffmpeg / libavutil / random.c @ b0973124

History | View | Annotate | Download (3.36 KB)

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_random_init(AVRandomState *state, unsigned int seed)
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
void av_init_random(unsigned int seed, AVRandomState *state)
59
{
60
    av_random_init(state, seed);
61
}
62

    
63
/** generate AV_RANDOM_N words at one time (which will then be tempered later) (av_random calls this; you shouldn't) */
64
void av_random_generate_untempered_numbers(AVRandomState *state)
65
{
66
    int kk;
67
    unsigned int y;
68

    
69
    for (kk = 0; kk < AV_RANDOM_N - M; kk++) {
70
        y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
71
        state->mt[kk] = state->mt[kk + M] ^ (y >> 1) ^ ((y&1)*A);
72
    }
73
    for (; kk < AV_RANDOM_N - 1; kk++) {
74
        y = (state->mt[kk] & UPPER_MASK) | (state->mt[kk + 1] & LOWER_MASK);
75
        state->mt[kk] = state->mt[kk + (M - AV_RANDOM_N)] ^ (y >> 1) ^ ((y&1)*A);
76
    }
77
    y = (state->mt[AV_RANDOM_N - 1] & UPPER_MASK) | (state->mt[0] & LOWER_MASK);
78
    state->mt[AV_RANDOM_N - 1] = state->mt[M - 1] ^ (y >> 1) ^ ((y&1)*A);
79
    state->index = 0;
80
}
81

    
82
#ifdef TEST
83
#include "common.h"
84
#include "log.h"
85
int main(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 < 10000; j++) {
93
        START_TIMER
94
        for (i = 0; i < 624; i++) {
95
            x+= av_random(&state);
96
        }
97
        STOP_TIMER("624 calls of av_random");
98
    }
99
    av_log(NULL, AV_LOG_ERROR, "final value:%X\n", x);
100
    return 0;
101
}
102
#endif