blob: cca9d120dfcaee290a97a95c319d68d2da6785f4 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
|
/* vi: set sw=4 ts=4: */
/*
* $RANDOM support.
*
* Copyright (C) 2009 Denys Vlasenko
*
* Licensed under GPLv2, see file LICENSE in this tarball for details.
*/
#include "libbb.h"
#include "random.h"
uint32_t FAST_FUNC
next_random(random_t *rnd)
{
/* Galois LFSR parameter */
/* Taps at 32 31 29 1: */
enum { MASK = 0x8000000b };
/* Another example - taps at 32 31 30 10: */
/* MASK = 0x00400007 */
uint32_t t;
/* LCG has period of 2^32 and alternating lowest bit */
rnd->LCG = 1664525 * rnd->LCG + 1013904223;
/* Galois LFSR has period of 2^32-1 = 3 * 5 * 17 * 257 * 65537 */
t = (rnd->galois_LFSR << 1);
if (rnd->galois_LFSR < 0) /* if we just shifted 1 out of msb... */
t ^= MASK;
rnd->galois_LFSR = t;
/* Both are weak, combining them gives better randomness
* and ~2^64 period. & 0x7fff is probably bash compat
* for $RANDOM range. Combining with subtraction is
* just for fun. + and ^ would work equally well. */
t = (t - rnd->LCG) & 0x7fff;
return t;
}
|
