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/* vi: set sw=4 ts=4: */
/*
* $RANDOM support.
*
* Copyright (C) 2009 Denys Vlasenko
*
* Licensed under GPLv2, see file LICENSE in this source tree.
*/
#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;
if (UNINITED_RANDOM_T(rnd)) {
/* Can use monotonic_ns() for better randomness but for now
* it is not used anywhere else in busybox... so avoid bloat
*/
INIT_RANDOM_T(rnd, getpid(), monotonic_us());
}
/* 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;
}
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