| /* 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; |
| |
| 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; |
| } |