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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; }