2009-10-10 00:29:04 +05:30
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/* vi: set sw=4 ts=4: */
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/*
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* $RANDOM support.
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*
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2009-10-10 03:05:30 +05:30
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* Copyright (C) 2009 Denys Vlasenko
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2009-10-10 00:29:04 +05:30
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*
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* Licensed under GPLv2, see file LICENSE in this tarball for details.
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*/
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#include "libbb.h"
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#include "random.h"
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uint32_t FAST_FUNC
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next_random(random_t *rnd)
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{
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/* Galois LFSR parameter */
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/* Taps at 32 31 29 1: */
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enum { MASK = 0x8000000b };
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/* Another example - taps at 32 31 30 10: */
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/* MASK = 0x00400007 */
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uint32_t t;
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/* LCG has period of 2^32 and alternating lowest bit */
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rnd->LCG = 1664525 * rnd->LCG + 1013904223;
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/* Galois LFSR has period of 2^32-1 = 3 * 5 * 17 * 257 * 65537 */
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t = (rnd->galois_LFSR << 1);
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if (rnd->galois_LFSR < 0) /* if we just shifted 1 out of msb... */
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t ^= MASK;
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rnd->galois_LFSR = t;
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/* Both are weak, combining them gives better randomness
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* and ~2^64 period. & 0x7fff is probably bash compat
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* for $RANDOM range. Combining with subtraction is
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* just for fun. + and ^ would work equally well. */
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t = (t - rnd->LCG) & 0x7fff;
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return t;
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}
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