76ace254e1
function old new delta next_random 46 68 +22 forkshell 248 263 +15 expand_vars_to_list 2118 2131 +13 run_pipe 1775 1782 +7 popstring 134 140 +6 builtin_umask 123 121 -2 ash_main 1356 1336 -20 get_local_var_value 125 104 -21 ------------------------------------------------------------------------------ (add/remove: 0/0 grow/shrink: 5/3 up/down: 63/-43) Total: 20 bytes Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
45 lines
1.2 KiB
C
45 lines
1.2 KiB
C
/* vi: set sw=4 ts=4: */
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/*
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* $RANDOM support.
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*
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* Copyright (C) 2009 Denys Vlasenko
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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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if (UNINITED_RANDOM_T(rnd)) {
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/* Can use monotonic_ns() for better randomness but for now
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* it is not used anywhere else in busybox... so avoid bloat
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*/
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INIT_RANDOM_T(rnd, getpid(), monotonic_us());
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}
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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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