Last change
on this file since 3734 was 3320, checked in by Bruno Cornec, 10 years ago |
- Re-add (thanks git BTW) the 2.2.9 branch which had been destroyed in
the move to 3.0
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-
Property svn:eol-style
set to
native
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File size:
1.2 KB
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Rev | Line | |
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[3320] | 1 | /* vi: set sw=4 ts=4: */
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| 2 | /*
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| 3 | * $RANDOM support.
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| 4 | *
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| 5 | * Copyright (C) 2009 Denys Vlasenko
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| 6 | *
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| 7 | * Licensed under GPLv2, see file LICENSE in this source tree.
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| 8 | */
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| 9 | #include "libbb.h"
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| 10 | #include "random.h"
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| 11 |
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| 12 | uint32_t FAST_FUNC
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| 13 | next_random(random_t *rnd)
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| 14 | {
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| 15 | /* Galois LFSR parameter */
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| 16 | /* Taps at 32 31 29 1: */
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| 17 | enum { MASK = 0x8000000b };
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| 18 | /* Another example - taps at 32 31 30 10: */
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| 19 | /* MASK = 0x00400007 */
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| 20 |
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| 21 | uint32_t t;
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| 22 |
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| 23 | if (UNINITED_RANDOM_T(rnd)) {
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| 24 | /* Can use monotonic_ns() for better randomness but for now
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| 25 | * it is not used anywhere else in busybox... so avoid bloat
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| 26 | */
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| 27 | INIT_RANDOM_T(rnd, getpid(), monotonic_us());
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| 28 | }
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| 29 |
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| 30 | /* LCG has period of 2^32 and alternating lowest bit */
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| 31 | rnd->LCG = 1664525 * rnd->LCG + 1013904223;
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| 32 | /* Galois LFSR has period of 2^32-1 = 3 * 5 * 17 * 257 * 65537 */
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| 33 | t = (rnd->galois_LFSR << 1);
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| 34 | if (rnd->galois_LFSR < 0) /* if we just shifted 1 out of msb... */
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| 35 | t ^= MASK;
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| 36 | rnd->galois_LFSR = t;
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| 37 | /* Both are weak, combining them gives better randomness
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| 38 | * and ~2^64 period. & 0x7fff is probably bash compat
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| 39 | * for $RANDOM range. Combining with subtraction is
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| 40 | * just for fun. + and ^ would work equally well. */
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| 41 | t = (t - rnd->LCG) & 0x7fff;
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| 42 |
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| 43 | return t;
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| 44 | }
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