Last change
on this file since 3735 was 2725, checked in by Bruno Cornec, 13 years ago |
- Update mindi-busybox to 1.18.3 to avoid problems with the tar command which is now failing on recent versions with busybox 1.7.3
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-
Property svn:eol-style
set to
native
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File size:
1.2 KB
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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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