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@@ -6,17 +6,51 @@
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*
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* Licensed under GPLv2, see file LICENSE in this source tree.
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*/
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-#include "libbb.h"
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-#include "random.h"
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+
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+/* For testing against dieharder, you need only random.{c,h}
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+ * Howto:
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+ * gcc -O2 -Wall -DRANDTEST random.c -o random
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+ * ./random | dieharder -g 200 -a
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+ */
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+
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+#if !defined RANDTEST
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+
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+# include "libbb.h"
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+# include "random.h"
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+# define RAND_BASH_MASK 0x7fff
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+
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+#else
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+# include <stdint.h>
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+# include <unistd.h>
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+# include <stdio.h>
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+# include <time.h>
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+# define RAND_BASH_MASK 0xffffffff /* off */
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+# define FAST_FUNC /* nothing */
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+# define PUSH_AND_SET_FUNCTION_VISIBILITY_TO_HIDDEN /* nothing */
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+# define POP_SAVED_FUNCTION_VISIBILITY /* nothing */
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+# define monotonic_us() time(NULL)
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+# include "random.h"
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+#endif
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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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+ /* Galois LFSR parameter:
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+ * Taps at 32 31 29 1:
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+ */
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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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+ /* enum { MASK = 0x00400007 }; */
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+
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+ /* Xorshift parameters:
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+ * Choices for a,b,c: 10,13,10; 8,9,22; 2,7,3; 23,3,24
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+ * (given by algorithm author)
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+ */
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+ enum {
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+ a = 2,
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+ b = 7,
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+ c = 3,
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+ };
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uint32_t t;
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@@ -27,18 +61,94 @@ next_random(random_t *rnd)
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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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+ /* LCG: period of 2^32, but quite weak:
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+ * bit 0 alternates beetween 0 and 1 (pattern of length 2)
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+ * bit 1 has a repeating pattern of length 4
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+ * bit 2 has a repeating pattern of length 8
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+ * etc...
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+ */
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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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+
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+ /* Galois LFSR:
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+ * period of 2^32-1 = 3 * 5 * 17 * 257 * 65537.
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+ * Successive values are right-shifted one bit
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+ * and possibly xored with a sparse constant.
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+ */
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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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+ /* http://en.wikipedia.org/wiki/Xorshift
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+ * Period 2^64-1 = 3 * 715827883 * 2147483647
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+ * Moderately good statistical properties:
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+ * fails the following "dieharder -g 200 -a" tests:
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+ * diehard_operm5| 0
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+ * diehard_oqso| 0
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+ * diehard_count_1s_byt| 0
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+ * diehard_3dsphere| 3
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+ * diehard_squeeze| 0
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+ * diehard_runs| 0
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+ * diehard_runs| 0
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+ * diehard_craps| 0
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+ * diehard_craps| 0
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+ * rgb_minimum_distance| 3
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+ * rgb_minimum_distance| 4
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+ * rgb_minimum_distance| 5
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+ * rgb_permutations| 3
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+ * rgb_permutations| 4
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+ * rgb_permutations| 5
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+ * dab_filltree| 32
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+ * dab_filltree| 32
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+ * dab_monobit2| 12
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+ */
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+ t = rnd->xs64_x ^ (rnd->xs64_x << a);
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+ rnd->xs64_x = rnd->xs64_y;
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+ rnd->xs64_y = rnd->xs64_y ^ (rnd->xs64_y >> c) ^ t ^ (t >> b);
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+
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+ /* Combined LCG + Galois LFSR have 2^32 * 2^32-1 period.
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+ * Strength:
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+ * individually, both are extremely weak cryptographycally;
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+ * when combined, they fail the following "dieharder -g 200 -a" tests:
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+ * diehard_rank_6x8| 0
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+ * diehard_oqso| 0
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+ * diehard_dna| 0
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+ * diehard_count_1s_byt| 0
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+ * rgb_bitdist| 2
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+ * dab_monobit2| 12
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+ *
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+ * Combining them with xorshift-64 increases period to
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+ * 2^32 * 2^32-1 * 2^64-1 / 3
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+ * (2^32-1 and 2^64-1 have one common divisor 3, hence "/ 3" part),
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+ * which is about 2^128 / 3, or in base 10 ~1.13*10^38.
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+ * Strength of the combination:
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+ * passes all "dieharder -g 200 -a" tests.
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+ *
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+ * Combining with subtraction and addition is just for fun.
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+ * It does not add meaningful strength, could use xor operation instead.
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+ */
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+ t = rnd->galois_LFSR - rnd->LCG + rnd->xs64_y;
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+
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+ /* bash compat $RANDOM range: */
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+ return t & RAND_BASH_MASK;
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}
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+
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+#ifdef RANDTEST
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+static random_t rnd;
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+
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+int main(int argc, char **argv)
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+{
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+ int i;
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+ int buf[4096];
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+
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+ for (;;) {
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+ for (i = 0; i < sizeof(buf) / sizeof(buf[0]); i++) {
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+ buf[i] = next_random(&rnd);
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+ }
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+ write(1, buf, sizeof(buf));
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+ }
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+
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+ return 0;
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+}
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+
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+#endif
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