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-rw-r--r--shell/random.c37
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diff --git a/shell/random.c b/shell/random.c
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+/* vi: set sw=4 ts=4: */
+/*
+ * $RANDOM support.
+ *
+ * Copyright (C) 2008 Denys Vlasenko
+ *
+ * Licensed under GPLv2, see file LICENSE in this tarball for details.
+ */
+#include "libbb.h"
+#include "random.h"
+
+uint32_t FAST_FUNC
+next_random(random_t *rnd)
+{
+ /* Galois LFSR parameter */
+ /* Taps at 32 31 29 1: */
+ enum { MASK = 0x8000000b };
+ /* Another example - taps at 32 31 30 10: */
+ /* MASK = 0x00400007 */
+
+ uint32_t t;
+
+ /* LCG has period of 2^32 and alternating lowest bit */
+ rnd->LCG = 1664525 * rnd->LCG + 1013904223;
+ /* Galois LFSR has period of 2^32-1 = 3 * 5 * 17 * 257 * 65537 */
+ t = (rnd->galois_LFSR << 1);
+ if (rnd->galois_LFSR < 0) /* if we just shifted 1 out of msb... */
+ t ^= MASK;
+ rnd->galois_LFSR = t;
+ /* Both are weak, combining them gives better randomness
+ * and ~2^64 period. & 0x7fff is probably bash compat
+ * for $RANDOM range. Combining with subtraction is
+ * just for fun. + and ^ would work equally well. */
+ t = (t - rnd->LCG) & 0x7fff;
+
+ return t;
+}