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authorDenys Vlasenko2009-10-09 20:59:04 +0200
committerDenys Vlasenko2009-10-09 20:59:04 +0200
commit3ea2e82dc7b32703f5043c3b30b049905a0d07aa (patch)
treeeae992a986a4ce66efa32845ecccc204a0c4c0e1 /shell/random.c
parenta05b2b856bf095dfd5f575c106d1a9e5ac6edd3c (diff)
downloadbusybox-3ea2e82dc7b32703f5043c3b30b049905a0d07aa.zip
busybox-3ea2e82dc7b32703f5043c3b30b049905a0d07aa.tar.gz
ash: factor out $RANDOM support
function old new delta next_random - 46 +46 ash_main 1361 1356 -5 change_random 132 97 -35 ------------------------------------------------------------------------------ (add/remove: 1/0 grow/shrink: 0/2 up/down: 46/-40) Total: 6 bytes Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Diffstat (limited to 'shell/random.c')
-rw-r--r--shell/random.c37
1 files changed, 37 insertions, 0 deletions
diff --git a/shell/random.c b/shell/random.c
new file mode 100644
index 0000000..a4dce84
--- /dev/null
+++ b/shell/random.c
@@ -0,0 +1,37 @@
+/* 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;
+}