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>

1 files changed, 37 insertions, 0 deletions

diff --git a/shell/random.c b/shell/random.c
new file mode 100644
index 000000000..a4dce846d
--- /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;
+}