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;
+}