sbase

commit 2677235ed7f86f26c817ca5eca7c730f2418638a
parent 83182aa959b2100ea0cf6766e6ef3a553877a710
Author: Elie Le Vaillant <eolien55@disroot.org>
Date:   Fri,  6 Dec 2024 10:37:35 +0100

libutil: add random.c

Some programs need a good PRNG, such as shuf(1), or cron(1).  This adds
to libutil a random_uniform function which simply solves the problem of
creating integers uniformly in a range.  random_seed seeds the
generator.  arc4random would probably be a better PRNG than random, but
it is less portable unfortunately.

Diffstat:
M
Makefile
 | 
1
+
A
libutil/random.c
 | 
46
++++++++++++++++++++++++++++++++++++++++++++++

2 files changed, 47 insertions(+), 0 deletions(-)
diff --git a/Makefile b/Makefile
@@ -86,6 +86,7 @@ LIBUTILOBJ =\
 	libutil/strsep.o\
 	libutil/strnsubst.o\
 	libutil/strtonum.o\
+	libutil/random.o\
 	libutil/unescape.o\
 	libutil/writeall.o
 
diff --git a/libutil/random.c b/libutil/random.c
@@ -0,0 +1,46 @@
+#include <stdint.h>
+#include <stdlib.h>
+#include <time.h>
+
+/*
+ * Uniformity is achieved by generating new random numbers until the one
+ * returned is outside the range [0, 2**32 % upper_bound).  This
+ * guarantees the selected random number will be inside
+ * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
+ * after reduction modulo upper_bound.
+ *
+ * Copied off OpenBSD (original is arc4random_uniform)
+ */
+uint32_t
+random_uniform(uint32_t upper_bound)
+{
+	uint32_t r, min;
+
+	if (upper_bound < 2)
+		return 0;
+
+	/* 2**32 % x == (2**32 - x) % x */
+	min = -upper_bound % upper_bound;
+
+	/*
+	 * This could theoretically loop forever but each retry has
+	 * p > 0.5 (worst case, usually far better) of selecting a
+	 * number inside the range we need, so it should rarely need
+	 * to re-roll.
+	 */
+	for (;;) {
+		r = random(); /* arc4random() is better, but we don't always have it */
+		if (r >= min)
+			break;
+	}
+
+	return r % upper_bound;
+}
+
+void
+random_seed(void)
+{
+	struct timespec ts;
+	clock_gettime(CLOCK_REALTIME, &ts);
+	srandom(ts.tv_nsec); /* not a good source of randomness, but eh */
+}