Add lrand48_uniform, based on OpenBSD arc4random_uniform

Makefile

@@ -19,8 +19,8 @@ V =	0.0
 
 LDLIBS =	-lm
 
-HDR =		err.h note.h synth.h
-OBJ =		err.o main.o note.o synth.o
+HDR =		err.h lrand48_uniform.h note.h synth.h
+OBJ =		err.o lrand48_uniform.o main.o note.o synth.o
 SRC =		${OBJ:.o=.c}
 DIST_FILES =	COPYING Makefile ${HDR} ${SRC}
 

lrand48_uniform.c

@@ -0,0 +1,70 @@
+/*	$OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $	*/
+
+/*
+ * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+/*
+ * arngment - random music generator
+ *
+ * Written in 2020 by Lucas
+ *
+ * To the extent possible under law, the author(s) have dedicated all
+ * copyright and related and neighboring rights to this software to the
+ * public domain worldwide. This software is distributed without any
+ * warranty.
+ *
+ * You should have received a copy of the CC0 Public Domain Dedication
+ * along with this software. If not, see
+ * <http://creativecommons.org/publicdomain/zero/1.0/>.
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+
+/*
+ * Calculate a uniformly distributed random number less than upper_bound
+ * avoiding "modulo bias".
+ *
+ * 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.
+ */
+long
+lrand48_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 = lrand48();
+		if (r >= min)
+			break;
+	}
+
+	return r % upper_bound;
+}

lrand48_uniform.h

@@ -0,0 +1,18 @@
+/*
+ * arngment - random music generator
+ *
+ * Written in 2020 by Lucas
+ *
+ * To the extent possible under law, the author(s) have dedicated all
+ * copyright and related and neighboring rights to this software to the
+ * public domain worldwide. This software is distributed without any
+ * warranty.
+ *
+ * You should have received a copy of the CC0 Public Domain Dedication
+ * along with this software. If not, see
+ * <http://creativecommons.org/publicdomain/zero/1.0/>.
+ */
+
+#include <stdint.h>
+
+long lrand48_uniform(uint32_t);

main.c

@@ -17,9 +17,11 @@
 #include <stdio.h>
 #include <stdint.h>
 #include <stdlib.h>
+#include <time.h>
 #include <unistd.h>
 
 #include "err.h"
+#include "lrand48_uniform.h"
 #include "note.h"
 #include "synth.h"
 
@@ -63,8 +65,9 @@ main(int argc, char *argv[])
 	if (!note_parse(&base_note, argv[0]))
 		errx(1, "Invalid note %s", argv[0]);
 
+	srand48(time(NULL));
 	for (;;) {
-		k = arc4random_uniform(nelems(scale));
+		k = lrand48_uniform(nelems(scale));
 		note = base_note;
 		note.offset += scale[k];
 		for (i = 0; i < RATE / 2; i += 2) {
@@ -77,7 +80,7 @@ main(int argc, char *argv[])
 			buf[i + 1] = r * INT16_MAX;
 		}
 
-		k = arc4random_uniform(nelems(scale));
+		k = lrand48_uniform(nelems(scale));
 		note = base_note;
 		note.offset += scale[k];
 		for (; i < RATE; i += 2) {