--- a/libmlk-core/core/util.c	Wed Dec 23 15:37:48 2020 +0100
+++ b/libmlk-core/core/util.c	Mon Jan 04 11:34:18 2021 +0100
@@ -48,5 +48,5 @@
 {
 	assert(upper <= RAND_MAX);
 
-	return (rand() % (upper - lower + 1)) + lower;
+	return (rand() % (upper - lower)) + lower;
 }

--- a/tests/CMakeLists.txt	Wed Dec 23 15:37:48 2020 +0100
+++ b/tests/CMakeLists.txt	Mon Jan 04 11:34:18 2021 +0100
@@ -40,3 +40,5 @@
 	SOURCES test-tileset.c
 	FLAGS DIRECTORY="${tests_SOURCE_DIR}/assets/maps/"
 )
+
+molko_define_test(TARGET util SOURCES test-util.c)

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-util.c	Mon Jan 04 11:34:18 2021 +0100
@@ -0,0 +1,63 @@
+/*
+ * test-util.c -- test utilities
+ *
+ * Copyright (c) 2020 David Demelier <markand@malikania.fr>
+ *
+ * Permission to use, copy, modify, and/or 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.
+ */
+
+#include <stdbool.h>
+
+#define GREATEST_USE_ABBREVS 0
+#include <greatest.h>
+
+#include <core/util.h>
+
+GREATEST_TEST
+nrand_simple(void)
+{
+	bool found[10] = {false};
+
+	/* Only try from [2..5], util_nrand does not include upper range. */
+	for (int i = 0; i < 100000; ++i)
+		found[util_nrand(2, 6)] = true;
+
+	GREATEST_ASSERT_EQ(false, found[0]);
+	GREATEST_ASSERT_EQ(false, found[1]);
+	GREATEST_ASSERT_EQ(true,  found[2]);
+	GREATEST_ASSERT_EQ(true,  found[3]);
+	GREATEST_ASSERT_EQ(true,  found[4]);
+	GREATEST_ASSERT_EQ(true,  found[5]);
+	GREATEST_ASSERT_EQ(false, found[7]);
+	GREATEST_ASSERT_EQ(false, found[8]);
+	GREATEST_ASSERT_EQ(false, found[9]);
+
+	GREATEST_PASS();
+}
+
+GREATEST_SUITE(suite_nrand)
+{
+	GREATEST_RUN_TEST(nrand_simple);
+}
+
+GREATEST_MAIN_DEFS();
+
+int
+main(int argc, char **argv)
+{
+	GREATEST_MAIN_BEGIN();
+	GREATEST_RUN_SUITE(suite_nrand);
+	GREATEST_MAIN_END();
+
+	return 0;
+}