1 files changed, 38 insertions, 0 deletions
diff --git a/libmisc/rand.c b/libmisc/rand.c
new file mode 100644
index 0000000..d1643ee
--- /dev/null
+++ b/libmisc/rand.c
@@ -0,0 +1,38 @@
+/* libmisc/rand.c - Non-crytpographic random-number utilities
+ *
+ * Copyright (C) 2024-2025  Luke T. Shumaker <lukeshu@lukeshu.com>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+#include <stdlib.h> /* for random() */
+
+#include <libmisc/assert.h>
+
+#include <libmisc/rand.h>
+
+uint64_t rand_uint63n(uint64_t cnt) {
+	assert(cnt != 0 && ((cnt-1) & 0x8000000000000000) == 0);
+	if (cnt <= UINT64_C(1)<<31) {
+		uint32_t fair_cnt = ((UINT32_C(1)<<31) / cnt) * cnt;
+		uint32_t rnd;
+		do {
+			rnd = random();
+		} while (rnd >= fair_cnt);
+		return rnd % cnt;
+	} else if (cnt <= UINT64_C(1)<<62) {
+		uint64_t fair_cnt = ((UINT64_C(1)<<62) / cnt) * cnt;
+		uint64_t rnd;
+		do {
+			rnd = (((uint64_t)random()) << 31) | random();
+		} while (rnd >= fair_cnt);
+		return rnd % cnt;
+	} else if (cnt <= UINT64_C(1)<<63) {
+		uint64_t fair_cnt = ((UINT64_C(1)<<63) / cnt) * cnt;
+		uint64_t rnd;
+		do {
+			rnd = (((uint64_t)random()) << 62) | (((uint64_t)random()) << 31) | random();
+		} while (rnd >= fair_cnt);
+		return rnd % cnt;
+	}
+	assert_notreached("cnt is out of bounds");
+}