1 files changed, 47 insertions, 0 deletions
diff --git a/lib/libc/gen/arc4random_uniform.c b/lib/libc/gen/arc4random_uniform.c
new file mode 100644
index 000000000000..23455e545899
--- /dev/null
+++ b/lib/libc/gen/arc4random_uniform.c
@@ -0,0 +1,47 @@
+/*-
+ * SPDX-License-Identifier: 0BSD
+ *
+ * Copyright (c) Robert Clausecker <fuz@FreeBSD.org>
+ * Based on a publication by Daniel Lemire.
+ * Public domain where applicable.
+ *
+ * Daniel Lemire, "Fast Random Integer Generation in an Interval",
+ * Association for Computing Machinery, ACM Trans. Model. Comput. Simul.,
+ * no. 1, vol. 29, pp. 1--12, New York, NY, USA, January 2019.
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+
+uint32_t
+arc4random_uniform(uint32_t upper_bound)
+{
+	uint64_t product;
+
+	/*
+	 * The paper uses these variable names:
+	 *
+	 * L -- log2(UINT32_MAX+1)
+	 * s -- upper_bound
+	 * x -- arc4random() return value
+	 * m -- product
+	 * l -- (uint32_t)product
+	 * t -- threshold
+	 */
+
+	if (upper_bound <= 1)
+		return (0);
+
+	product = upper_bound * (uint64_t)arc4random();
+
+	if ((uint32_t)product < upper_bound) {
+		uint32_t threshold;
+
+		/* threshold = (2**32 - upper_bound) % upper_bound */
+		threshold = -upper_bound % upper_bound;
+		while ((uint32_t)product < threshold)
+			product = upper_bound * (uint64_t)arc4random();
+	}
+
+	return (product >> 32);
+}