1 files changed, 56 insertions, 0 deletions

diff --git a/fuzz/uniform_random.c b/fuzz/uniform_random.c
new file mode 100644
index 0000000..7711d69
--- /dev/null
+++ b/fuzz/uniform_random.c
@@ -0,0 +1,56 @@
+/*
+ * 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.
+ */
+
+#include <stdint.h>
+#include <stdlib.h>
+
+uint32_t uniform_random(uint32_t);
+
+/*
+ * 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.
+ */
+uint32_t
+uniform_random(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 = (uint32_t)random();
+                if (r >= min)
+                        break;
+        }
+
+        return r % upper_bound;
+}