From: Vladimír Čunát <vladimir.cunat@nic.cz>
Date: Mon, 17 Sep 2018 13:54:01 +0000 (+0200)
Subject: add kr_random_coin() to make some code more readable
X-Git-Tag: v3.2.0~13^2~3
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f8867141dbeb24ffad44a1059f9a4596eeae1c7c;p=thirdparty%2Fknot-resolver.git

add kr_random_coin() to make some code more readable
---

diff --git a/lib/nsrep.c b/lib/nsrep.c
index cf08c71a5..ab46eea68 100644
--- a/lib/nsrep.c
+++ b/lib/nsrep.c
@@ -255,8 +255,8 @@ static int eval_nsrep(const knot_dname_t *owner, const pack_t *addr_set, struct
 	   (score < KR_NS_LONG  || qry->flags.NO_THROTTLE)) {
 		update_nsrep_set(ns, owner, addr_choice, score);
 		ns->reputation = reputation;
-	} else if ((kr_rand_bytes(1) <= 256 / 10) &&
-		   (kr_rand_bytes(2) * KR_NS_MAX_SCORE / (1<<16) >= score)) {
+	} else if (kr_rand_coin(1, 10) &&
+		   !kr_rand_coin(score, KR_NS_MAX_SCORE)) {
 		/* With 10% chance probe server with a probability
 		 * given by its RTT / MAX_RTT. */
 		update_nsrep_set(ns, owner, addr_choice, score);
diff --git a/lib/utils.h b/lib/utils.h
index 448842005..fc435bb78 100644
--- a/lib/utils.h
+++ b/lib/utils.h
@@ -214,6 +214,24 @@ static inline uint64_t kr_rand_bytes(int size)
 /** non-static variant of kr_rand_bytes() */
 KR_EXPORT uint64_t kr_rand_bytes_nonstatic(int size);
 
+/** Throw a pseudo-random coin, succeeding approximately with probability nomin/denomin.
+ * - low precision, only one byte of randomness
+ * - the most extreme bias is 1:255
+ * - tip: use !kr_rand_coin() to get the complementary probability
+ */
+static inline bool kr_rand_coin(int nomin, int denomin)
+{
+	if (nomin < 0 || denomin <= 0 || nomin > denomin) {
+		kr_log_error("kr_rand_coin(): EINVAL\n");
+		abort();
+	}
+	/* threshold = how many parts from 256 are a success */
+	int threshold = (nomin * 256 + /*rounding*/ denomin / 2) / denomin;
+	if (threshold == 0) threshold = 1;
+	if (threshold == 256) threshold = 255;
+	return (kr_rand_bytes(1) < threshold);
+}
+
 /** Memory reservation routine for knot_mm_t */
 KR_EXPORT
 int kr_memreserve(void *baton, char **mem, size_t elm_size, size_t want, size_t *have);
diff --git a/modules/stats/stats.c b/modules/stats/stats.c
index 5f5139de6..32c02219d 100644
--- a/modules/stats/stats.c
+++ b/modules/stats/stats.c
@@ -141,7 +141,7 @@ static void collect_sample(struct stat_data *data, struct kr_rplan *rplan, knot_
 		}
 		/* Consider 1 in N for frequent sampling.
 		 * TODO: redesign the sampling approach. */
-		if (kr_rand_bytes(1) * FREQUENT_PSAMPLE / 256 <= 1) {
+		if (kr_rand_coin(1, FREQUENT_PSAMPLE)) {
 			int key_len = collect_key(key, qry->sname, qry->stype);
 			if (key_len < 0) {
 				assert(false);