From: Alan T. DeKok <aland@freeradius.org>
Date: Mon, 22 Jan 2024 19:31:07 +0000 (-0500)
Subject: document source of fast rng
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=999e596b434d30487e95752a86f7eb4013958161;p=thirdparty%2Ffreeradius-server.git

document source of fast rng
---

diff --git a/src/lib/util/rand.c b/src/lib/util/rand.c
index db226720036..fbe9ecca743 100644
--- a/src/lib/util/rand.c
+++ b/src/lib/util/rand.c
@@ -258,6 +258,25 @@ while (p < end) { \
 	}
 }
 
+
+/*
+ *	http://www.cse.yorku.ca/~oz/marsaglia-rng.html
+ *
+ *	We implement MWC here, which uses 2 32-bit numbers for a
+ *	state, and has a period of 2^60.
+ *
+ *	We could extend this to a larger RNG with 4 32-bit state
+ *	numbers {a, b, c, d} and use KISS, which has a period of about
+ *	2^123.
+ *
+ *	a' = 36969 * (a & 65535) + (a >> 16)
+ *	b' = 18000 * (b & 65535) + (b >> 16))
+ *
+ *	MWC	(a' << 16) + b'
+ *	SHR3	(c ^= (c <<17); c ^= ( c>>13); c ^= (c << 5))
+ *	CONG	d' = 69069 * d + 1234567
+ *	KISS	((MWC^CONG)+SHR3)
+ */
 uint32_t fr_fast_rand(fr_fast_rand_t *ctx)
 {
 	ctx->a = (36969 * (ctx->a & 0xffff)) + (ctx->a >> 16);