From 1b8dc7f05e05c3e2a4620a171e413249caa6807c Mon Sep 17 00:00:00 2001
From: "Nicholas J. Kain" <nicholas@kain.us>
Date: Thu, 13 Oct 2022 08:43:11 -0400
Subject: [PATCH] random: Use sfc64 instead of gjrand.

---
 nk/random.c | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

diff --git a/nk/random.c b/nk/random.c
index df30272..5500fc9 100644
--- a/nk/random.c
+++ b/nk/random.c
@@ -1,17 +1,16 @@
-// Copyright 2013-2018 Nicholas J. Kain <njkain at gmail dot com>
+// Copyright 2013-2022 Nicholas J. Kain <njkain at gmail dot com>
 // SPDX-License-Identifier: MIT
 #include <stdint.h>
 #include "nk/hwrng.h"
 #include "nk/random.h"
 
-// GJrand64: https://gjrand.sourceforge.net
+// SFC64 modified to use a Weyl counter.
 
 void nk_random_init(struct nk_random_state *s)
 {
-    nk_hwrng_bytes(s->seed, sizeof(uint64_t) * 2);
-    s->seed[2] = 2000001;
-    s->seed[3] = 0;
-    for (size_t i = 0; i < 14; ++i) nk_random_u64(s);
+    nk_hwrng_bytes(s->seed, sizeof(uint64_t) * 3);
+    s->seed[3] = 1;
+    for (size_t i = 0; i < 12; ++i) nk_random_u64(s);
 }
 
 static inline uint64_t rotl64(const uint64_t x, int k) {
@@ -20,17 +19,11 @@ static inline uint64_t rotl64(const uint64_t x, int k) {
 
 uint64_t nk_random_u64(struct nk_random_state *s)
 {
-    s->seed[1] += s->seed[2];
-    s->seed[0] = rotl64(s->seed[0], 32);
-    s->seed[2] ^= s->seed[1];
-    s->seed[3] += 0x55aa96a5;
-    s->seed[0] += s->seed[1];
-    s->seed[2] = rotl64(s->seed[2], 23);
-    s->seed[1] ^= s->seed[0];
-    s->seed[0] += s->seed[2];
-    s->seed[1] = rotl64(s->seed[1], 19);
-    s->seed[2] += s->seed[0];
-    s->seed[1] += s->seed[3];
-    return s->seed[0];
+    const uint64_t t = (s->seed[0] + s->seed[1]) ^ s->seed[3];
+    s->seed[3] += 0x6a09e667a7541669ull;
+    s->seed[0] = s->seed[1] ^ (s->seed[1] >> 11);
+    s->seed[1] = s->seed[2] + (s->seed[2] << 3);
+    s->seed[2] = rotl64(s->seed[2], 24) + t;
+    return t;
 }