From 6280be46a6fb62b8833a9d55bff590fb209b0fc6 Mon Sep 17 00:00:00 2001
From: Chaosus <chaosus89@gmail.com>
Date: Sun, 17 Mar 2019 20:51:51 +0300
Subject: [PATCH] Properly setup seed in RNG

---
 core/math/random_pcg.cpp |  3 +--
 core/math/random_pcg.h   |  6 +++---
 thirdparty/misc/pcg.cpp  | 10 ++++++++++
 thirdparty/misc/pcg.h    |  1 +
 4 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/core/math/random_pcg.cpp b/core/math/random_pcg.cpp
index 45467b32b23..8351bd138e2 100644
--- a/core/math/random_pcg.cpp
+++ b/core/math/random_pcg.cpp
@@ -34,8 +34,7 @@
 
 RandomPCG::RandomPCG(uint64_t p_seed, uint64_t p_inc) :
 		pcg(),
-		current_seed(DEFAULT_SEED) {
-	pcg.inc = p_inc;
+		current_inc(p_inc) {
 	seed(p_seed);
 }
 
diff --git a/core/math/random_pcg.h b/core/math/random_pcg.h
index 230eb9a11b9..cd721ef4d1a 100644
--- a/core/math/random_pcg.h
+++ b/core/math/random_pcg.h
@@ -38,18 +38,18 @@
 class RandomPCG {
 	pcg32_random_t pcg;
 	uint64_t current_seed; // seed with this to get the same state
+	uint64_t current_inc;
 
 public:
 	static const uint64_t DEFAULT_SEED = 12047754176567800795U;
 	static const uint64_t DEFAULT_INC = PCG_DEFAULT_INC_64;
 	static const uint64_t RANDOM_MAX = 0xFFFFFFFF;
 
-	RandomPCG(uint64_t p_seed = DEFAULT_SEED, uint64_t p_inc = PCG_DEFAULT_INC_64);
+	RandomPCG(uint64_t p_seed = DEFAULT_SEED, uint64_t p_inc = DEFAULT_INC);
 
 	_FORCE_INLINE_ void seed(uint64_t p_seed) {
 		current_seed = p_seed;
-		pcg.state = p_seed;
-		pcg32_random_r(&pcg); // Force changing internal state to avoid initial 0
+		pcg32_srandom_r(&pcg, current_seed, current_inc);
 	}
 	_FORCE_INLINE_ uint64_t get_seed() { return current_seed; }
 
diff --git a/thirdparty/misc/pcg.cpp b/thirdparty/misc/pcg.cpp
index eac3b36d368..c421e16f89e 100644
--- a/thirdparty/misc/pcg.cpp
+++ b/thirdparty/misc/pcg.cpp
@@ -13,3 +13,13 @@ uint32_t pcg32_random_r(pcg32_random_t* rng)
     uint32_t rot = oldstate >> 59u;
     return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
 }
+
+// Source from http://www.pcg-random.org/downloads/pcg-c-basic-0.9.zip
+void pcg32_srandom_r(pcg32_random_t* rng, uint64_t initstate, uint64_t initseq)
+{
+    rng->state = 0U;
+    rng->inc = (initseq << 1u) | 1u;
+    pcg32_random_r(rng);
+    rng->state += initstate;
+    pcg32_random_r(rng);
+}
diff --git a/thirdparty/misc/pcg.h b/thirdparty/misc/pcg.h
index e2d66d51d56..6f42b3b0949 100644
--- a/thirdparty/misc/pcg.h
+++ b/thirdparty/misc/pcg.h
@@ -10,5 +10,6 @@
 
 typedef struct { uint64_t state;  uint64_t inc; } pcg32_random_t;
 uint32_t pcg32_random_r(pcg32_random_t* rng);
+void pcg32_srandom_r(pcg32_random_t* rng, uint64_t initstate, uint64_t initseq);
 
 #endif // RANDOM_H