remove use of <random> in stats (#628)

the previous implementation was quite slow due to use of mt and uniform_distribution.

1 files changed, 43 insertions, 5 deletions

diff --git a/src/zencore/stats.cpp b/src/zencore/stats.cpp
index 087319fa4..7c1a9e086 100644
--- a/src/zencore/stats.cpp
+++ b/src/zencore/stats.cpp
@@ -178,10 +178,49 @@ Meter::Mark(uint64_t Count)
 
 //////////////////////////////////////////////////////////////////////////
 
-// TODO: should consider a cheaper RNG here, this will run for every thread
-//		 that gets created
+uint64_t
+rol64(uint64_t x, int k)
+{
+	return (x << k) | (x >> (64 - k));
+}
+
+struct xoshiro256ss_state
+{
+	uint64_t s[4];
+};
+
+uint64_t
+xoshiro256ss(struct xoshiro256ss_state* state)
+{
+	uint64_t*	   s	  = state->s;
+	uint64_t const result = rol64(s[1] * 5, 7) * 9;
+	uint64_t const t	  = s[1] << 17;
+
+	s[2] ^= s[0];
+	s[3] ^= s[1];
+	s[1] ^= s[2];
+	s[0] ^= s[3];
 
-thread_local std::mt19937_64 ThreadLocalRng;
+	s[2] ^= t;
+	s[3] = rol64(s[3], 45);
+
+	return result;
+}
+
+class xoshiro256
+{
+public:
+	uint64_t				  operator()() { return xoshiro256ss(&m_State); }
+	static constexpr uint64_t min() { return 0; }
+	static constexpr uint64_t max() { return ~(0ull); }
+
+private:
+	xoshiro256ss_state m_State{0xf0fefaf9, 0xbeeb5238, 0x48472397, 0x58858558};
+};
+
+thread_local xoshiro256 ThreadLocalRng;
+
+//////////////////////////////////////////////////////////////////////////
 
 UniformSample::UniformSample(uint32_t ReservoirSize) : m_Values(ReservoirSize)
 {
@@ -222,8 +261,7 @@ UniformSample::Update(int64_t Value)
 		// Randomly choose an old entry to potentially replace (the probability
 		// of replacing an entry diminishes with time)
 
-		std::uniform_int_distribution<uint64_t> UniformDist(0, Count);
-		uint64_t								SampleIndex = UniformDist(ThreadLocalRng);
+		const uint64_t SampleIndex = ThreadLocalRng() % Count;
 
 		if (SampleIndex < Size)
 		{