From cab8627ffdd41966d76c12ed32eb804ac45a2e7f Mon Sep 17 00:00:00 2001
From: Jamie Reece Wilson <me@reece.sx>
Date: Fri, 1 Dec 2023 03:04:26 +0000
Subject: [PATCH] [*] (AuRandom) Resolve bad floating point distribution

---
 Include/Aurora/RNG/IRandomDevice.hpp |  4 +-
 Source/RNG/AuRandomDevice.cpp        | 88 ++++++++++++++++++++++++++--
 Source/RNG/AuRandomDevice.hpp        |  7 ++-
 3 files changed, 90 insertions(+), 9 deletions(-)

diff --git a/Include/Aurora/RNG/IRandomDevice.hpp b/Include/Aurora/RNG/IRandomDevice.hpp
index abf8bd86..0bae5585 100644
--- a/Include/Aurora/RNG/IRandomDevice.hpp
+++ b/Include/Aurora/RNG/IRandomDevice.hpp
@@ -22,9 +22,9 @@ namespace Aurora::RNG
         virtual AuUInt32 NextU32() = 0;
         virtual AuUInt32 NextU32(AuUInt32 uMin, AuUInt32 uMax) = 0;
         virtual AuUInt64 NextU64() = 0;
-        virtual AuInt32  NextInt(AuInt32 uMin, AuInt32 uMax) = 0;
+        virtual AuInt32  NextInt(AuInt32 iMin, AuInt32 iMax) = 0;
         virtual double   NextDecimal() = 0;
-        virtual float    NextNumber(float min, float max) = 0;
+        virtual double   NextNumber(double dMin, double dMax) = 0;
         virtual AuUInt32 NextIndex(AuUInt32 uCount /* = max + 1*/) = 0;
 
         virtual Memory::MemoryViewRead ToSeed() = 0;
diff --git a/Source/RNG/AuRandomDevice.cpp b/Source/RNG/AuRandomDevice.cpp
index 23eeec39..015c2aba 100644
--- a/Source/RNG/AuRandomDevice.cpp
+++ b/Source/RNG/AuRandomDevice.cpp
@@ -202,9 +202,87 @@ namespace Aurora::RNG
         return uMin + uNext;
     }
 
+    // Source: https://stackoverflow.com/a/5016867
+    bool RandomDevice::DecGeometric(int x)
+    {
+        if (x <= 0)
+        {
+            return true;
+        }
+
+        while (true)
+        {
+            auto r = this->NextByte();
+
+            if (x < 8)
+            {
+                return (r & ((1 << x) - 1)) == 0;
+            }
+            else if (r != 0)
+            {
+                return false;
+            }
+
+            x -= 8;
+        }
+    }
+
+    // Source: https://stackoverflow.com/a/5016867
+    double RandomDevice::UniformFloatInRange(double udMin, double udMax)
+    {
+    #if defined(AURNG_USE_GARBAGE_DECIMALS)
+        return (double(this->NextU32()) * (double(1.0) / double(AuNumericLimits<AuUInt32>::max()))) * udMax;
+    #else
+        union
+        {
+            double f;
+            AuUInt64 u;
+        } convert;
+
+        convert.f = udMin;
+        auto uABits = convert.u;
+        convert.f = udMax;
+        auto uBBits = convert.u;
+        
+        auto uMask = uBBits - uABits;
+        uMask |= uMask >> 1;
+        uMask |= uMask >> 2;
+        uMask |= uMask >> 4;
+        uMask |= uMask >> 8;
+        uMask |= uMask >> 16;
+        uMask |= uMask >> 32;
+
+        int bExp {};
+        frexp(udMax, &bExp);
+
+        while (true)
+        {
+            int iXExp {};
+            double x;
+
+            auto uXBits = this->NextU64();
+            uXBits &= uMask;
+            uXBits += uABits;
+            if (uXBits >= uBBits)
+            {
+                continue;
+            }
+
+            convert.u = uXBits;
+            x         = convert.f;
+            frexp(x, &iXExp);
+
+            if (DecGeometric(bExp - iXExp))
+            {
+                return x;
+            }
+        }
+    #endif
+    }
+
     double   RandomDevice::NextDecimal() 
     {
-        return double(NextU32()) * (double(1.0) / double(AuNumericLimits<AuUInt32>::max()));
+        return this->UniformFloatInRange(0.0, 1.0);
     }
 
     AuUInt32 RandomDevice::NextIndex(AuUInt32 uCount /* = max + 1*/)
@@ -218,11 +296,11 @@ namespace Aurora::RNG
         }
         return uNext;
     }
-    
-    float    RandomDevice::NextNumber(float fMin, float fMax)
+
+    double   RandomDevice::NextNumber(double dMin, double dMax)
     {
-        auto fRange = fMax - fMin;
-        return NextDecimal() * fRange + fMin;
+        auto dRange = dMin - dMin;
+        return this->UniformFloatInRange(0.0, dRange) + dMin;
     }
 
     AuMemoryViewRead RandomDevice::ToSeed()
diff --git a/Source/RNG/AuRandomDevice.hpp b/Source/RNG/AuRandomDevice.hpp
index cc5237bb..898735e6 100644
--- a/Source/RNG/AuRandomDevice.hpp
+++ b/Source/RNG/AuRandomDevice.hpp
@@ -24,10 +24,10 @@ namespace Aurora::RNG
         bool     NextBoolean() override;
         AuUInt32 NextU32() override;
         AuUInt64 NextU64() override;
-        AuInt32  NextInt(AuInt32 uMin, AuInt32 uMax) override;
+        AuInt32  NextInt(AuInt32 iMin, AuInt32 iMax) override;
         AuUInt32 NextU32(AuUInt32 uMin, AuUInt32 uMax)    override;
         double   NextDecimal() override;
-        float    NextNumber(float min, float max) override;
+        double   NextNumber(double dMin, double dMax) override;
         AuUInt32 NextIndex(AuUInt32 uCount /* = max + 1*/) override;
 
         AuMemoryViewRead ToSeed() override;
@@ -37,5 +37,8 @@ namespace Aurora::RNG
     private:
         RandomDef def_;
         WELLRand fast_;
+
+        bool DecGeometric(int x);
+        double UniformFloatInRange(double udMin, double udMax);
     };
 }
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