Fix CryptoRNG birthday paradox and Math.Abs overflow issues

Co-authored-by: philstopford <1983851+philstopford@users.noreply.github.com>

Author: Copilot

Common/entropyRNG/crypto.cs

@@ -19,7 +19,7 @@ public static class Crypto_RNG
         new(() => RandomNumberGenerator.Create());
 
     private static readonly ThreadLocal<byte[]> _threadLocalBuffer = 
-        new(() => new byte[sizeof(long)]); // Use long buffer for efficiency
+        new(() => new byte[sizeof(uint)]); // Buffer for uint conversion
 
     // Performance optimization: Box-Muller cached value for better efficiency
     [ThreadStatic] private static double _cachedGaussian;
@@ -60,16 +60,19 @@ private static double[] GenerateGaussianPair()
         double U1, U2;
 
         // Performance optimization: Use do-while for better branch prediction
+        // Use uint to avoid Math.Abs overflow issue with int.MinValue
         do
         {
-            random.GetBytes(tmp.AsSpan(0, sizeof(int)));
-            U1 = Math.Abs(BitConverter.ToInt32(tmp, 0)) / maxInt;
+            random.GetBytes(tmp.AsSpan(0, sizeof(uint)));
+            uint value1 = BitConverter.ToUInt32(tmp, 0);
+            U1 = value1 / (double)uint.MaxValue;
         } while (U1 < MIN_RANDOM_VALUE);
 
         do
         {
-            random.GetBytes(tmp.AsSpan(0, sizeof(int)));
-            U2 = Math.Abs(BitConverter.ToInt32(tmp, 0)) / maxInt;
+            random.GetBytes(tmp.AsSpan(0, sizeof(uint)));
+            uint value2 = BitConverter.ToUInt32(tmp, 0);
+            U2 = value2 / (double)uint.MaxValue;
         } while (U2 < MIN_RANDOM_VALUE);
 
         // Performance optimization: Pre-calculate common sub-expressions
@@ -90,7 +93,9 @@ public static double nextdouble()
         var tmp = _threadLocalBuffer.Value!;
 
         random.GetBytes(tmp.AsSpan(0, sizeof(int)));
-        return Math.Abs(BitConverter.ToInt32(tmp, 0)) / maxInt;
+        // Use uint to avoid Math.Abs overflow issue with int.MinValue
+        uint value = BitConverter.ToUInt32(tmp, 0);
+        return value / (double)uint.MaxValue;
     }
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]

UnitTests/RNGTests.cs

@@ -64,7 +64,9 @@ public static void CryptoRNGTest()
             .GroupBy(x => x)               // group matching items
             .Where(g => g.Skip(1).Any())   // where the group contains more than one item
             .SelectMany(g => g).ToArray();           // re-expand the groups with more than one item
-        Assert.That(values3.Distinct().Count(), Is.EqualTo(sampleCount));
+        // Allow for occasional duplicates due to birthday paradox - with 25000 samples from uint.MaxValue space,
+        // probability of collision is ~7%. Require at least 99.5% distinct values (at most 125 duplicates).
+        Assert.That(values3.Distinct().Count(), Is.GreaterThanOrEqualTo((int)(sampleCount * 0.995)));
     }
 
     [Test]