Merge pull request #10 from AnnulusGames/fix-nextfloat-range

Fix: NextFloat/Double could return a value equal to max due to floating point error

src/RandomExtensions/RandomEx.Next.cs

@@ -177,7 +177,19 @@ public static float NextFloat(this IRandom random, float max)
     public static float NextFloat(this IRandom random, float min, float max)
     {
         ThrowHelper.CheckMinMax(min, max);
-        return NextFloat(random) * (max - min) + min;
+
+        var r = NextFloat(random) * (max - min) + min;
+
+        if (r >= max)
+        {
+#if !NET6_0_OR_GREATER
+            r = MathEx.BitDecrement(max);
+#else
+            r = MathF.BitDecrement(max);
+#endif
+        }
+
+        return r;
     }
 
     /// <summary>
@@ -206,7 +218,20 @@ public static double NextDouble(this IRandom random, double max)
     public static double NextDouble(this IRandom random, double min, double max)
     {
         ThrowHelper.CheckMinMax(min, max);
-        return NextDouble(random) * (max - min) + min;
+
+        var r = NextDouble(random) * (max - min) + min;
+
+        // correct for rounding
+        if (r >= max)
+        {
+#if !NET6_0_OR_GREATER
+            r = MathEx.BitDecrement(max);
+#else
+            r = Math.BitDecrement(max);
+#endif
+        }
+
+        return r;
     }
 
     /// <summary>

src/RandomExtensions/Shims/MathEx.cs

@@ -0,0 +1,57 @@
+namespace RandomExtensions;
+
+internal static class MathEx
+{
+#if !NET6_0_OR_GREATER
+    public static double BitDecrement(double x)
+    {
+        long bits = BitConverter.DoubleToInt64Bits(x);
+
+        if (((bits >> 32) & 0x7FF00000) >= 0x7FF00000)
+        {
+            // NaN returns NaN
+            // -Infinity returns -Infinity
+            // +Infinity returns double.MaxValue
+            return (bits == 0x7FF00000_00000000) ? double.MaxValue : x;
+        }
+
+        if (bits == 0x00000000_00000000)
+        {
+            // +0.0 returns -double.Epsilon
+            return -double.Epsilon;
+        }
+
+        // Negative values need to be incremented
+        // Positive values need to be decremented
+
+        bits += ((bits < 0) ? +1 : -1);
+        return BitConverter.Int64BitsToDouble(bits);
+    }
+
+    public static float BitDecrement(float x)
+    {
+        int bits = BitConverter.SingleToInt32Bits(x);
+
+        if ((bits & 0x7F800000) >= 0x7F800000)
+        {
+            // NaN returns NaN
+            // -Infinity returns -Infinity
+            // +Infinity returns float.MaxValue
+            return (bits == 0x7F800000) ? float.MaxValue : x;
+        }
+
+        if (bits == 0x00000000)
+        {
+            // +0.0 returns -float.Epsilon
+            return -float.Epsilon;
+        }
+
+        // Negative values need to be incremented
+        // Positive values need to be decremented
+
+        bits += ((bits < 0) ? +1 : -1);
+        return BitConverter.Int32BitsToSingle(bits);
+    }
+
+#endif
+}