Fix machine epsilon for double

src/Avalonia.Base/Utilities/MathUtilities.cs

@@ -8,6 +8,9 @@ namespace Avalonia.Utilities
     /// </summary>
     public static class MathUtilities
     {
+        // smallest such that 1.0+DoubleEpsilon != 1.0
+        private const double DoubleEpsilon = 2.2204460492503131e-016;
+
         /// <summary>
         /// AreClose - Returns whether or not two doubles are "close".  That is, whether or 
         /// not they are within epsilon of each other.
@@ -18,7 +21,7 @@ namespace Avalonia.Utilities
         {
             //in case they are Infinities (then epsilon check does not work)
             if (value1 == value2) return true;
-            double eps = (Math.Abs(value1) + Math.Abs(value2) + 10.0) * double.Epsilon;
+            double eps = (Math.Abs(value1) + Math.Abs(value2) + 10.0) * DoubleEpsilon;
             double delta = value1 - value2;
             return (-eps < delta) && (eps > delta);
         }
@@ -78,7 +81,7 @@ namespace Avalonia.Utilities
         /// <param name="value"> The double to compare to 1. </param>
         public static bool IsOne(double value)
         {
-            return Math.Abs(value - 1.0) < 10.0 * double.Epsilon;
+            return Math.Abs(value - 1.0) < 10.0 * DoubleEpsilon;
         }
 
         /// <summary>
@@ -88,7 +91,7 @@ namespace Avalonia.Utilities
         /// <param name="value"> The double to compare to 0. </param>
         public static bool IsZero(double value)
         {
-            return Math.Abs(value) < 10.0 * double.Epsilon;
+            return Math.Abs(value) < 10.0 * DoubleEpsilon;
         }
 
         /// <summary>

tests/Avalonia.Base.UnitTests/Utilities/MathUtilitiesTests.cs

@@ -0,0 +1,51 @@
+using Avalonia.Utilities;
+using Xunit;
+
+namespace Avalonia.Base.UnitTests.Utilities
+{
+    public class MathUtilitiesTests
+    {
+        [Fact]
+        public void Two_Equivalent_Double_Values_Are_Close()
+        {
+            const int N = 10;
+            var x = 42.42;
+            var y = 0.0;
+            var dx = x / N;
+
+            for (var i = 0; i < N; ++i)
+                y += dx;
+            var actual = MathUtilities.AreClose(x, y);
+
+            Assert.True(actual);
+        }
+
+        [Fact]
+        public void Calculated_Double_One_Is_One()
+        {
+            const int N = 10;
+            var dx = 1.0 / N;
+            var x = 0.0;
+            
+            for (var i = 0; i < N; ++i)
+                x += dx;
+            var actual = MathUtilities.IsOne(x);
+
+            Assert.True(actual);
+        }
+
+        [Fact]
+        public void Calculated_Double_Zero_Is_Zero()
+        {
+            const int N = 10;
+            var x = 1.0;
+            var dx = x / N;
+            
+            for (var i = 0; i < N; ++i)
+                x -= dx;
+            var actual = MathUtilities.IsZero(x);
+
+            Assert.True(actual);
+        }
+    }
+}