Fix negative FeetInches and StonePounds with negative values

UnitsNet.Tests/CustomCode/LengthTests.cs

@@ -153,5 +153,21 @@ public void MinValueIsCorrectForUnitWithBaseTypeDouble()
         {
             Assert.Equal(double.MinValue, Length.MinValue.Meters);
         }
+
+        [Fact]
+        public void NegativeLengthToStonePoundsReturnsCorrectValues()
+        {
+            var negativeLength = Length.FromInches(-1.0);
+            var feetInches = negativeLength.FeetInches;
+
+            Assert.Equal(0, feetInches.Feet);
+            Assert.Equal(-1.0, feetInches.Inches);
+
+            negativeLength = Length.FromInches(-25.0);
+            feetInches = negativeLength.FeetInches;
+
+            Assert.Equal(-2.0, feetInches.Feet);
+            Assert.Equal(-1.0, feetInches.Inches);
+        }
     }
 }

UnitsNet.Tests/CustomCode/MassTests.cs

@@ -20,6 +20,7 @@
 // THE SOFTWARE.
 
 using System;
+using UnitsNet.Units;
 using Xunit;
 
 namespace UnitsNet.Tests.CustomCode
@@ -106,5 +107,21 @@ public void MassTimesAccelerationEqualsForce()
             Force force = Mass.FromKilograms(18)*Acceleration.FromMetersPerSecondSquared(3);
             Assert.Equal(force, Force.FromNewtons(54));
         }
+
+        [Fact]
+        public void NegativeMassToStonePoundsReturnsCorrectValues()
+        {
+            var negativeMass = Mass.FromPounds(-1.0);
+            var stonePounds = negativeMass.StonePounds;
+
+            Assert.Equal(0, stonePounds.Stone);
+            Assert.Equal(-1.0, stonePounds.Pounds);
+
+            negativeMass = Mass.FromPounds(-25.0);
+            stonePounds = negativeMass.StonePounds;
+
+            Assert.Equal(-1.0, stonePounds.Stone);
+            Assert.Equal(-11.0, stonePounds.Pounds);
+        }
     }
 }

UnitsNet/CustomCode/Quantities/Length.extra.cs

@@ -42,7 +42,7 @@ public sealed partial class Length
     public partial struct Length
 #endif
     {
-        private const double FeetToInches = 12;
+        private const double InchesInOneFoot = 12;
 
         /// <summary>
         ///     Converts the length to a customary feet/inches combination.
@@ -51,11 +51,11 @@ public FeetInches FeetInches
         {
             get
             {
-                double totalInches = Inches;
-                double wholeFeet = Math.Floor(totalInches/FeetToInches);
-                double inches = totalInches%FeetToInches;
+                var inInches = Inches;
+                var feet = Math.Truncate(inInches / InchesInOneFoot);
+                var inches = inInches % InchesInOneFoot;
 
-                return new FeetInches(wholeFeet, inches);
+                return new FeetInches(feet, inches);
             }
         }
 
@@ -64,7 +64,7 @@ public FeetInches FeetInches
         /// </summary>
         public static Length FromFeetInches(double feet, double inches)
         {
-            return FromInches(FeetToInches*feet + inches);
+            return FromInches(InchesInOneFoot*feet + inches);
         }
 
         // Windows Runtime Component does not allow operator overloads: https://msdn.microsoft.com/en-us/library/br230301.aspx

UnitsNet/CustomCode/Quantities/Mass.extra.cs

@@ -50,7 +50,7 @@ public static Mass FromGravitationalForce(Force f)
         ///     StonePounds related code makes it easier to work with Stone/Pound combination, which are customarily used in the UK
         ///     to express body weight. For example, someone weighs 11 stone 4 pounds (about 72 kilograms).
         /// </summary>
-        private const double StoneToPounds = 14;
+        private const double StonesInOnePound = 14.0;
 
         /// <summary>
         ///     Converts the mass to a customary stone/pounds combination.
@@ -59,11 +59,12 @@ public StonePounds StonePounds
         {
             get
             {
-                double totalPounds = Pounds;
-                double wholeStone = Math.Floor(totalPounds/StoneToPounds);
-                double pounds = totalPounds%StoneToPounds;
+                var inPounds = Pounds;
 
-                return new StonePounds(wholeStone, pounds);
+                var stones = Math.Truncate(inPounds / StonesInOnePound);
+                var pounds = inPounds % StonesInOnePound;
+
+                return new StonePounds(stones, pounds);
             }
         }
 
@@ -72,7 +73,7 @@ public StonePounds StonePounds
         /// </summary>
         public static Mass FromStonePounds(double stone, double pounds)
         {
-            return FromPounds(StoneToPounds*stone + pounds);
+            return FromPounds(StonesInOnePound*stone + pounds);
         }
 
         // Windows Runtime Component does not allow operator overloads: https://msdn.microsoft.com/en-us/library/br230301.aspx