Static OS check to select clock (#577)

* static check

* move epsilon

* missing file

---------

Author: bitfaster

BitFaster.Caching.Benchmarks/TimeBenchmarks.cs

@@ -7,6 +7,7 @@
 namespace BitFaster.Caching.Benchmarks
 {
 #if Windows
+    [DisassemblyDiagnoser(printSource: true, maxDepth: 5)]
     [SimpleJob(RuntimeMoniker.Net48)]
 #endif
     [SimpleJob(RuntimeMoniker.Net60)]

BitFaster.Caching.UnitTests/DurationTests.cs

@@ -10,6 +10,8 @@ namespace BitFaster.Caching.UnitTests
 {
     public class DurationTests
     {
+        public static readonly ulong epsilon = (ulong)Duration.FromMilliseconds(20).raw;
+
         private readonly ITestOutputHelper testOutputHelper;
 
         public DurationTests(ITestOutputHelper testOutputHelper)

BitFaster.Caching.UnitTests/Lru/AfterAccessPolicyTests.cs

@@ -53,7 +53,7 @@ public void CreateItemInitializesTimestampToNow()
         {
             var item = this.policy.CreateItem(1, 2);
 
-            item.TickCount.Should().BeCloseTo(Duration.SinceEpoch().raw, Duration.epsilon);
+            item.TickCount.Should().BeCloseTo(Duration.SinceEpoch().raw, DurationTests.epsilon);
         }
 
         [Fact]

BitFaster.Caching.UnitTests/Lru/DiscretePolicyTests.cs

@@ -48,7 +48,7 @@ public void CreateItemInitializesKeyValueAndTicks()
             item.Key.Should().Be(1);
             item.Value.Should().Be(2);
 
-            item.TickCount.Should().BeCloseTo(timeToExpire.raw + Duration.SinceEpoch().raw, Duration.epsilon);
+            item.TickCount.Should().BeCloseTo(timeToExpire.raw + Duration.SinceEpoch().raw, DurationTests.epsilon);
         }
 
         [Fact]

BitFaster.Caching.UnitTests/Lru/TLruTickCount64PolicyTests .cs

@@ -56,7 +56,7 @@ public void CreateItemInitializesTimestampToNow()
         {
             var item = this.policy.CreateItem(1, 2);
 
-            item.TickCount.Should().BeCloseTo(Duration.SinceEpoch().raw, Duration.epsilon);
+            item.TickCount.Should().BeCloseTo(Duration.SinceEpoch().raw, DurationTests.epsilon);
         }
 
         [Fact]

BitFaster.Caching/Duration.cs

@@ -26,7 +26,9 @@ public readonly struct Duration
 
         internal static readonly Duration Zero = new Duration(0);
 
-        internal static readonly ulong epsilon = (ulong)Duration.FromMilliseconds(20).raw;
+#if NETCOREAPP3_0_OR_GREATER
+        private static readonly bool IsMacOS = RuntimeInformation.IsOSPlatform(OSPlatform.OSX);
+#endif
 
         internal Duration(long raw)
         { 
@@ -41,7 +43,7 @@ internal Duration(long raw)
         public static Duration SinceEpoch()
         {
 #if NETCOREAPP3_0_OR_GREATER
-            if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+            if (IsMacOS)
             {
                 return new Duration(Stopwatch.GetTimestamp());
             }
@@ -61,8 +63,8 @@ public static Duration SinceEpoch()
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public TimeSpan ToTimeSpan()
         {
-#if NETCOREAPP3_0_OR_GREATER    
-            if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+#if NETCOREAPP3_0_OR_GREATER
+            if (IsMacOS)
             {
                 return StopwatchTickConverter.FromTicks(raw);
             }
@@ -72,7 +74,7 @@ public TimeSpan ToTimeSpan()
             }
 #else
             return StopwatchTickConverter.FromTicks(raw);
-#endif    
+#endif
         }
 
         /// <summary>
@@ -84,7 +86,7 @@ public TimeSpan ToTimeSpan()
         public static Duration FromTimeSpan(TimeSpan timeSpan)
         {
 #if NETCOREAPP3_0_OR_GREATER
-            if (RuntimeInformation.IsOSPlatform(OSPlatform.OSX))
+            if (IsMacOS)
             {
                 return new Duration(StopwatchTickConverter.ToTicks(timeSpan));
             }
@@ -94,7 +96,7 @@ public static Duration FromTimeSpan(TimeSpan timeSpan)
             }
 #else
             return new Duration(StopwatchTickConverter.ToTicks(timeSpan));
-#endif       
+#endif
         }
 
         /// <summary>