refactor(render): extract frame presentation diagnostics

tests/AcDream.App.Tests/Rendering/RollingTimingSampleWindowTests.cs

@@ -0,0 +1,66 @@
+using AcDream.App.Rendering;
+
+namespace AcDream.App.Tests.Rendering;
+
+public sealed class RollingTimingSampleWindowTests
+{
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-1)]
+    public void Constructor_RejectsNonPositiveCapacity(int capacity)
+    {
+        Assert.Throws<ArgumentOutOfRangeException>(
+            () => new RollingTimingSampleWindow(capacity));
+    }
+
+    [Fact]
+    public void Snapshot_EmptyOrNonPositiveSamplesReturnsZeroes()
+    {
+        var samples = new RollingTimingSampleWindow(4);
+        samples.PushHundredthsMicroseconds(0);
+        samples.PushHundredthsMicroseconds(-10);
+
+        Assert.Equal(default, samples.Snapshot());
+    }
+
+    [Fact]
+    public void Snapshot_UsesTheAcceptedUpperMedianAndP95Rules()
+    {
+        var samples = new RollingTimingSampleWindow(8);
+        samples.PushHundredthsMicroseconds(100);
+        samples.PushHundredthsMicroseconds(200);
+
+        RollingTimingPercentiles snapshot = samples.Snapshot();
+
+        Assert.Equal(200, snapshot.MedianHundredthsMicroseconds);
+        Assert.Equal(200, snapshot.Percentile95HundredthsMicroseconds);
+    }
+
+    [Fact]
+    public void Push_WrapsAndRetainsOnlyTheNewestCapacitySamples()
+    {
+        var samples = new RollingTimingSampleWindow(3);
+        samples.PushHundredthsMicroseconds(100);
+        samples.PushHundredthsMicroseconds(200);
+        samples.PushHundredthsMicroseconds(300);
+        samples.PushHundredthsMicroseconds(400);
+
+        RollingTimingPercentiles snapshot = samples.Snapshot();
+
+        Assert.Equal(3, samples.Capacity);
+        Assert.Equal(300, snapshot.MedianHundredthsMicroseconds);
+        Assert.Equal(400, snapshot.Percentile95HundredthsMicroseconds);
+    }
+
+    [Fact]
+    public void PushStopwatchTicks_UsesHundredthsOfAMicrosecondUnits()
+    {
+        var samples = new RollingTimingSampleWindow(1);
+
+        samples.PushStopwatchTicks(System.Diagnostics.Stopwatch.Frequency);
+
+        Assert.Equal(
+            new RollingTimingPercentiles(100_000_000, 100_000_000),
+            samples.Snapshot());
+    }
+}