refactor(render): extract frame presentation diagnostics

src/AcDream.App/Rendering/RollingTimingSampleWindow.cs

@@ -0,0 +1,54 @@
+namespace AcDream.App.Rendering;
+
+internal readonly record struct RollingTimingPercentiles(
+    long MedianHundredthsMicroseconds,
+    long Percentile95HundredthsMicroseconds);
+
+/// <summary>
+/// Fixed-capacity rolling timing distribution. Zero and negative values remain
+/// empty padding and do not dilute the diagnostic percentiles.
+/// </summary>
+internal sealed class RollingTimingSampleWindow
+{
+    private readonly long[] _samples;
+    private int _cursor;
+
+    public RollingTimingSampleWindow(int capacity)
+    {
+        if (capacity <= 0)
+            throw new ArgumentOutOfRangeException(nameof(capacity));
+        _samples = new long[capacity];
+    }
+
+    public int Capacity => _samples.Length;
+
+    public void PushHundredthsMicroseconds(long sample)
+    {
+        _samples[_cursor] = sample;
+        _cursor = (_cursor + 1) % _samples.Length;
+    }
+
+    public void PushStopwatchTicks(long ticks) =>
+        PushHundredthsMicroseconds(
+            (long)(ticks * 100_000_000.0 / System.Diagnostics.Stopwatch.Frequency));
+
+    public RollingTimingPercentiles Snapshot()
+    {
+        var copy = (long[])_samples.Clone();
+        Array.Sort(copy);
+        int populated = 0;
+        foreach (long sample in copy)
+        {
+            if (sample > 0)
+                populated++;
+        }
+
+        if (populated == 0)
+            return default;
+
+        long median = copy[copy.Length - 1 - (populated - 1) / 2];
+        int p95Offset = (int)((populated - 1) * 0.05);
+        long percentile95 = copy[copy.Length - 1 - p95Offset];
+        return new RollingTimingPercentiles(median, percentile95);
+    }
+}