TASK 21 slice 2a: native-FPS probe seam (ffprobe parse + derive sibling)

- FfmpegFrameRateParser (pure): prefers avg_frame_rate= then r_frame_rate=, rational N/N/M, unknown/0 -> null. - IFrameRateProbe + FfmpegFrameRateProbe: derives sibling ffprobe.exe from the located ffmpeg dir, reuses the IDecodeProcess seam for the ffprobe subprocess text; null if ffprobe absent. - FfmpegLocator now also extracts ffprobe.exe (ProbeFileName) from the pinned archive, conditional so old caches without it degrade to no pacing. - Tests: 6 pure parser units + 1 probe integration via fake locator/process; FfmpegLocatorTests still green. 15/15, 0 warnings. Pacing (probe->delay) is slice 2b. Derivative reference: standard ffprobe avg_frame_rate probing used across OBS/media tooling.
2026-08-31 19:45:48 -07:00
parent 071ec8be0c
commit 8fa78423e6
7 changed files with 267 additions and 4 deletions
@@ -0,0 +1,67 @@
+using System.Diagnostics;
+using System.IO;
+using ytLive.Services.Encoder;
+
+namespace ytLive.Services;
+
+/// <summary>
+/// TASK 21 (slice 2): resolves the native frame rate of a media file so the
+/// decoder can pace frames to real time. Seam so <see cref="MediaVideoSource"/>
+/// never touches ffprobe directly and tests can fake the whole probe.
+/// </summary>
+public interface IFrameRateProbe
+{
+    /// <summary>Native video FPS for <paramref name="path"/>, or null when unknown
+    /// (no video stream, ffprobe unavailable, unparsable).</summary>
+    Task<double?> ProbeAsync(string path, CancellationToken cancellationToken = default);
+}
+
+/// <summary>
+/// Default probe: locates ffmpeg (via <see cref="IFfmpegLocator"/>), derives the
+/// sibling <c>ffprobe.exe</c> from its directory, and asks ffprobe for the
+/// <c>avg_frame_rate</c>/<c>r_frame_rate</c>. Reuses the binary-stdout
+/// <see cref="IDecodeProcess"/> seam for the subprocess (ffprobe text is read from
+/// the same raw stdout). Returns null (no pacing) if ffprobe is unavailable.
+/// </summary>
+public sealed class FfmpegFrameRateProbe : IFrameRateProbe
+{
+    private readonly IFfmpegLocator _locator;
+    private readonly Func<IDecodeProcess> _probeProcessFactory;
+
+    public FfmpegFrameRateProbe(IFfmpegLocator locator, Func<IDecodeProcess> probeProcessFactory)
+    {
+        _locator = locator;
+        _probeProcessFactory = probeProcessFactory;
+    }
+
+    public async Task<double?> ProbeAsync(string path, CancellationToken cancellationToken = default)
+    {
+        if (string.IsNullOrWhiteSpace(path)) return null;
+
+        var ffmpegPath = await _locator.LocateAsync(cancellationToken).ConfigureAwait(false);
+        var ffprobePath = Path.Combine(Path.GetDirectoryName(ffmpegPath) ?? string.Empty, "ffprobe.exe");
+        if (!File.Exists(ffprobePath)) return null;
+
+        var startInfo = new ProcessStartInfo
+        {
+            FileName = ffprobePath,
+            Arguments =
+                $"-v error -select_streams v:0 " +
+                $"-show_entries stream=avg_frame_rate,r_frame_rate " +
+                $"-of default=noprint_wrappers=1 \"{path}\"",
+            UseShellExecute = false,
+            RedirectStandardOutput = true,
+            RedirectStandardError = true,
+            CreateNoWindow = true,
+        };
+
+        using var process = _probeProcessFactory();
+        process.Start(startInfo);
+        string output;
+        using (var reader = new StreamReader(process.StandardOutput))
+            output = reader.ReadToEnd();
+        await process.WaitForExitAsync(cancellationToken).ConfigureAwait(false);
+
+        return FfmpegFrameRateParser.ParseFrameRate(output);
+    }
+}