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);
    }
}