using System.Globalization;

namespace ytLive.Services;

/// <summary>
/// TASK 21 (slice 2): parses the native frame rate of a video from ffprobe
/// <c>-show_entries stream=avg_frame_rate,r_frame_rate</c> output, which reports
/// each as a rational like <c>30/1</c> or <c>60000/1001</c> (or <c>0/0</c> when
/// unknown). Prefers <c>avg_frame_rate</c> (the true display rate over the whole
/// file, correct for VFR) and falls back to <c>r_frame_rate</c> (the container/
/// codec rate). Pure: no ffprobe, no I/O, unit-testable.
/// </summary>
public static class FfmpegFrameRateParser
{
    public static double? ParseFrameRate(string probeOutput)
    {
        foreach (var key in new[] { "avg_frame_rate=", "r_frame_rate=" })
        {
            var value = FindValue(probeOutput, key);
            if (value is not null && TryParseRational(value, out var fps))
                return fps;
        }
        return null;
    }

    /// <summary>The trimmed value of the first line beginning with <paramref name="key"/>.</summary>
    private static string? FindValue(string text, string key)
    {
        foreach (var line in text.Split('\n'))
        {
            var trimmed = line.Trim();
            if (trimmed.StartsWith(key, StringComparison.Ordinal))
                return trimmed.Substring(key.Length).Trim();
        }
        return null;
    }

    /// <summary>Parses <c>v</c> as <c>N</c> (integer) or <c>N/M</c> (rational) into
    /// a frames-per-second double. <c>0</c> and <c>0/M</c> count as unknown.</summary>
    private static bool TryParseRational(string value, out double fps)
    {
        fps = 0;
        var parts = value.Split('/');
        if (parts.Length == 0 || !int.TryParse(parts[0], NumberStyles.None, CultureInfo.InvariantCulture, out var num))
            return false;
        var den = 1;
        if (parts.Length > 1 && !int.TryParse(parts[1], NumberStyles.None, CultureInfo.InvariantCulture, out den))
            return false;
        if (num <= 0 || den <= 0) return false;
        fps = (double)num / den;
        return true;
    }
}