src/Jellyfin.MediaEncoding.Keyframes/Matroska/MatroskaKeyframeExtractor.cs


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using System;
using System.Collections.Generic;
using System.IO;
using Jellyfin.MediaEncoding.Keyframes.Matroska.Extensions;
using NEbml.Core;

namespace Jellyfin.MediaEncoding.Keyframes.Matroska
{
    /// <summary>
    /// The keyframe extractor for the matroska container.
    /// </summary>
    public static class MatroskaKeyframeExtractor
    {
        /// <summary>
        /// Extracts the keyframes in ticks (scaled using the container timestamp scale) from the matroska container.
        /// </summary>
        /// <param name="filePath">The file path.</param>
        /// <returns>An instance of <see cref="KeyframeData"/>.</returns>
        public static KeyframeData GetKeyframeData(string filePath)
        {
            using var stream = File.OpenRead(filePath);
            using var reader = new EbmlReader(stream);

            var seekHead = reader.ReadSeekHead();
            var info = reader.ReadInfo(seekHead.InfoPosition);
            var videoTrackNumber = reader.FindFirstTrackNumberByType(seekHead.TracksPosition, MatroskaConstants.TrackTypeVideo);

            var keyframes = new List<long>();
            reader.ReadAt(seekHead.CuesPosition);
            reader.EnterContainer();

            while (reader.FindElement(MatroskaConstants.CuePoint))
            {
                reader.EnterContainer();
                ulong? trackNumber = null;
                // Mandatory element
                reader.FindElement(MatroskaConstants.CueTime);
                var cueTime = reader.ReadUInt();

                // Mandatory element
                reader.FindElement(MatroskaConstants.CueTrackPositions);
                reader.EnterContainer();
                if (reader.FindElement(MatroskaConstants.CuePointTrackNumber))
                {
                    trackNumber = reader.ReadUInt();
                }

                reader.LeaveContainer();

                if (trackNumber == videoTrackNumber)
                {
                    keyframes.Add(ScaleToTicks(cueTime, info.TimestampScale));
                }

                reader.LeaveContainer();
            }

            reader.LeaveContainer();

            var result = new KeyframeData(ScaleToTicks(info.Duration ?? 0, info.TimestampScale), keyframes);
            return result;
        }

        private static long ScaleToTicks(ulong unscaledValue, long timestampScale)
        {
            // TimestampScale is in nanoseconds, scale it to get the value in ticks, 1 tick == 100 ns
            return (long)unscaledValue * timestampScale / 100;
        }

        private static long ScaleToTicks(double unscaledValue, long timestampScale)
        {
            // TimestampScale is in nanoseconds, scale it to get the value in ticks, 1 tick == 100 ns
            return Convert.ToInt64(unscaledValue * timestampScale / 100);
        }
    }
}