jellyfin-server/Jellyfin.Api/Helpers/DynamicHlsHelper.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Net;
using System.Security.Claims;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Jellyfin.Api.Extensions;
using Jellyfin.Api.Models.StreamingDtos;
using Jellyfin.Data.Enums;
using Jellyfin.Extensions;
using MediaBrowser.Common.Configuration;
using MediaBrowser.Common.Extensions;
using MediaBrowser.Common.Net;
using MediaBrowser.Controller.Configuration;
using MediaBrowser.Controller.Devices;
using MediaBrowser.Controller.Dlna;
using MediaBrowser.Controller.Library;
using MediaBrowser.Controller.MediaEncoding;
using MediaBrowser.Model.Dlna;
using MediaBrowser.Model.Entities;
using MediaBrowser.Model.Net;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;
using Microsoft.Net.Http.Headers;

namespace Jellyfin.Api.Helpers;

/// <summary>
/// Dynamic hls helper.
/// </summary>
public class DynamicHlsHelper
{
    private readonly ILibraryManager _libraryManager;
    private readonly IUserManager _userManager;
    private readonly IDlnaManager _dlnaManager;
    private readonly IMediaSourceManager _mediaSourceManager;
    private readonly IServerConfigurationManager _serverConfigurationManager;
    private readonly IMediaEncoder _mediaEncoder;
    private readonly IDeviceManager _deviceManager;
    private readonly TranscodingJobHelper _transcodingJobHelper;
    private readonly INetworkManager _networkManager;
    private readonly ILogger<DynamicHlsHelper> _logger;
    private readonly IHttpContextAccessor _httpContextAccessor;
    private readonly EncodingHelper _encodingHelper;

    /// <summary>
    /// Initializes a new instance of the <see cref="DynamicHlsHelper"/> class.
    /// </summary>
    /// <param name="libraryManager">Instance of the <see cref="ILibraryManager"/> interface.</param>
    /// <param name="userManager">Instance of the <see cref="IUserManager"/> interface.</param>
    /// <param name="dlnaManager">Instance of the <see cref="IDlnaManager"/> interface.</param>
    /// <param name="mediaSourceManager">Instance of the <see cref="IMediaSourceManager"/> interface.</param>
    /// <param name="serverConfigurationManager">Instance of the <see cref="IServerConfigurationManager"/> interface.</param>
    /// <param name="mediaEncoder">Instance of the <see cref="IMediaEncoder"/> interface.</param>
    /// <param name="deviceManager">Instance of the <see cref="IDeviceManager"/> interface.</param>
    /// <param name="transcodingJobHelper">Instance of <see cref="TranscodingJobHelper"/>.</param>
    /// <param name="networkManager">Instance of the <see cref="INetworkManager"/> interface.</param>
    /// <param name="logger">Instance of the <see cref="ILogger{DynamicHlsHelper}"/> interface.</param>
    /// <param name="httpContextAccessor">Instance of the <see cref="IHttpContextAccessor"/> interface.</param>
    /// <param name="encodingHelper">Instance of <see cref="EncodingHelper"/>.</param>
    public DynamicHlsHelper(
        ILibraryManager libraryManager,
        IUserManager userManager,
        IDlnaManager dlnaManager,
        IMediaSourceManager mediaSourceManager,
        IServerConfigurationManager serverConfigurationManager,
        IMediaEncoder mediaEncoder,
        IDeviceManager deviceManager,
        TranscodingJobHelper transcodingJobHelper,
        INetworkManager networkManager,
        ILogger<DynamicHlsHelper> logger,
        IHttpContextAccessor httpContextAccessor,
        EncodingHelper encodingHelper)
    {
        _libraryManager = libraryManager;
        _userManager = userManager;
        _dlnaManager = dlnaManager;
        _mediaSourceManager = mediaSourceManager;
        _serverConfigurationManager = serverConfigurationManager;
        _mediaEncoder = mediaEncoder;
        _deviceManager = deviceManager;
        _transcodingJobHelper = transcodingJobHelper;
        _networkManager = networkManager;
        _logger = logger;
        _httpContextAccessor = httpContextAccessor;
        _encodingHelper = encodingHelper;
    }

    /// <summary>
    /// Get master hls playlist.
    /// </summary>
    /// <param name="transcodingJobType">Transcoding job type.</param>
    /// <param name="streamingRequest">Streaming request dto.</param>
    /// <param name="enableAdaptiveBitrateStreaming">Enable adaptive bitrate streaming.</param>
    /// <returns>A <see cref="Task"/> containing the resulting <see cref="ActionResult"/>.</returns>
    public async Task<ActionResult> GetMasterHlsPlaylist(
        TranscodingJobType transcodingJobType,
        StreamingRequestDto streamingRequest,
        bool enableAdaptiveBitrateStreaming)
    {
        var isHeadRequest = _httpContextAccessor.HttpContext?.Request.Method == WebRequestMethods.Http.Head;
        // CTS lifecycle is managed internally.
        var cancellationTokenSource = new CancellationTokenSource();
        return await GetMasterPlaylistInternal(
            streamingRequest,
            isHeadRequest,
            enableAdaptiveBitrateStreaming,
            transcodingJobType,
            cancellationTokenSource).ConfigureAwait(false);
    }

    private async Task<ActionResult> GetMasterPlaylistInternal(
        StreamingRequestDto streamingRequest,
        bool isHeadRequest,
        bool enableAdaptiveBitrateStreaming,
        TranscodingJobType transcodingJobType,
        CancellationTokenSource cancellationTokenSource)
    {
        if (_httpContextAccessor.HttpContext is null)
        {
            throw new ResourceNotFoundException(nameof(_httpContextAccessor.HttpContext));
        }

        using var state = await StreamingHelpers.GetStreamingState(
                streamingRequest,
                _httpContextAccessor.HttpContext,
                _mediaSourceManager,
                _userManager,
                _libraryManager,
                _serverConfigurationManager,
                _mediaEncoder,
                _encodingHelper,
                _dlnaManager,
                _deviceManager,
                _transcodingJobHelper,
                transcodingJobType,
                cancellationTokenSource.Token)
            .ConfigureAwait(false);

        _httpContextAccessor.HttpContext.Response.Headers.Add(HeaderNames.Expires, "0");
        if (isHeadRequest)
        {
            return new FileContentResult(Array.Empty<byte>(), MimeTypes.GetMimeType("playlist.m3u8"));
        }

        var totalBitrate = (state.OutputAudioBitrate ?? 0) + (state.OutputVideoBitrate ?? 0);

        var builder = new StringBuilder();

        builder.AppendLine("#EXTM3U");

        var isLiveStream = state.IsSegmentedLiveStream;

        var queryString = _httpContextAccessor.HttpContext.Request.QueryString.ToString();

        // from universal audio service
        if (!string.IsNullOrWhiteSpace(state.Request.SegmentContainer)
            && !queryString.Contains("SegmentContainer", StringComparison.OrdinalIgnoreCase))
        {
            queryString += "&SegmentContainer=" + state.Request.SegmentContainer;
        }

        // from universal audio service
        if (!string.IsNullOrWhiteSpace(state.Request.TranscodeReasons)
            && !queryString.Contains("TranscodeReasons=", StringComparison.OrdinalIgnoreCase))
        {
            queryString += "&TranscodeReasons=" + state.Request.TranscodeReasons;
        }

        // Main stream
        var playlistUrl = isLiveStream ? "live.m3u8" : "main.m3u8";

        playlistUrl += queryString;

        var subtitleStreams = state.MediaSource
            .MediaStreams
            .Where(i => i.IsTextSubtitleStream)
            .ToList();

        var subtitleGroup = subtitleStreams.Count > 0 && (state.SubtitleDeliveryMethod == SubtitleDeliveryMethod.Hls || state.VideoRequest!.EnableSubtitlesInManifest)
            ? "subs"
            : null;

        // If we're burning in subtitles then don't add additional subs to the manifest
        if (state.SubtitleStream is not null && state.SubtitleDeliveryMethod == SubtitleDeliveryMethod.Encode)
        {
            subtitleGroup = null;
        }

        if (!string.IsNullOrWhiteSpace(subtitleGroup))
        {
            AddSubtitles(state, subtitleStreams, builder, _httpContextAccessor.HttpContext.User);
        }

        var basicPlaylist = AppendPlaylist(builder, state, playlistUrl, totalBitrate, subtitleGroup);

        if (state.VideoStream is not null && state.VideoRequest is not null)
        {
            // Provide a workaround for the case issue between flac and fLaC.
            var flacWaPlaylist = ApplyFlacCaseWorkaround(state, basicPlaylist.ToString());
            if (!string.IsNullOrEmpty(flacWaPlaylist))
            {
                builder.Append(flacWaPlaylist);
            }

            var encodingOptions = _serverConfigurationManager.GetEncodingOptions();

            // Provide SDR HEVC entrance for backward compatibility.
            if (encodingOptions.AllowHevcEncoding
                && !encodingOptions.AllowAv1Encoding
                && EncodingHelper.IsCopyCodec(state.OutputVideoCodec)
                && state.VideoStream.VideoRange == VideoRange.HDR
                && string.Equals(state.ActualOutputVideoCodec, "hevc", StringComparison.OrdinalIgnoreCase))
            {
                var requestedVideoProfiles = state.GetRequestedProfiles("hevc");
                if (requestedVideoProfiles is not null && requestedVideoProfiles.Length > 0)
                {
                    // Force HEVC Main Profile and disable video stream copy.
                    state.OutputVideoCodec = "hevc";
                    var sdrVideoUrl = ReplaceProfile(playlistUrl, "hevc", string.Join(',', requestedVideoProfiles), "main");
                    sdrVideoUrl += "&AllowVideoStreamCopy=false";

                    var sdrOutputVideoBitrate = _encodingHelper.GetVideoBitrateParamValue(state.VideoRequest, state.VideoStream, state.OutputVideoCodec);
                    var sdrOutputAudioBitrate = 0;
                    if (EncodingHelper.LosslessAudioCodecs.Contains(state.VideoRequest.AudioCodec, StringComparison.OrdinalIgnoreCase))
                    {
                        sdrOutputAudioBitrate = state.AudioStream.BitRate ?? 0;
                    }
                    else
                    {
                        sdrOutputAudioBitrate = _encodingHelper.GetAudioBitrateParam(state.VideoRequest, state.AudioStream, state.OutputAudioChannels) ?? 0;
                    }

                    var sdrTotalBitrate = sdrOutputAudioBitrate + sdrOutputVideoBitrate;
                    var sdrPlaylist = AppendPlaylist(builder, state, sdrVideoUrl, sdrTotalBitrate, subtitleGroup);

                    // Provide a workaround for the case issue between flac and fLaC.
                    flacWaPlaylist = ApplyFlacCaseWorkaround(state, sdrPlaylist.ToString());
                    if (!string.IsNullOrEmpty(flacWaPlaylist))
                    {
                        builder.Append(flacWaPlaylist);
                    }

                    // Restore the video codec
                    state.OutputVideoCodec = "copy";
                }
            }

            // Provide Level 5.0 entrance for backward compatibility.
            // e.g. Apple A10 chips refuse the master playlist containing SDR HEVC Main Level 5.1 video,
            // but in fact it is capable of playing videos up to Level 6.1.
            if (encodingOptions.AllowHevcEncoding
                && !encodingOptions.AllowAv1Encoding
                && EncodingHelper.IsCopyCodec(state.OutputVideoCodec)
                && state.VideoStream.Level.HasValue
                && state.VideoStream.Level > 150
                && state.VideoStream.VideoRange == VideoRange.SDR
                && string.Equals(state.ActualOutputVideoCodec, "hevc", StringComparison.OrdinalIgnoreCase))
            {
                var playlistCodecsField = new StringBuilder();
                AppendPlaylistCodecsField(playlistCodecsField, state);

                // Force the video level to 5.0.
                var originalLevel = state.VideoStream.Level;
                state.VideoStream.Level = 150;
                var newPlaylistCodecsField = new StringBuilder();
                AppendPlaylistCodecsField(newPlaylistCodecsField, state);

                // Restore the video level.
                state.VideoStream.Level = originalLevel;
                var newPlaylist = ReplacePlaylistCodecsField(basicPlaylist, playlistCodecsField, newPlaylistCodecsField);
                builder.Append(newPlaylist);

                // Provide a workaround for the case issue between flac and fLaC.
                flacWaPlaylist = ApplyFlacCaseWorkaround(state, newPlaylist);
                if (!string.IsNullOrEmpty(flacWaPlaylist))
                {
                    builder.Append(flacWaPlaylist);
                }
            }
        }

        if (EnableAdaptiveBitrateStreaming(state, isLiveStream, enableAdaptiveBitrateStreaming, _httpContextAccessor.HttpContext.GetNormalizedRemoteIP()))
        {
            var requestedVideoBitrate = state.VideoRequest is null ? 0 : state.VideoRequest.VideoBitRate ?? 0;

            // By default, vary by just 200k
            var variation = GetBitrateVariation(totalBitrate);

            var newBitrate = totalBitrate - variation;
            var variantUrl = ReplaceVideoBitrate(playlistUrl, requestedVideoBitrate, requestedVideoBitrate - variation);
            AppendPlaylist(builder, state, variantUrl, newBitrate, subtitleGroup);

            variation *= 2;
            newBitrate = totalBitrate - variation;
            variantUrl = ReplaceVideoBitrate(playlistUrl, requestedVideoBitrate, requestedVideoBitrate - variation);
            AppendPlaylist(builder, state, variantUrl, newBitrate, subtitleGroup);
        }

        return new FileContentResult(Encoding.UTF8.GetBytes(builder.ToString()), MimeTypes.GetMimeType("playlist.m3u8"));
    }

    private StringBuilder AppendPlaylist(StringBuilder builder, StreamState state, string url, int bitrate, string? subtitleGroup)
    {
        var playlistBuilder = new StringBuilder();
        playlistBuilder.Append("#EXT-X-STREAM-INF:BANDWIDTH=")
            .Append(bitrate.ToString(CultureInfo.InvariantCulture))
            .Append(",AVERAGE-BANDWIDTH=")
            .Append(bitrate.ToString(CultureInfo.InvariantCulture));

        AppendPlaylistVideoRangeField(playlistBuilder, state);

        AppendPlaylistCodecsField(playlistBuilder, state);

        AppendPlaylistResolutionField(playlistBuilder, state);

        AppendPlaylistFramerateField(playlistBuilder, state);

        if (!string.IsNullOrWhiteSpace(subtitleGroup))
        {
            playlistBuilder.Append(",SUBTITLES=\"")
                .Append(subtitleGroup)
                .Append('"');
        }

        playlistBuilder.Append(Environment.NewLine);
        playlistBuilder.AppendLine(url);
        builder.Append(playlistBuilder);

        return playlistBuilder;
    }

    /// <summary>
    /// Appends a VIDEO-RANGE field containing the range of the output video stream.
    /// </summary>
    /// <seealso cref="AppendPlaylist(StringBuilder, StreamState, string, int, string)"/>
    /// <param name="builder">StringBuilder to append the field to.</param>
    /// <param name="state">StreamState of the current stream.</param>
    private void AppendPlaylistVideoRangeField(StringBuilder builder, StreamState state)
    {
        if (state.VideoStream is not null && state.VideoStream.VideoRange != VideoRange.Unknown)
        {
            var videoRange = state.VideoStream.VideoRange;
            if (EncodingHelper.IsCopyCodec(state.OutputVideoCodec))
            {
                if (videoRange == VideoRange.SDR)
                {
                    builder.Append(",VIDEO-RANGE=SDR");
                }

                if (videoRange == VideoRange.HDR)
                {
                    builder.Append(",VIDEO-RANGE=PQ");
                }
            }
            else
            {
                // Currently we only encode to SDR.
                builder.Append(",VIDEO-RANGE=SDR");
            }
        }
    }

    /// <summary>
    /// Appends a CODECS field containing formatted strings of
    /// the active streams output video and audio codecs.
    /// </summary>
    /// <seealso cref="AppendPlaylist(StringBuilder, StreamState, string, int, string)"/>
    /// <seealso cref="GetPlaylistVideoCodecs(StreamState, string, int)"/>
    /// <seealso cref="GetPlaylistAudioCodecs(StreamState)"/>
    /// <param name="builder">StringBuilder to append the field to.</param>
    /// <param name="state">StreamState of the current stream.</param>
    private void AppendPlaylistCodecsField(StringBuilder builder, StreamState state)
    {
        // Video
        string videoCodecs = string.Empty;
        int? videoCodecLevel = GetOutputVideoCodecLevel(state);
        if (!string.IsNullOrEmpty(state.ActualOutputVideoCodec) && videoCodecLevel.HasValue)
        {
            videoCodecs = GetPlaylistVideoCodecs(state, state.ActualOutputVideoCodec, videoCodecLevel.Value);
        }

        // Audio
        string audioCodecs = string.Empty;
        if (!string.IsNullOrEmpty(state.ActualOutputAudioCodec))
        {
            audioCodecs = GetPlaylistAudioCodecs(state);
        }

        StringBuilder codecs = new StringBuilder();

        codecs.Append(videoCodecs);

        if (!string.IsNullOrEmpty(videoCodecs) && !string.IsNullOrEmpty(audioCodecs))
        {
            codecs.Append(',');
        }

        codecs.Append(audioCodecs);

        if (codecs.Length > 1)
        {
            builder.Append(",CODECS=\"")
                .Append(codecs)
                .Append('"');
        }
    }

    /// <summary>
    /// Appends a RESOLUTION field containing the resolution of the output stream.
    /// </summary>
    /// <seealso cref="AppendPlaylist(StringBuilder, StreamState, string, int, string)"/>
    /// <param name="builder">StringBuilder to append the field to.</param>
    /// <param name="state">StreamState of the current stream.</param>
    private void AppendPlaylistResolutionField(StringBuilder builder, StreamState state)
    {
        if (state.OutputWidth.HasValue && state.OutputHeight.HasValue)
        {
            builder.Append(",RESOLUTION=")
                .Append(state.OutputWidth.GetValueOrDefault())
                .Append('x')
                .Append(state.OutputHeight.GetValueOrDefault());
        }
    }

    /// <summary>
    /// Appends a FRAME-RATE field containing the framerate of the output stream.
    /// </summary>
    /// <seealso cref="AppendPlaylist(StringBuilder, StreamState, string, int, string)"/>
    /// <param name="builder">StringBuilder to append the field to.</param>
    /// <param name="state">StreamState of the current stream.</param>
    private void AppendPlaylistFramerateField(StringBuilder builder, StreamState state)
    {
        double? framerate = null;
        if (state.TargetFramerate.HasValue)
        {
            framerate = Math.Round(state.TargetFramerate.GetValueOrDefault(), 3);
        }
        else if (state.VideoStream?.RealFrameRate is not null)
        {
            framerate = Math.Round(state.VideoStream.RealFrameRate.GetValueOrDefault(), 3);
        }

        if (framerate.HasValue)
        {
            builder.Append(",FRAME-RATE=")
                .Append(framerate.Value.ToString(CultureInfo.InvariantCulture));
        }
    }

    private bool EnableAdaptiveBitrateStreaming(StreamState state, bool isLiveStream, bool enableAdaptiveBitrateStreaming, IPAddress ipAddress)
    {
        // Within the local network this will likely do more harm than good.
        if (_networkManager.IsInLocalNetwork(ipAddress))
        {
            return false;
        }

        if (!enableAdaptiveBitrateStreaming)
        {
            return false;
        }

        if (isLiveStream || string.IsNullOrWhiteSpace(state.MediaPath))
        {
            // Opening live streams is so slow it's not even worth it
            return false;
        }

        if (EncodingHelper.IsCopyCodec(state.OutputVideoCodec))
        {
            return false;
        }

        if (EncodingHelper.IsCopyCodec(state.OutputAudioCodec))
        {
            return false;
        }

        if (!state.IsOutputVideo)
        {
            return false;
        }

        // Having problems in android
        return false;
        // return state.VideoRequest.VideoBitRate.HasValue;
    }

    private void AddSubtitles(StreamState state, IEnumerable<MediaStream> subtitles, StringBuilder builder, ClaimsPrincipal user)
    {
        if (state.SubtitleDeliveryMethod == SubtitleDeliveryMethod.Drop)
        {
            return;
        }

        var selectedIndex = state.SubtitleStream is null || state.SubtitleDeliveryMethod != SubtitleDeliveryMethod.Hls ? (int?)null : state.SubtitleStream.Index;
        const string Format = "#EXT-X-MEDIA:TYPE=SUBTITLES,GROUP-ID=\"subs\",NAME=\"{0}\",DEFAULT={1},FORCED={2},AUTOSELECT=YES,URI=\"{3}\",LANGUAGE=\"{4}\"";

        foreach (var stream in subtitles)
        {
            var name = stream.DisplayTitle;

            var isDefault = selectedIndex.HasValue && selectedIndex.Value == stream.Index;
            var isForced = stream.IsForced;

            var url = string.Format(
                CultureInfo.InvariantCulture,
                "{0}/Subtitles/{1}/subtitles.m3u8?SegmentLength={2}&api_key={3}",
                state.Request.MediaSourceId,
                stream.Index.ToString(CultureInfo.InvariantCulture),
                30.ToString(CultureInfo.InvariantCulture),
                user.GetToken());

            var line = string.Format(
                CultureInfo.InvariantCulture,
                Format,
                name,
                isDefault ? "YES" : "NO",
                isForced ? "YES" : "NO",
                url,
                stream.Language ?? "Unknown");

            builder.AppendLine(line);
        }
    }

    /// <summary>
    /// Get the H.26X level of the output video stream.
    /// </summary>
    /// <param name="state">StreamState of the current stream.</param>
    /// <returns>H.26X level of the output video stream.</returns>
    private int? GetOutputVideoCodecLevel(StreamState state)
    {
        string levelString = string.Empty;
        if (EncodingHelper.IsCopyCodec(state.OutputVideoCodec)
            && state.VideoStream is not null
            && state.VideoStream.Level.HasValue)
        {
            levelString = state.VideoStream.Level.ToString() ?? string.Empty;
        }
        else
        {
            if (string.Equals(state.ActualOutputVideoCodec, "h264", StringComparison.OrdinalIgnoreCase))
            {
                levelString = state.GetRequestedLevel(state.ActualOutputVideoCodec) ?? "41";
                levelString = EncodingHelper.NormalizeTranscodingLevel(state, levelString);
            }

            if (string.Equals(state.ActualOutputVideoCodec, "h265", StringComparison.OrdinalIgnoreCase)
                || string.Equals(state.ActualOutputVideoCodec, "hevc", StringComparison.OrdinalIgnoreCase))
            {
                levelString = state.GetRequestedLevel("h265") ?? state.GetRequestedLevel("hevc") ?? "120";
                levelString = EncodingHelper.NormalizeTranscodingLevel(state, levelString);
            }

            if (string.Equals(state.ActualOutputVideoCodec, "av1", StringComparison.OrdinalIgnoreCase))
            {
                levelString = state.GetRequestedLevel("av1") ?? "19";
                levelString = EncodingHelper.NormalizeTranscodingLevel(state, levelString);
            }
        }

        if (int.TryParse(levelString, NumberStyles.Integer, CultureInfo.InvariantCulture, out var parsedLevel))
        {
            return parsedLevel;
        }

        return null;
    }

    /// <summary>
    /// Get the profile of the output video stream.
    /// </summary>
    /// <param name="state">StreamState of the current stream.</param>
    /// <param name="codec">Video codec.</param>
    /// <returns>Profile of the output video stream.</returns>
    private string GetOutputVideoCodecProfile(StreamState state, string codec)
    {
        string profileString = string.Empty;
        if (EncodingHelper.IsCopyCodec(state.OutputVideoCodec)
            && !string.IsNullOrEmpty(state.VideoStream.Profile))
        {
            profileString = state.VideoStream.Profile;
        }
        else if (!string.IsNullOrEmpty(codec))
        {
            profileString = state.GetRequestedProfiles(codec).FirstOrDefault() ?? string.Empty;
            if (string.Equals(state.ActualOutputVideoCodec, "h264", StringComparison.OrdinalIgnoreCase))
            {
                profileString ??= "high";
            }

            if (string.Equals(state.ActualOutputVideoCodec, "h265", StringComparison.OrdinalIgnoreCase)
                || string.Equals(state.ActualOutputVideoCodec, "hevc", StringComparison.OrdinalIgnoreCase)
                || string.Equals(state.ActualOutputVideoCodec, "av1", StringComparison.OrdinalIgnoreCase))
            {
                profileString ??= "main";
            }
        }

        return profileString;
    }

    /// <summary>
    /// Gets a formatted string of the output audio codec, for use in the CODECS field.
    /// </summary>
    /// <seealso cref="AppendPlaylistCodecsField(StringBuilder, StreamState)"/>
    /// <seealso cref="GetPlaylistVideoCodecs(StreamState, string, int)"/>
    /// <param name="state">StreamState of the current stream.</param>
    /// <returns>Formatted audio codec string.</returns>
    private string GetPlaylistAudioCodecs(StreamState state)
    {
        if (string.Equals(state.ActualOutputAudioCodec, "aac", StringComparison.OrdinalIgnoreCase))
        {
            string? profile = state.GetRequestedProfiles("aac").FirstOrDefault();
            return HlsCodecStringHelpers.GetAACString(profile);
        }

        if (string.Equals(state.ActualOutputAudioCodec, "mp3", StringComparison.OrdinalIgnoreCase))
        {
            return HlsCodecStringHelpers.GetMP3String();
        }

        if (string.Equals(state.ActualOutputAudioCodec, "ac3", StringComparison.OrdinalIgnoreCase))
        {
            return HlsCodecStringHelpers.GetAC3String();
        }

        if (string.Equals(state.ActualOutputAudioCodec, "eac3", StringComparison.OrdinalIgnoreCase))
        {
            return HlsCodecStringHelpers.GetEAC3String();
        }

        if (string.Equals(state.ActualOutputAudioCodec, "flac", StringComparison.OrdinalIgnoreCase))
        {
            return HlsCodecStringHelpers.GetFLACString();
        }

        if (string.Equals(state.ActualOutputAudioCodec, "alac", StringComparison.OrdinalIgnoreCase))
        {
            return HlsCodecStringHelpers.GetALACString();
        }

        if (string.Equals(state.ActualOutputAudioCodec, "opus", StringComparison.OrdinalIgnoreCase))
        {
            return HlsCodecStringHelpers.GetOPUSString();
        }

        return string.Empty;
    }

    /// <summary>
    /// Gets a formatted string of the output video codec, for use in the CODECS field.
    /// </summary>
    /// <seealso cref="AppendPlaylistCodecsField(StringBuilder, StreamState)"/>
    /// <seealso cref="GetPlaylistAudioCodecs(StreamState)"/>
    /// <param name="state">StreamState of the current stream.</param>
    /// <param name="codec">Video codec.</param>
    /// <param name="level">Video level.</param>
    /// <returns>Formatted video codec string.</returns>
    private string GetPlaylistVideoCodecs(StreamState state, string codec, int level)
    {
        if (level == 0)
        {
            // This is 0 when there's no requested level in the device profile
            // and the source is not encoded in H.26X or AV1
            _logger.LogError("Got invalid level when building CODECS field for HLS master playlist");
            return string.Empty;
        }

        if (string.Equals(codec, "h264", StringComparison.OrdinalIgnoreCase))
        {
            string profile = GetOutputVideoCodecProfile(state, "h264");
            return HlsCodecStringHelpers.GetH264String(profile, level);
        }

        if (string.Equals(codec, "h265", StringComparison.OrdinalIgnoreCase)
            || string.Equals(codec, "hevc", StringComparison.OrdinalIgnoreCase))
        {
            string profile = GetOutputVideoCodecProfile(state, "hevc");
            return HlsCodecStringHelpers.GetH265String(profile, level);
        }

        if (string.Equals(codec, "av1", StringComparison.OrdinalIgnoreCase))
        {
            string profile = GetOutputVideoCodecProfile(state, "av1");

            // Currently we only transcode to 8 bits AV1
            int bitDepth = 8;
            if (EncodingHelper.IsCopyCodec(state.OutputVideoCodec)
                && state.VideoStream != null
                && state.VideoStream.BitDepth.HasValue)
            {
                bitDepth = state.VideoStream.BitDepth.Value;
            }

            return HlsCodecStringHelpers.GetAv1String(profile, level, false, bitDepth);
        }

        return string.Empty;
    }

    private int GetBitrateVariation(int bitrate)
    {
        // By default, vary by just 50k
        var variation = 50000;

        if (bitrate >= 10000000)
        {
            variation = 2000000;
        }
        else if (bitrate >= 5000000)
        {
            variation = 1500000;
        }
        else if (bitrate >= 3000000)
        {
            variation = 1000000;
        }
        else if (bitrate >= 2000000)
        {
            variation = 500000;
        }
        else if (bitrate >= 1000000)
        {
            variation = 300000;
        }
        else if (bitrate >= 600000)
        {
            variation = 200000;
        }
        else if (bitrate >= 400000)
        {
            variation = 100000;
        }

        return variation;
    }

    private string ReplaceVideoBitrate(string url, int oldValue, int newValue)
    {
        return url.Replace(
            "videobitrate=" + oldValue.ToString(CultureInfo.InvariantCulture),
            "videobitrate=" + newValue.ToString(CultureInfo.InvariantCulture),
            StringComparison.OrdinalIgnoreCase);
    }

    private string ReplaceProfile(string url, string codec, string oldValue, string newValue)
    {
        string profileStr = codec + "-profile=";
        return url.Replace(
            profileStr + oldValue,
            profileStr + newValue,
            StringComparison.OrdinalIgnoreCase);
    }

    private string ReplacePlaylistCodecsField(StringBuilder playlist, StringBuilder oldValue, StringBuilder newValue)
    {
        var oldPlaylist = playlist.ToString();
        return oldPlaylist.Replace(
            oldValue.ToString(),
            newValue.ToString(),
            StringComparison.Ordinal);
    }

    private string ApplyFlacCaseWorkaround(StreamState state, string srcPlaylist)
    {
        if (!string.Equals(state.ActualOutputAudioCodec, "flac", StringComparison.OrdinalIgnoreCase))
        {
            return string.Empty;
        }

        var newPlaylist = srcPlaylist.Replace(",flac\"", ",fLaC\"", StringComparison.Ordinal);

        return newPlaylist.Contains(",fLaC\"", StringComparison.Ordinal) ? newPlaylist : string.Empty;
    }
}