ersatztv/ErsatzTV.Application/Channels/Commands/CreateChannelFromLineupHandler.cs

using System.Globalization;
using System.Text.RegularExpressions;
using System.Threading.Channels;
using ErsatzTV.Application.Playouts;
using ErsatzTV.Core;
using ErsatzTV.Core.Api.Channels;
using ErsatzTV.Core.Api.LibraryBrowse;
using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Domain.Filler;
using ErsatzTV.Core.Errors;
using ErsatzTV.Core.Interfaces.Search;
using ErsatzTV.Core.Scheduling;
using ErsatzTV.Infrastructure.Data;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Logging;
using Channel = ErsatzTV.Core.Domain.Channel;

namespace ErsatzTV.Application.Channels;

public class CreateChannelFromLineupHandler(
    ChannelWriter<IBackgroundServiceRequest> workerChannel,
    IDbContextFactory<TvContext> dbContextFactory,
    ISearchTargets searchTargets,
    ILogger<CreateChannelFromLineupHandler> logger)
    : IRequestHandler<CreateChannelFromLineup, Either<BaseError, CreateChannelFromLineupResponseModel>>
{
    // The single system playlist group that holds every generated channel-lineup playlist.
    // Matches the Trakt "Trakt Lists" precedent (DbInitializer + delete guards on IsSystem).
    private const string SystemPlaylistGroupName = "Channel Lineups";

    public async Task<Either<BaseError, CreateChannelFromLineupResponseModel>> Handle(
        CreateChannelFromLineup request,
        CancellationToken cancellationToken)
    {
        await using TvContext dbContext = await dbContextFactory.CreateDbContextAsync(cancellationToken);

        Either<BaseError, PreparedCreate> validation = await Validate(dbContext, request, cancellationToken);
        return await validation.Match(
            Left: error => Task.FromResult<Either<BaseError, CreateChannelFromLineupResponseModel>>(error),
            Right: prepared => PersistAndDispatch(dbContext, prepared, cancellationToken));
    }

    private async Task<Either<BaseError, CreateChannelFromLineupResponseModel>> PersistAndDispatch(
        TvContext dbContext,
        PreparedCreate prepared,
        CancellationToken cancellationToken)
    {
        await using var transaction = await dbContext.Database.BeginTransactionAsync(cancellationToken);
        try
        {
            dbContext.Channels.Add(prepared.Channel);
            if (prepared.Playlist is not null)
            {
                dbContext.Playlists.Add(prepared.Playlist);
            }

            dbContext.ProgramSchedules.Add(prepared.ProgramSchedule);
            dbContext.Playouts.Add(prepared.Playout);

            await dbContext.SaveChangesAsync(cancellationToken);
            await transaction.CommitAsync(cancellationToken);
        }
        catch (DbUpdateException ex)
        {
            await transaction.RollbackAsync(cancellationToken);
            logger.LogError(ex, "Failed to persist channel created from lineup");
            return BaseError.New("Unable to create channel from lineup");
        }

        searchTargets.SearchTargetsChanged();

        // post-commit side effect runs on CancellationToken.None so a late request cancellation
        // can't abort it after the commit landed (#254)
        await workerChannel.WriteAsync(
            new BuildPlayout(prepared.Playout.Id, PlayoutBuildMode.Reset),
            CancellationToken.None);

        // Mirror CreateClassicPlayoutHandler: on-demand playouts must be time-shifted to "now" after build.
        if (prepared.Channel.PlayoutMode is ChannelPlayoutMode.OnDemand)
        {
            await workerChannel.WriteAsync(
                new TimeShiftOnDemandPlayout(prepared.Playout.Id, DateTimeOffset.Now, false),
                CancellationToken.None);
        }

        await workerChannel.WriteAsync(new RefreshChannelList(), CancellationToken.None);

        return new CreateChannelFromLineupResponseModel(
            prepared.Channel.Id,
            prepared.Playlist?.Id,
            prepared.ProgramSchedule.Id,
            prepared.Playout.Id);
    }

    private static async Task<Either<BaseError, PreparedCreate>> Validate(
        TvContext dbContext,
        CreateChannelFromLineup request,
        CancellationToken cancellationToken)
    {
        string name = (request.Name ?? string.Empty).Trim();
        string number = (request.Number ?? string.Empty).Trim();
        string group = (request.Group ?? string.Empty).Trim();
        string categories = (request.Categories ?? string.Empty).Trim();
        CreateChannelFromLineupAdvancedOptions advanced = request.Advanced ?? new CreateChannelFromLineupAdvancedOptions();

        if (string.IsNullOrWhiteSpace(name))
        {
            return BaseError.New("Channel name is required");
        }

        if (name.Length > 50)
        {
            return BaseError.New("Channel name must be 50 characters or fewer");
        }

        if (string.IsNullOrWhiteSpace(group))
        {
            return BaseError.New("Channel group is required");
        }

        if (!Regex.IsMatch(number, Channel.NumberValidator))
        {
            return BaseError.New("Invalid channel number; two decimals are allowed for subchannels");
        }

        if (await dbContext.Channels.AnyAsync(c => c.Number == number, cancellationToken))
        {
            return BaseError.New("Channel number must be unique");
        }

        if (!request.IsEnabled && request.ShowInEpg)
        {
            return BaseError.New("Disabled channels cannot be shown in EPG");
        }

        if (!string.IsNullOrWhiteSpace(request.Logo?.Path) &&
            Uri.TryCreate(request.Logo.Path, UriKind.Absolute, out _) &&
            !Artwork.IsExternalUrl(request.Logo.Path))
        {
            return BaseError.New("External logo url is invalid");
        }

        if (request.Lineup is null || request.Lineup.Count == 0)
        {
            return BaseError.New("Lineup must contain at least one item");
        }

        ChannelTemplate template = await dbContext.ChannelTemplates
            .AsNoTracking()
            .SingleOrDefaultAsync(t => t.Id == request.TemplateId, cancellationToken);
        if (template is null)
        {
            return new NotFoundError($"Channel template {request.TemplateId} does not exist.");
        }

        int ffmpegProfileId = advanced.FFmpegProfileId ?? template.FFmpegProfileId;
        int? fallbackFillerId = advanced.FallbackFillerId ?? template.FallbackFillerId;
        int? preRollFillerId = advanced.PreRollFillerId ?? template.PreRollFillerId;
        int? midRollFillerId = advanced.MidRollFillerId ?? template.MidRollFillerId;
        int? postRollFillerId = advanced.PostRollFillerId ?? template.PostRollFillerId;
        PlaybackOrder playbackOrder = advanced.PlaybackOrder ?? PlaybackOrder.Chronological;
        ChannelPlayoutSource playoutSource = advanced.PlayoutSource ?? template.PlayoutSource;

        // Mirror channels need special MirrorSourceChannelId plumbing (see CreateChannelHandler);
        // this endpoint only builds generated playouts.
        if (playoutSource is ChannelPlayoutSource.Mirror)
        {
            return BaseError.New("Mirror playout source is not supported by this endpoint");
        }

        Either<BaseError, Unit> referenceValidation = await ValidateReferences(
            dbContext,
            advanced,
            template,
            cancellationToken);
        foreach (BaseError error in referenceValidation.LeftToSeq())
        {
            return error;
        }

        // Normalize + validate every lineup entry once, so validation and build see the same data.
        var normalized = new List<NormalizedLineupItem>();
        for (int i = 0; i < request.Lineup.Count; i++)
        {
            Either<BaseError, NormalizedLineupItem> itemValidation =
                await NormalizeLineupItem(dbContext, request.Lineup[i], i, cancellationToken);
            foreach (BaseError error in itemValidation.LeftToSeq())
            {
                return error;
            }

            foreach (NormalizedLineupItem item in itemValidation.RightToSeq())
            {
                normalized.Add(item);
            }
        }

        bool multiItem = normalized.Count >= 2;

        // MultiCollection entries only support Shuffle / ShuffleInOrder / WeightedShuffle
        // (mirrors PlayoutModeMustBeValid -- keep the two lists in step).
        if (normalized.Any(i => i.CollectionType is CollectionType.MultiCollection) &&
            playbackOrder is not (PlaybackOrder.Shuffle or PlaybackOrder.ShuffleInOrder
                or PlaybackOrder.WeightedShuffle))
        {
            return BaseError.New($"Invalid playback order for multi collection: '{playbackOrder}'");
        }

        // A lineup of 2+ entries is persisted as a Playlist, and PlaylistEnumerator has no default arm: an
        // order it doesn't know leaves the enumerator null and the items are dropped from the playlist with
        // nothing reported. This is the second (and less obvious) persisting writer of
        // PlaylistItem.PlaybackOrder, alongside ReplacePlaylistItems (#70; the silent fallbacks are #403).
        if (multiItem && playbackOrder is PlaybackOrder.WeightedShuffle)
        {
            return BaseError.New(
                $"Playback order '{playbackOrder}' is not supported for a multi-item lineup; it is available on classic schedule items");
        }

        if (multiItem)
        {
            // The generated playlist cannot express rerun collections or nested playlists
            // (PlaylistItem + CollectionKey.ForPlaylistItem lack those fields).
            for (int i = 0; i < normalized.Count; i++)
            {
                if (normalized[i].CollectionType is CollectionType.RerunFirstRun or CollectionType.Playlist)
                {
                    return BaseError.New(
                        $"lineup[{normalized[i].Index}]: rerun collections and playlists are only " +
                        "supported as a single-item lineup");
                }
            }
        }

        Channel channel = BuildChannel(
            request,
            template,
            advanced,
            name,
            number,
            group,
            categories,
            ffmpegProfileId,
            fallbackFillerId);

        string scheduleName = await DeCollideName(
            GeneratedName(number, name, "Schedule"),
            (candidate, ct) => dbContext.ProgramSchedules.AnyAsync(ps => ps.Name == candidate, ct),
            cancellationToken);

        ProgramSchedule schedule = BuildProgramSchedule(scheduleName, template, advanced);

        Playlist playlist = null;
        ProgramScheduleItemFlood floodItem = BuildFloodBase(
            playbackOrder,
            advanced,
            template,
            fallbackFillerId,
            preRollFillerId,
            midRollFillerId,
            postRollFillerId);

        if (multiItem)
        {
            playlist = await BuildPlaylist(dbContext, number, name, normalized, playbackOrder, cancellationToken);
            floodItem.CollectionType = CollectionType.Playlist;
            floodItem.Playlist = playlist;
        }
        else
        {
            NormalizedLineupItem only = normalized[0];
            floodItem.CollectionType = only.CollectionType;
            floodItem.CollectionId = only.CollectionId;
            floodItem.MultiCollectionId = only.MultiCollectionId;
            floodItem.SmartCollectionId = only.SmartCollectionId;
            floodItem.RerunCollectionId = only.RerunCollectionId;
            floodItem.MediaItemId = only.MediaItemId;
            floodItem.PlaylistId = only.PlaylistId;
        }

        schedule.Items = [floodItem];

        var playout = new Playout
        {
            Channel = channel,
            ProgramSchedule = schedule,
            ScheduleKind = PlayoutScheduleKind.Classic
        };

        return new PreparedCreate(channel, playlist, schedule, playout);
    }

    private static async Task<Playlist> BuildPlaylist(
        TvContext dbContext,
        string channelNumber,
        string channelName,
        List<NormalizedLineupItem> lineup,
        PlaybackOrder playbackOrder,
        CancellationToken cancellationToken)
    {
        // Reuse the single system playlist group, creating it on first use.
        PlaylistGroup playlistGroup = await dbContext.PlaylistGroups
            .FirstOrDefaultAsync(pg => pg.IsSystem && pg.Name == SystemPlaylistGroupName, cancellationToken);
        playlistGroup ??= new PlaylistGroup { Name = SystemPlaylistGroupName, IsSystem = true };

        string playlistName = await DeCollideName(
            GeneratedName(channelNumber, channelName, "Lineup"),
            (candidate, ct) => dbContext.Playlists.AnyAsync(
                p => p.PlaylistGroupId == playlistGroup.Id && p.Name == candidate,
                ct),
            cancellationToken);

        var playlist = new Playlist
        {
            Name = playlistName,
            IsSystem = true,
            PlaylistGroup = playlistGroup,
            PlaylistGroupId = playlistGroup.Id,
            Items = []
        };

        int index = 1;
        foreach (NormalizedLineupItem item in lineup)
        {
            playlist.Items.Add(new PlaylistItem
            {
                Playlist = playlist,
                Index = index++,
                CollectionType = item.CollectionType,
                CollectionId = item.CollectionId,
                MultiCollectionId = item.MultiCollectionId,
                SmartCollectionId = item.SmartCollectionId,
                MediaItemId = item.MediaItemId,
                PlaybackOrder = playbackOrder,

                // Play every item in each entry before advancing so lineup order is honored
                // (PlaylistEnumerator round-robins one-per-entry unless PlayAll/Count).
                PlayAll = true,
                IncludeInProgramGuide = true
            });
        }

        return playlist;
    }

    private static Channel BuildChannel(
        CreateChannelFromLineup request,
        ChannelTemplate template,
        CreateChannelFromLineupAdvancedOptions advanced,
        string name,
        string number,
        string group,
        string categories,
        int ffmpegProfileId,
        int? fallbackFillerId)
    {
        var artwork = new List<Artwork>();
        if (!string.IsNullOrWhiteSpace(request.Logo?.Path))
        {
            string logo = request.Logo.Path;
            if (logo.StartsWith("iptv/logos/", StringComparison.Ordinal))
            {
                logo = logo.Replace("iptv/logos/", string.Empty);
            }

            artwork.Add(new Artwork
            {
                Path = logo,
                ArtworkKind = ArtworkKind.Logo,
                OriginalContentType = !string.IsNullOrEmpty(request.Logo.ContentType) ? request.Logo.ContentType : null,
                DateAdded = DateTime.UtcNow,
                DateUpdated = DateTime.UtcNow
            });
        }

        return new Channel(Guid.NewGuid())
        {
            Name = name,
            Number = number,
            SortNumber = double.Parse(number, CultureInfo.InvariantCulture),
            Group = group,
            Categories = categories,
            FFmpegProfileId = ffmpegProfileId,
            SlugSeconds = null,
            PlayoutSource = advanced.PlayoutSource ?? template.PlayoutSource,
            PlayoutMode = advanced.PlayoutMode ?? template.PlayoutMode,
            StreamingMode = advanced.StreamingMode ?? template.StreamingMode,
            WatermarkId = advanced.WatermarkId ?? template.WatermarkId,
            FallbackFillerId = fallbackFillerId,
            Artwork = artwork,
            StreamSelectorMode = advanced.StreamSelectorMode ?? template.StreamSelectorMode,
            StreamSelector = advanced.StreamSelector ?? template.StreamSelector ?? string.Empty,
            PreferredAudioLanguageCode =
                advanced.PreferredAudioLanguageCode ?? template.PreferredAudioLanguageCode ?? string.Empty,
            PreferredAudioTitle = advanced.PreferredAudioTitle ?? template.PreferredAudioTitle ?? string.Empty,
            PreferredSubtitleLanguageCode =
                advanced.PreferredSubtitleLanguageCode ?? template.PreferredSubtitleLanguageCode ?? string.Empty,
            SubtitleMode = advanced.SubtitleMode ?? template.SubtitleMode,
            MusicVideoCreditsMode = advanced.MusicVideoCreditsMode ?? template.MusicVideoCreditsMode,
            MusicVideoCreditsTemplate =
                advanced.MusicVideoCreditsTemplate ?? template.MusicVideoCreditsTemplate ?? string.Empty,
            SongVideoMode = advanced.SongVideoMode ?? template.SongVideoMode,
            TranscodeMode = advanced.TranscodeMode ?? template.TranscodeMode,
            IdleBehavior = advanced.IdleBehavior ?? template.IdleBehavior,
            IsEnabled = request.IsEnabled,
            ShowInEpg = request.IsEnabled && request.ShowInEpg
        };
    }

    private static ProgramSchedule BuildProgramSchedule(
        string scheduleName,
        ChannelTemplate template,
        CreateChannelFromLineupAdvancedOptions advanced) =>
        new()
        {
            Name = scheduleName,
            KeepMultiPartEpisodesTogether = true,
            TreatCollectionsAsShows = true,
            ShuffleScheduleItems = advanced.ShuffleScheduleItems ?? template.ShuffleScheduleItems,
            RandomStartPoint = advanced.RandomStartPoint ?? template.RandomStartPoint,
            FixedStartTimeBehavior = advanced.FixedStartTimeBehavior ?? template.FixedStartTimeBehavior,
            Items = []
        };

    private static ProgramScheduleItemFlood BuildFloodBase(
        PlaybackOrder playbackOrder,
        CreateChannelFromLineupAdvancedOptions advanced,
        ChannelTemplate template,
        int? fallbackFillerId,
        int? preRollFillerId,
        int? midRollFillerId,
        int? postRollFillerId) =>
        new()
        {
            Index = 1,
            PlaybackOrder = playbackOrder,
            GuideMode = GuideMode.Normal,
            CustomTitle = string.Empty,
            SearchTitle = string.Empty,
            SearchQuery = string.Empty,
            PreRollFillerId = preRollFillerId,
            MidRollFillerId = midRollFillerId,
            PostRollFillerId = postRollFillerId,
            FallbackFillerId = fallbackFillerId,
            PreferredAudioLanguageCode =
                advanced.PreferredAudioLanguageCode ?? template.PreferredAudioLanguageCode ?? string.Empty,
            PreferredAudioTitle = advanced.PreferredAudioTitle ?? template.PreferredAudioTitle ?? string.Empty,
            PreferredSubtitleLanguageCode =
                advanced.PreferredSubtitleLanguageCode ?? template.PreferredSubtitleLanguageCode ?? string.Empty,
            SubtitleMode = advanced.SubtitleMode ?? template.SubtitleMode
        };

    private static string GeneratedName(string channelNumber, string channelName, string suffix)
    {
        string prefix = $"{channelNumber} {channelName}".Trim();
        int maxPrefixLength = Math.Max(0, 50 - suffix.Length - 1);
        if (prefix.Length > maxPrefixLength)
        {
            prefix = prefix[..maxPrefixLength].TrimEnd();
        }

        return $"{prefix} {suffix}".Trim();
    }

    // De-collide a generated (already <= 50 char) name against a unique index by appending " 2", " 3", ...
    // rather than leaking a raw UNIQUE-constraint failure.
    private static async Task<string> DeCollideName(
        string baseName,
        Func<string, CancellationToken, Task<bool>> exists,
        CancellationToken cancellationToken)
    {
        if (!await exists(baseName, cancellationToken))
        {
            return baseName;
        }

        for (int n = 2; ; n++)
        {
            string suffix = $" {n}";
            string candidate = baseName.Length + suffix.Length > 50
                ? baseName[..(50 - suffix.Length)].TrimEnd() + suffix
                : baseName + suffix;

            if (!await exists(candidate, cancellationToken))
            {
                return candidate;
            }
        }
    }

    private static async Task<Either<BaseError, Unit>> ValidateReferences(
        TvContext dbContext,
        CreateChannelFromLineupAdvancedOptions advanced,
        ChannelTemplate template,
        CancellationToken cancellationToken)
    {
        int ffmpegProfileId = advanced.FFmpegProfileId ?? template.FFmpegProfileId;
        if (!await dbContext.FFmpegProfiles.AnyAsync(p => p.Id == ffmpegProfileId, cancellationToken))
        {
            return new NotFoundError($"FFmpegProfile {ffmpegProfileId} does not exist.");
        }

        Either<BaseError, Unit> channelReferences = await ValidateChannelReferences(
            dbContext,
            advanced.WatermarkId ?? template.WatermarkId,
            advanced.FallbackFillerId ?? template.FallbackFillerId,
            cancellationToken);
        foreach (BaseError error in channelReferences.LeftToSeq())
        {
            return error;
        }

        Either<BaseError, Unit> itemFillers = await ValidateItemFillers(
            dbContext,
            advanced.PreRollFillerId ?? template.PreRollFillerId,
            advanced.MidRollFillerId ?? template.MidRollFillerId,
            advanced.PostRollFillerId ?? template.PostRollFillerId,
            cancellationToken);
        foreach (BaseError error in itemFillers.LeftToSeq())
        {
            return error;
        }

        return Unit.Default;
    }

    private static async Task<Either<BaseError, Unit>> ValidateChannelReferences(
        TvContext dbContext,
        int? watermarkId,
        int? fallbackFillerId,
        CancellationToken cancellationToken)
    {
        if (watermarkId.HasValue &&
            !await dbContext.ChannelWatermarks.AnyAsync(w => w.Id == watermarkId.Value, cancellationToken))
        {
            return new NotFoundError($"Watermark {watermarkId.Value} does not exist.");
        }

        if (fallbackFillerId.HasValue && !await dbContext.FillerPresets.AnyAsync(
                fp => fp.Id == fallbackFillerId.Value && fp.FillerKind == FillerKind.Fallback,
                cancellationToken))
        {
            return new NotFoundError($"Fallback filler {fallbackFillerId.Value} does not exist.");
        }

        return Unit.Default;
    }

    private static async Task<Either<BaseError, Unit>> ValidateItemFillers(
        TvContext dbContext,
        int? preRollFillerId,
        int? midRollFillerId,
        int? postRollFillerId,
        CancellationToken cancellationToken)
    {
        if (preRollFillerId.HasValue && !await FillerExists(dbContext, preRollFillerId.Value, FillerKind.PreRoll, cancellationToken))
        {
            return new NotFoundError($"Pre-roll filler {preRollFillerId.Value} does not exist.");
        }

        if (midRollFillerId.HasValue && !await FillerExists(dbContext, midRollFillerId.Value, FillerKind.MidRoll, cancellationToken))
        {
            return new NotFoundError($"Mid-roll filler {midRollFillerId.Value} does not exist.");
        }

        if (postRollFillerId.HasValue && !await FillerExists(dbContext, postRollFillerId.Value, FillerKind.PostRoll, cancellationToken))
        {
            return new NotFoundError($"Post-roll filler {postRollFillerId.Value} does not exist.");
        }

        return Unit.Default;
    }

    private static Task<bool> FillerExists(
        TvContext dbContext,
        int id,
        FillerKind fillerKind,
        CancellationToken cancellationToken) =>
        dbContext.FillerPresets.AnyAsync(fp => fp.Id == id && fp.FillerKind == fillerKind, cancellationToken);

    private static async Task<Either<BaseError, NormalizedLineupItem>> NormalizeLineupItem(
        TvContext dbContext,
        CreateChannelFromLineupItem item,
        int index,
        CancellationToken cancellationToken)
    {
        int providedIds = new int?[]
        {
            item.CollectionId,
            item.MultiCollectionId,
            item.SmartCollectionId,
            item.RerunCollectionId,
            item.MediaItemId,
            item.PlaylistId
        }.Count(id => id.HasValue);

        if (providedIds != 1)
        {
            return BaseError.New($"lineup[{index}] must provide exactly one typed id.");
        }

        switch (item.MediaType)
        {
            case LibraryBrowseMediaType.Movie:
                return await NormalizeMediaItem(
                    dbContext.Movies, item, CollectionType.Movie, index, "Movie", cancellationToken);
            case LibraryBrowseMediaType.TelevisionShow:
                return await NormalizeMediaItem(
                    dbContext.Shows, item, CollectionType.TelevisionShow, index, "TelevisionShow", cancellationToken);
            case LibraryBrowseMediaType.TelevisionSeason:
                return await NormalizeMediaItem(
                    dbContext.Seasons, item, CollectionType.TelevisionSeason, index, "TelevisionSeason", cancellationToken);
            case LibraryBrowseMediaType.Artist:
                return await NormalizeMediaItem(
                    dbContext.Artists, item, CollectionType.Artist, index, "Artist", cancellationToken);
            case LibraryBrowseMediaType.Collection:
                if (item.CollectionType != CollectionType.Collection)
                {
                    return Mismatch(index, item);
                }

                if (!item.CollectionId.HasValue)
                {
                    return WrongId(index, item.MediaType, "collectionId");
                }

                return await ExistsThen(
                    dbContext.Collections,
                    item.CollectionId.Value,
                    $"lineup[{index}] Collection",
                    new NormalizedLineupItem(index, CollectionType.Collection, CollectionId: item.CollectionId),
                    cancellationToken);
            case LibraryBrowseMediaType.SmartCollection:
                if (item.CollectionType != CollectionType.SmartCollection)
                {
                    return Mismatch(index, item);
                }

                if (!item.SmartCollectionId.HasValue)
                {
                    return WrongId(index, item.MediaType, "smartCollectionId");
                }

                return await ExistsThen(
                    dbContext.SmartCollections,
                    item.SmartCollectionId.Value,
                    $"lineup[{index}] SmartCollection",
                    new NormalizedLineupItem(index, CollectionType.SmartCollection, SmartCollectionId: item.SmartCollectionId),
                    cancellationToken);
            case LibraryBrowseMediaType.MultiCollection:
                if (item.CollectionType != CollectionType.MultiCollection)
                {
                    return Mismatch(index, item);
                }

                if (!item.MultiCollectionId.HasValue)
                {
                    return WrongId(index, item.MediaType, "multiCollectionId");
                }

                return await ExistsThen(
                    dbContext.MultiCollections,
                    item.MultiCollectionId.Value,
                    $"lineup[{index}] MultiCollection",
                    new NormalizedLineupItem(index, CollectionType.MultiCollection, MultiCollectionId: item.MultiCollectionId),
                    cancellationToken);
            case LibraryBrowseMediaType.RerunCollection:
                if (item.CollectionType != CollectionType.RerunFirstRun)
                {
                    return Mismatch(index, item);
                }

                if (!item.RerunCollectionId.HasValue)
                {
                    return WrongId(index, item.MediaType, "rerunCollectionId");
                }

                return await ExistsThen(
                    dbContext.RerunCollections,
                    item.RerunCollectionId.Value,
                    $"lineup[{index}] RerunCollection",
                    new NormalizedLineupItem(index, CollectionType.RerunFirstRun, RerunCollectionId: item.RerunCollectionId),
                    cancellationToken);
            case LibraryBrowseMediaType.Playlist:
                if (item.CollectionType != CollectionType.Playlist)
                {
                    return Mismatch(index, item);
                }

                if (!item.PlaylistId.HasValue)
                {
                    return WrongId(index, item.MediaType, "playlistId");
                }

                return await ExistsThen(
                    dbContext.Playlists,
                    item.PlaylistId.Value,
                    $"lineup[{index}] Playlist",
                    new NormalizedLineupItem(index, CollectionType.Playlist, PlaylistId: item.PlaylistId),
                    cancellationToken);
            default:
                return BaseError.New($"lineup[{index}] has an unsupported media type '{item.MediaType}'.");
        }
    }

    private static async Task<Either<BaseError, NormalizedLineupItem>> NormalizeMediaItem<TEntity>(
        DbSet<TEntity> set,
        CreateChannelFromLineupItem item,
        CollectionType expectedType,
        int index,
        string label,
        CancellationToken cancellationToken)
        where TEntity : class
    {
        if (item.CollectionType != expectedType)
        {
            return Mismatch(index, item);
        }

        if (!item.MediaItemId.HasValue)
        {
            return WrongId(index, item.MediaType, "mediaItemId");
        }

        return await ExistsThen(
            set,
            item.MediaItemId.Value,
            $"lineup[{index}] {label}",
            new NormalizedLineupItem(index, expectedType, MediaItemId: item.MediaItemId),
            cancellationToken);
    }

    private static BaseError Mismatch(int index, CreateChannelFromLineupItem item) =>
        BaseError.New(
            $"lineup[{index}]: media type '{item.MediaType}' does not match collection type '{item.CollectionType}'.");

    private static BaseError WrongId(int index, LibraryBrowseMediaType mediaType, string expectedField) =>
        BaseError.New($"lineup[{index}]: media type '{mediaType}' requires a {expectedField}.");

    private static async Task<Either<BaseError, NormalizedLineupItem>> ExistsThen<TEntity>(
        DbSet<TEntity> set,
        int id,
        string label,
        NormalizedLineupItem normalized,
        CancellationToken cancellationToken)
        where TEntity : class
    {
        bool exists = await set.AsNoTracking()
            .AnyAsync(e => EF.Property<int>(e, "Id") == id, cancellationToken);
        return exists
            ? normalized
            : new NotFoundError($"{label} {id} does not exist.");
    }

    private sealed record NormalizedLineupItem(
        int Index,
        CollectionType CollectionType,
        int? CollectionId = null,
        int? MultiCollectionId = null,
        int? SmartCollectionId = null,
        int? RerunCollectionId = null,
        int? MediaItemId = null,
        int? PlaylistId = null);

    private sealed record PreparedCreate(
        Channel Channel,
        Playlist Playlist,
        ProgramSchedule ProgramSchedule,
        Playout Playout);
}