ersatztv/ErsatzTV.Application/Emby/Commands/UpdateEmbyPathReplacementsHandler.cs

using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Interfaces.Repositories;

namespace ErsatzTV.Application.Emby;

public class UpdateEmbyPathReplacementsHandler : IRequestHandler<UpdateEmbyPathReplacements,
    Either<BaseError, Unit>>
{
    private readonly IMediaSourceRepository _mediaSourceRepository;

    public UpdateEmbyPathReplacementsHandler(IMediaSourceRepository mediaSourceRepository) =>
        _mediaSourceRepository = mediaSourceRepository;

    public async Task<Either<BaseError, Unit>> Handle(
        UpdateEmbyPathReplacements request,
        CancellationToken cancellationToken)
    {
        Option<EmbyMediaSource> maybeSource =
            await _mediaSourceRepository.GetEmby(request.EmbyMediaSourceId, cancellationToken);

        return await maybeSource.Match(
            Some: async embyMediaSource =>
            {
                Option<BaseError> maybeError = ValidateItems(request, embyMediaSource);
                return await maybeError.Match(
                    Some: error => Task.FromResult(Left<BaseError, Unit>(error)),
                    None: async () =>
                    {
                        await MergePathReplacements(request, embyMediaSource);
                        return Right<BaseError, Unit>(Unit.Default);
                    });
            },
            None: () => Task.FromResult(
                Left<BaseError, Unit>(
                    BaseError.New($"Emby media source {request.EmbyMediaSourceId} does not exist."))));
    }

    private Task<Unit> MergePathReplacements(
        UpdateEmbyPathReplacements request,
        EmbyMediaSource embyMediaSource)
    {
        embyMediaSource.PathReplacements ??= new List<EmbyPathReplacement>();

        var incoming = request.PathReplacements.Map(Project).ToList();

        var toAdd = incoming.Filter(r => r.Id < 1).ToList();
        var toRemove = embyMediaSource.PathReplacements.Filter(r => incoming.All(pr => pr.Id != r.Id)).ToList();
        var toUpdate = incoming.Except(toAdd).ToList();

        return _mediaSourceRepository.UpdatePathReplacements(embyMediaSource.Id, toAdd, toUpdate, toRemove);
    }

    private static EmbyPathReplacement Project(EmbyPathReplacementItem vm) =>
        new() { Id = vm.Id, EmbyPath = vm.EmbyPath, LocalPath = vm.LocalPath };

    // Defense-in-depth for design #202 findings 2c/8 — the repo UPDATE is scoped by
    // EmbyMediaSourceId, but reject a foreign/blank/null row here too, before any write, so the
    // mutation is all-or-nothing.
    private static Option<BaseError> ValidateItems(
        UpdateEmbyPathReplacements request,
        EmbyMediaSource embyMediaSource)
    {
        List<EmbyPathReplacementItem> items = request.PathReplacements ?? [];

        if (items.Any(item => item is null))
        {
            return BaseError.New("Path replacement items must not be null.");
        }

        if (items.Any(item => string.IsNullOrWhiteSpace(item.EmbyPath) || string.IsNullOrWhiteSpace(item.LocalPath)))
        {
            return BaseError.New("Each path replacement requires a non-blank Emby path and local path.");
        }

        var existingIds = (embyMediaSource.PathReplacements ?? new List<EmbyPathReplacement>())
            .Map(pr => pr.Id)
            .ToList();
        var foreignIds = items.Filter(item => item.Id > 0 && !existingIds.Contains(item.Id))
            .Map(item => item.Id)
            .ToList();
        if (foreignIds.Count > 0)
        {
            return BaseError.New(
                $"Path replacement id(s) {string.Join(", ", foreignIds)} do not belong to Emby media source " +
                $"{request.EmbyMediaSourceId}.");
        }

        return Option<BaseError>.None;
    }
}