using System.Threading.Channels;
using ErsatzTV.Core;
using ErsatzTV.Core.Errors;
using ErsatzTV.Core.Interfaces.Search;
using ErsatzTV.Infrastructure.Data;
using Microsoft.EntityFrameworkCore;
using static ErsatzTV.Application.Channels.ChannelValidations;
using Channel = ErsatzTV.Core.Domain.Channel;

namespace ErsatzTV.Application.Channels;

public class BulkMoveChannelsToGroupHandler(
    IDbContextFactory<TvContext> dbContextFactory,
    ChannelWriter<IBackgroundServiceRequest> workerChannel,
    ISearchTargets searchTargets)
    : IRequestHandler<BulkMoveChannelsToGroup, Either<BaseError, Unit>>
{
    public async Task<Either<BaseError, Unit>> Handle(
        BulkMoveChannelsToGroup request,
        CancellationToken cancellationToken)
    {
        if (request.ChannelIds.Count == 0)
        {
            return Left<BaseError, Unit>(BaseError.New("At least one channel id is required"));
        }

        Validation<BaseError, string> groupValidation = ValidateGroup(request.Group);
        if (groupValidation.IsFail)
        {
            return Left<BaseError, Unit>(groupValidation.FailToSeq().Head());
        }

        await using TvContext dbContext = await dbContextFactory.CreateDbContextAsync(cancellationToken);
        await using var transaction = await dbContext.Database.BeginTransactionAsync(cancellationToken);

        List<int> channelIds = request.ChannelIds.Distinct().ToList();
        List<Channel> channels = await dbContext.Channels
            .Where(c => channelIds.Contains(c.Id))
            .ToListAsync(cancellationToken);

        if (channels.Count != channelIds.Count)
        {
            var found = channels.Select(c => c.Id).ToHashSet();
            int missingId = channelIds.First(id => !found.Contains(id));
            return Left<BaseError, Unit>(new NotFoundError($"Channel {missingId} does not exist."));
        }

        foreach (Channel channel in channels)
        {
            channel.Group = request.Group;
        }

        await dbContext.SaveChangesAsync(cancellationToken);
        await transaction.CommitAsync(cancellationToken);

        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 RefreshChannelList(), CancellationToken.None);

        return Right<BaseError, Unit>(Unit.Default);
    }
}