using System.Threading.Channels;
using ErsatzTV.Application.Playouts;
using ErsatzTV.Core;
using ErsatzTV.Core.Domain;
using ErsatzTV.Core.Errors;
using ErsatzTV.Core.Scheduling;
using ErsatzTV.Infrastructure.Data;
using Microsoft.EntityFrameworkCore;
using static ErsatzTV.Application.ProgramSchedules.Mapper;

namespace ErsatzTV.Application.ProgramSchedules;

public class ReplaceProgramScheduleItemsHandler(
    IDbContextFactory<TvContext> dbContextFactory,
    ChannelWriter<IBackgroundServiceRequest> channel) : ProgramScheduleItemCommandBase,
    IRequestHandler<ReplaceProgramScheduleItems, Either<BaseError, IEnumerable<ProgramScheduleItemViewModel>>>
{
    public async Task<Either<BaseError, IEnumerable<ProgramScheduleItemViewModel>>> Handle(
        ReplaceProgramScheduleItems request,
        CancellationToken cancellationToken)
    {
        await using TvContext dbContext = await dbContextFactory.CreateDbContextAsync(cancellationToken);
        Option<ProgramSchedule> maybeProgramSchedule =
            await ProgramScheduleMustExist(dbContext, request.ProgramScheduleId, cancellationToken);
        return await maybeProgramSchedule.Match(
            Some: async programSchedule =>
            {
                Validation<BaseError, ProgramSchedule> validation = await Validate(dbContext, request, programSchedule);

                // Introduce the optimistic-concurrency check as a standalone Either AFTER the validation
                // pipeline (never via Apply) so a PreconditionFailedError survives to a 412 and is not
                // flattened to a generic 422 by Join() (issue #253 / api-conventions §7a).
                Either<BaseError, ProgramSchedule> validated = LanguageExtensions.ToEither(validation)
                    .Bind(ps => ps.CheckVersion(request.ExpectedVersion));

                return await validated.Match(
                    Right: ps => PersistItems(dbContext, request, ps, cancellationToken),
                    Left: error =>
                        Task.FromResult<Either<BaseError, IEnumerable<ProgramScheduleItemViewModel>>>(error));
            },
            None: () => Task.FromResult<Either<BaseError, IEnumerable<ProgramScheduleItemViewModel>>>(
                new NotFoundError("[ProgramScheduleId] does not exist.")));
    }

    private async Task<Either<BaseError, IEnumerable<ProgramScheduleItemViewModel>>> PersistItems(
        TvContext dbContext,
        ReplaceProgramScheduleItems request,
        ProgramSchedule programSchedule,
        CancellationToken cancellationToken)
    {
        // Positional in-place reconcile (rather than delete-and-reinsert): a schedule item owns the
        // persisted fill-group/shuffle enumerator state via PlayoutScheduleItemFillGroupIndex, whose
        // ProgramScheduleItemId FK is OnDelete(Cascade). Removing and re-inserting every item — as the
        // original handler did on every save, including a no-op PUT-back — cascade-deleted that state for
        // all playouts using the schedule (#252). Reusing the existing item row for the same-typed slot
        // keeps its id, so the cascade never fires and progression survives. The request DTO carries no
        // stable item id, so position is the only key available here; true content-aware stable identity
        // is deferred to the shared concurrency/round-trip contract in #253.
        var orderedItems = request.Items.OrderBy(i => i.Index).ToList();
        List<ProgramScheduleItem> existingItems = programSchedule.Items.OrderBy(i => i.Index).ToList();

        // load the watermark/graphics join rows for the existing items so they can be rebuilt in place
        await dbContext.Entry(programSchedule)
            .Collection(ps => ps.Items)
            .Query()
            .Include(i => i.ProgramScheduleItemWatermarks)
            .Include(i => i.ProgramScheduleItemGraphicsElements)
            .LoadAsync(cancellationToken);

        int commonCount = Math.Min(existingItems.Count, orderedItems.Count);
        for (var i = 0; i < commonCount; i++)
        {
            ProgramScheduleItem existing = existingItems[i];
            ProgramScheduleItem built = BuildItem(programSchedule, i, orderedItems[i]);
            if (existing.GetType() == built.GetType())
            {
                // same TPT subtype: copy all scalar values in place (BuildItem is the single source of
                // item construction, so no field is silently dropped) and rebuild the join rows, keeping
                // the item's id — and with it the fill-group index that would otherwise cascade away.
                built.Id = existing.Id;
                dbContext.Entry(existing).CurrentValues.SetValues(built);
                RebuildChildren(existing, orderedItems[i]);
            }
            else
            {
                // EF can't change a TPT row's type in place; this slot must be replaced (its fill-group
                // index resets, which is acceptable — the item fundamentally changed).
                dbContext.Remove(existing);
                programSchedule.Items.Add(built);
            }
        }

        // remove surplus existing items
        for (int i = commonCount; i < existingItems.Count; i++)
        {
            dbContext.Remove(existingItems[i]);
        }

        // add surplus incoming items
        for (int i = commonCount; i < orderedItems.Count; i++)
        {
            programSchedule.Items.Add(BuildItem(programSchedule, i, orderedItems[i]));
        }

        // Unconditional bump: this handler frequently saves with only CHILD changes and no root-scalar
        // change, so without an explicit bump EF would emit no root UPDATE and the concurrency token
        // would never fire (nor rotate other clients' ETags). Bumping guarantees both on every save,
        // including a no-op same-items PUT-back (issue #253 / api-conventions §7a).
        programSchedule.Version++;

        // Save through the guard so an EF concurrency failure (a racing writer won between our load and
        // save) maps to 412 rather than surfacing as a 500. On failure, propagate the error WITHOUT
        // running the post-save reload/enqueue below.
        Either<BaseError, Unit> saved = await dbContext.SaveChangesWithConcurrencyGuard(cancellationToken);
        if (saved.IsLeft)
        {
            return saved.Map(_ => (IEnumerable<ProgramScheduleItemViewModel>)[]);
        }

        // refresh any playouts that use this schedule
        // post-commit side effect runs on CancellationToken.None so a late request cancellation
        // can't abort it after the commit landed (#254)
        foreach (Playout playout in programSchedule.Playouts)
        {
            await channel.WriteAsync(new BuildPlayout(playout.Id, PlayoutBuildMode.Refresh), CancellationToken.None);
        }

        // reload with the full navigation graph before projecting: BuildItem creates the watermark/graphics
        // join rows with only their foreign-key ids set, so the tracked entities have null Watermark /
        // GraphicsElement navs that ProjectToViewModel dereferences (would 500 on PUT — see #229).
        List<ProgramScheduleItem> persisted = await dbContext.ProgramScheduleItems
            .Filter(psi => psi.ProgramScheduleId == programSchedule.Id)
            .IncludeScheduleItemDetails()
            .OrderBy(i => i.Index)
            .ToListAsync(cancellationToken);

        return persisted.Map(ProjectToViewModel).ToList();
    }

    // Rebuild the watermark/graphics join rows for an item updated in place. Clearing severs the required
    // relationship so EF deletes the orphaned join rows (same net effect the delete-and-reinsert path had),
    // then re-add from the request. Only the parent item row is preserved — the join rows carry no state.
    private static void RebuildChildren(ProgramScheduleItem item, ReplaceProgramScheduleItem request)
    {
        item.ProgramScheduleItemWatermarks ??= [];
        item.ProgramScheduleItemWatermarks.Clear();
        foreach (int watermarkId in request.WatermarkIds)
        {
            item.ProgramScheduleItemWatermarks.Add(
                new ProgramScheduleItemWatermark
                {
                    ProgramScheduleItem = item,
                    WatermarkId = watermarkId
                });
        }

        item.ProgramScheduleItemGraphicsElements ??= [];
        item.ProgramScheduleItemGraphicsElements.Clear();
        foreach (int graphicsElementId in request.GraphicsElementIds)
        {
            item.ProgramScheduleItemGraphicsElements.Add(
                new ProgramScheduleItemGraphicsElement
                {
                    ProgramScheduleItem = item,
                    GraphicsElementId = graphicsElementId
                });
        }
    }

    private static async Task<Validation<BaseError, ProgramSchedule>> Validate(
        TvContext dbContext,
        ReplaceProgramScheduleItems request,
        ProgramSchedule programSchedule)
    {
        Validation<BaseError, ProgramSchedule> validation = PlayoutModesMustBeValid(request, programSchedule)
            .Bind(programSchedule => CollectionTypesMustBeValid(request, programSchedule))
            .Bind(programSchedule => PlaybackOrdersMustBeValid(request, programSchedule));

        return await validation.ToEither().Match(
            Left: error => Task.FromResult<Validation<BaseError, ProgramSchedule>>(
                Fail<BaseError, ProgramSchedule>(error)),
            Right: validProgramSchedule => FillerConfigurationsMustBeValid(dbContext, request, validProgramSchedule));
    }

    private static Validation<BaseError, ProgramSchedule> PlayoutModesMustBeValid(
        ReplaceProgramScheduleItems request,
        ProgramSchedule programSchedule) =>
        request.Items.Map(item => PlayoutModeMustBeValid(item, programSchedule)).Sequence()
            .Map(_ => programSchedule);

    private static Validation<BaseError, ProgramSchedule> CollectionTypesMustBeValid(
        ReplaceProgramScheduleItems request,
        ProgramSchedule programSchedule) =>
        request.Items.Map(item => CollectionTypeMustBeValid(item, programSchedule)).Sequence()
            .Map(_ => programSchedule);

    private static async Task<Validation<BaseError, ProgramSchedule>> FillerConfigurationsMustBeValid(
        TvContext dbContext,
        ReplaceProgramScheduleItems request,
        ProgramSchedule programSchedule)
    {
        foreach (ReplaceProgramScheduleItem item in request.Items)
        {
            Either<BaseError, ProgramSchedule> result = await FillerConfigurationMustBeValid(
                dbContext,
                item,
                programSchedule);
            if (result.IsLeft)
            {
                return result.ToValidation();
            }
        }

        return programSchedule;
    }

    private static Validation<BaseError, ProgramSchedule> PlaybackOrdersMustBeValid(
        ReplaceProgramScheduleItems request,
        ProgramSchedule programSchedule)
    {
        var keyOrders = new Dictionary<CollectionKey, System.Collections.Generic.HashSet<PlaybackOrder>>();
        foreach (ReplaceProgramScheduleItem item in request.Items)
        {
            if (item.PlaybackOrder is PlaybackOrder.ShuffleInOrder &&
                item.FillWithGroupMode is not FillWithGroupMode.None)
            {
                return new BaseError("Shuffle in Order cannot be used with Fill With Group Mode");
            }

            var key = new CollectionKey(
                item.CollectionType,
                item.CollectionId,
                item.MediaItemId,
                item.MultiCollectionId,
                item.SmartCollectionId,
                item.RerunCollectionId,
                item.PlaylistId,
                item.SearchQuery);

            if (keyOrders.TryGetValue(key, out System.Collections.Generic.HashSet<PlaybackOrder> playbackOrders))
            {
                playbackOrders.Add(item.PlaybackOrder);
                keyOrders[key] = playbackOrders;
            }
            else
            {
                keyOrders.Add(key, new System.Collections.Generic.HashSet<PlaybackOrder> { item.PlaybackOrder });
            }
        }

        return Optional(keyOrders.Values.Count(set => set.Count != 1))
            .Filter(count => count == 0)
            .Map(_ => programSchedule)
            .ToValidation<BaseError>("A collection must not use multiple playback orders");
    }

    private sealed record CollectionKey(
        CollectionType CollectionType,
        int? CollectionId,
        int? MediaItemId,
        int? MultiCollectionId,
        int? SmartCollectionId,
        int? RerunCollectionId,
        int? PlaylistId,
        string SearchQuery);
}