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.ExpectedVersions));

                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)
    {
        // In-place reconcile (rather than delete-and-reinsert): a schedule item owns 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 keeps its id, so the cascade
        // never fires and progression survives.
        //
        // Two reconcile modes (#259):
        //   * id-based (preferred) — when the client round-trips each existing item's server id, map request
        //     items to existing rows BY id, so per-child state follows the LOGICAL item across reorders and
        //     mid-list inserts rather than staying with whatever previously occupied a slot.
        //   * positional (legacy fallback) — a fully id-less payload keeps the original by-array-position
        //     reconcile. This preserves today's misattribution on legacy reorders; it is temporary and
        //     retires with the §7a Phase-2 If-Match flip.
        // Index (ordering) is always derived from array position; identity (id) is a separate axis.
        var orderedItems = request.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);

        List<ProgramScheduleItem> existingItems = programSchedule.Items.ToList();
        List<int> requestIds = orderedItems.Where(i => i.Id.HasValue).Select(i => i.Id.Value).ToList();

        if (requestIds.Count == 0)
        {
            // ---- positional fallback (verbatim pre-#259 behavior) ----
            List<ProgramScheduleItem> orderedExisting = existingItems.OrderBy(i => i.Index).ToList();
            int commonCount = Math.Min(orderedExisting.Count, orderedItems.Count);
            for (var i = 0; i < commonCount; i++)
            {
                ProgramScheduleItem existing = orderedExisting[i];
                ProgramScheduleItem built = BuildItem(programSchedule, i, orderedItems[i]);
                if (existing.GetType() == built.GetType())
                {
                    built.Id = existing.Id;
                    dbContext.Entry(existing).CurrentValues.SetValues(built);
                    RebuildChildren(existing, orderedItems[i]);
                }
                else
                {
                    dbContext.Remove(existing);
                    programSchedule.Items.Add(built);
                }
            }

            for (int i = commonCount; i < orderedExisting.Count; i++)
            {
                dbContext.Remove(orderedExisting[i]);
            }

            for (int i = commonCount; i < orderedItems.Count; i++)
            {
                programSchedule.Items.Add(BuildItem(programSchedule, i, orderedItems[i]));
            }
        }
        else
        {
            // ---- id-based reconcile ----
            // Guards run HERE (inside PersistItems, after CheckVersion in Handle) so a client that is BOTH
            // version-stale and id-stale gets 412 (reload signal) — not 422, which reads as a payload bug
            // (§7c). Both guards persist nothing (plain BaseError → 422).

            // 7b: a duplicated id would map two request items onto one row (SetValues twice, last-writer-wins,
            // one item's config silently lost). Always a client bug.
            int? duplicateId = requestIds.GroupBy(id => id).Where(g => g.Count() > 1).Select(g => (int?)g.Key)
                .FirstOrDefault();
            if (duplicateId.HasValue)
            {
                return new BaseError($"Schedule item id {duplicateId.Value} appears more than once in the request.");
            }

            // 7a: an id not belonging to THIS schedule is a stale/foreign identity. Under Phase-1 (a missing
            // If-Match force-writes) it is a live lost-update signal; inserting-as-new would silently duplicate
            // the item and return a different id than the client sent. Reject loudly rather than mask it.
            var existingIds = existingItems.Select(e => e.Id).ToHashSet();
            foreach (int id in requestIds)
            {
                if (!existingIds.Contains(id))
                {
                    return new BaseError($"Schedule item id {id} does not belong to this schedule.");
                }
            }

            // delete pass: existing rows the request no longer references (their fill-group state cascades —
            // correct, the logical item is gone)
            var referencedIds = requestIds.ToHashSet();
            foreach (ProgramScheduleItem existing in existingItems)
            {
                if (!referencedIds.Contains(existing.Id))
                {
                    dbContext.Remove(existing);
                }
            }

            // match/insert pass in array order (Index = i)
            var existingById = existingItems.ToDictionary(e => e.Id);
            for (var i = 0; i < orderedItems.Count; i++)
            {
                ReplaceProgramScheduleItem requestItem = orderedItems[i];
                ProgramScheduleItem built = BuildItem(programSchedule, i, requestItem);
                if (requestItem.Id.HasValue && existingById.TryGetValue(requestItem.Id.Value, out ProgramScheduleItem existing))
                {
                    if (existing.GetType() == built.GetType())
                    {
                        // same TPT subtype: copy all scalars in place (BuildItem is the single source of item
                        // construction, so no field is dropped) and rebuild join rows, keeping the id — and
                        // with it the fill-group index that would otherwise cascade away. State follows the
                        // logical item regardless of its new position.
                        built.Id = existing.Id;
                        dbContext.Entry(existing).CurrentValues.SetValues(built);
                        RebuildChildren(existing, requestItem);
                    }
                    else
                    {
                        // EF can't retype a TPT row in place; replace it. Fill-group state resets and the
                        // response returns a NEW id (the item fundamentally changed — clients re-sync from it).
                        dbContext.Remove(existing);
                        programSchedule.Items.Add(built);
                    }
                }
                else
                {
                    // id-less request item → new (unknown ids were already rejected above)
                    programSchedule.Items.Add(built);
                }
            }
        }

        // 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);
}