import Foundation
import ManaCore
import Observation
import SwiftData

/// Persistente Offline-Queue für Grade-Aktionen. Drain-Loop kann
/// vom UI ausgelöst werden (bei Reconnect oder App-Foreground).
@MainActor
@Observable
final class GradeQueue {
    private(set) var isDraining = false
    private(set) var lastDrainError: String?

    private let api: CardsAPI
    private let context: ModelContext

    init(api: CardsAPI, context: ModelContext) {
        self.api = api
        self.context = context
    }

    /// Enqueue + sofort versuchen zu senden. Bei Fehler bleibt der
    /// Eintrag in der Queue.
    func submit(cardId: String, subIndex: Int, rating: Rating, reviewedAt: Date = .now) async {
        let grade = PendingGrade(
            cardId: cardId,
            subIndex: subIndex,
            rating: rating,
            reviewedAt: reviewedAt
        )
        context.insert(grade)
        try? context.save()
        Log.study.info(
            "Queued grade for \(cardId, privacy: .public)/\(subIndex, privacy: .public): \(rating.rawValue, privacy: .public)"
        )
        await drain()
    }

    /// Schickt alle pending grades in FIFO-Reihenfolge ab. Bei Server-
    /// Erfolg: aus Queue löschen. Bei Netzfehler: Loop abbrechen
    /// (kommender Drain probiert es nochmal).
    func drain() async {
        guard !isDraining else { return }
        isDraining = true
        defer { isDraining = false }

        let descriptor = FetchDescriptor<PendingGrade>(
            sortBy: [SortDescriptor(\.queuedAt, order: .forward)]
        )
        let pending = (try? context.fetch(descriptor)) ?? []
        guard !pending.isEmpty else {
            lastDrainError = nil
            return
        }

        for grade in pending {
            guard let rating = grade.rating else {
                context.delete(grade)
                continue
            }
            do {
                _ = try await api.gradeReview(
                    cardId: grade.cardId,
                    subIndex: grade.subIndex,
                    rating: rating,
                    reviewedAt: grade.reviewedAt
                )
                context.delete(grade)
                try? context.save()
            } catch {
                let msg = (error as? LocalizedError)?.errorDescription ?? String(describing: error)
                grade.lastTryAt = .now
                grade.lastError = msg
                try? context.save()
                lastDrainError = msg
                Log.study.notice(
                    "Drain stopped for \(grade.cardId, privacy: .public)/\(grade.subIndex, privacy: .public): \(msg, privacy: .public)"
                )
                return
            }
        }
        lastDrainError = nil
        Log.study.info("Drain complete")
    }

    /// Wie viele Grades hängen aktuell offline?
    func pendingCount() -> Int {
        let descriptor = FetchDescriptor<PendingGrade>()
        return (try? context.fetchCount(descriptor)) ?? 0
    }
}