import Foundation
import SwiftData

/// Snapshot eines `DueReview` (Review + Card-Subset) zum Zeitpunkt des
/// letzten Sync. Wird verwendet, wenn `StudySession` keine Verbindung
/// zum Server bekommt — der User lernt dann die Karten, die zum Sync-
/// Zeitpunkt fällig waren.
///
/// Server-authoritative-FSRS bleibt: die `due`/`stability`/… Werte
/// kommen vom Server, lokal wird nie gerechnet. Beim nächsten Sync
/// liefert der Server eine neue Due-Liste.
@Model
final class CachedDueReview {
    /// Eindeutiger Schlüssel: `"<cardId>-<subIndex>"`. SwiftData braucht
    /// einen primären Identifier pro Model, das natürliche compound-
    /// Schlüssel auf cardId+subIndex.
    @Attribute(.unique) var compoundId: String

    var cardId: String
    var subIndex: Int
    var deckId: String
    var userId: String

    // Review-State (server-authoritative, hier nur Snapshot)
    var due: Date
    var stability: Double
    var difficulty: Double
    var elapsedDays: Double
    var scheduledDays: Double
    var learningSteps: Int
    var reps: Int
    var lapses: Int
    var stateRaw: String
    var lastReview: Date?

    // Card-Snapshot (für offline-Rendering — was die Study-View braucht)
    var cardType: String
    var cardFields: [String: String]

    var snapshottedAt: Date

    init(dueReview: DueReview, deckId: String, userId: String) {
        compoundId = "\(dueReview.review.cardId)-\(dueReview.review.subIndex)"
        cardId = dueReview.review.cardId
        subIndex = dueReview.review.subIndex
        self.deckId = deckId
        self.userId = userId
        due = dueReview.review.due
        stability = dueReview.review.stability
        difficulty = dueReview.review.difficulty
        elapsedDays = dueReview.review.elapsedDays
        scheduledDays = dueReview.review.scheduledDays
        learningSteps = dueReview.review.learningSteps
        reps = dueReview.review.reps
        lapses = dueReview.review.lapses
        stateRaw = dueReview.review.state.rawValue
        lastReview = dueReview.review.lastReview
        cardType = dueReview.card.type.rawValue
        cardFields = dueReview.card.fields
        snapshottedAt = .now
    }

    /// Rekonstruiert einen `DueReview` für die `StudySession`-Queue.
    /// Gibt `nil` zurück, wenn der Type/State im Enum nicht mehr existiert
    /// (z.B. nach Schema-Migration).
    func toDueReview() -> DueReview? {
        guard let state = ReviewState(rawValue: stateRaw),
              let type = CardType(rawValue: cardType)
        else { return nil }
        let review = Review(
            cardId: cardId,
            subIndex: subIndex,
            userId: userId,
            due: due,
            stability: stability,
            difficulty: difficulty,
            elapsedDays: elapsedDays,
            scheduledDays: scheduledDays,
            learningSteps: learningSteps,
            reps: reps,
            lapses: lapses,
            state: state,
            lastReview: lastReview
        )
        let card = ReviewCard(id: cardId, deckId: deckId, type: type, fields: cardFields)
        return DueReview(review: review, card: card)
    }
}