wordeck-native/Sources/Features/Study/StudySession.swift

import Foundation
import ManaCore
import Observation
import SwiftData

/// State-Machine für eine Lern-Session. Lädt Due-Reviews beim Start,
/// rendert eine Karte nach der anderen, schickt Grades via GradeQueue ab.
///
/// Seit ζ-1 (2026-05-18): wenn der Server-Call scheitert, fällt die
/// Session auf den `CachedDueReview`-Snapshot vom letzten Sync zurück.
/// Der User lernt dann offline. Grades laufen wie immer in die
/// `GradeQueue` und drainen beim Reconnect.
@MainActor
@Observable
final class StudySession {
    enum Phase {
        case loading
        case studying
        case finished
        case failed(String)
    }

    private(set) var phase: Phase = .loading
    private(set) var queue: [DueReview] = []
    private(set) var currentIndex: Int = 0
    private(set) var isFlipped: Bool = false
    private(set) var totalGraded: Int = 0
    /// `true` wenn die Session aus dem lokalen Snapshot statt vom Server
    /// gestartet wurde. View kann ein Offline-Banner zeigen.
    private(set) var isOfflineSession: Bool = false

    let deckId: String
    let deckName: String

    private let api: WordeckAPI
    private let context: ModelContext
    private let gradeQueue: GradeQueue

    init(deckId: String, deckName: String, auth: AuthClient, context: ModelContext) {
        self.deckId = deckId
        self.deckName = deckName
        self.context = context
        api = WordeckAPI(auth: auth)
        gradeQueue = GradeQueue(api: api, context: context)
    }

    var current: DueReview? {
        guard queue.indices.contains(currentIndex) else { return nil }
        return queue[currentIndex]
    }

    var remaining: Int {
        max(0, queue.count - currentIndex)
    }

    func start() async {
        phase = .loading
        do {
            queue = try await api.dueReviews(deckId: deckId, limit: 500)
            currentIndex = 0
            isFlipped = false
            totalGraded = 0
            isOfflineSession = false
            if queue.isEmpty {
                phase = .finished
            } else {
                phase = .studying
            }
            let count = queue.count
            let id = deckId
            Log.study.info("Session start — \(count, privacy: .public) due in deck \(id, privacy: .public)")
        } catch {
            // Server nicht erreichbar oder Auth-Fehler → Cache-Fallback.
            queue = loadFromCache()
            currentIndex = 0
            isFlipped = false
            totalGraded = 0
            if queue.isEmpty {
                let msg = (error as? LocalizedError)?.errorDescription ?? String(describing: error)
                phase = .failed(msg)
                Log.study.error("Session start failed (no cache): \(msg, privacy: .public)")
            } else {
                isOfflineSession = true
                phase = .studying
                let count = queue.count
                let id = deckId
                Log.study
                    .notice("Offline-Session — \(count, privacy: .public) cached due in deck \(id, privacy: .public)")
            }
        }
    }

    private func loadFromCache() -> [DueReview] {
        let deckId = deckId
        var descriptor = FetchDescriptor<CachedDueReview>(
            predicate: #Predicate<CachedDueReview> { $0.deckId == deckId },
            sortBy: [SortDescriptor(\.due, order: .forward)]
        )
        descriptor.fetchLimit = 500
        let cached = (try? context.fetch(descriptor)) ?? []
        return cached.compactMap { $0.toDueReview() }
    }

    func flip() {
        guard case .studying = phase else { return }
        isFlipped.toggle()
    }

    func grade(_ rating: Rating) async {
        guard case .studying = phase, let card = current else { return }
        let reviewedAt = Date.now
        await gradeQueue.submit(
            cardId: card.review.cardId,
            subIndex: card.review.subIndex,
            rating: rating,
            reviewedAt: reviewedAt
        )
        totalGraded += 1
        advance()
    }

    private func advance() {
        currentIndex += 1
        isFlipped = false
        if currentIndex >= queue.count {
            phase = .finished
            let count = totalGraded
            Log.study.info("Session finished — graded \(count, privacy: .public)")
        }
    }
}