import Foundation
import os.log

private let log = Logger(subsystem: "com.albert.nasbackup", category: "ManifestCache")

/// Persists a copy of the NAS backup manifest on-device so reconciliation can
/// skip the NAS network round-trip when the cache is still fresh.
///
/// Cache is updated after every successful manifest download and after every
/// completed backup. Stale threshold = 5 minutes. The actor serialises all
/// reads and writes so the cache is safe to use across tasks and threads.
actor NASManifestCache {
    static let shared = NASManifestCache()

    private var cached: CachedEntry?
    private let diskURL: URL
    private static let staleInterval: TimeInterval = 5 * 60

    private struct CachedEntry: Codable {
        var manifest: BackupManifest
        var savedAt: Date
    }

    private init() {
        let caches = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)[0]
        diskURL = caches.appendingPathComponent("kisani_manifest_cache.json")
        // Load from disk immediately so the first `manifest` access is instant.
        if let data = try? Data(contentsOf: diskURL),
           let entry = try? JSONDecoder.kisani.decode(CachedEntry.self, from: data) {
            cached = entry
            log.info("ManifestCache: loaded \(entry.manifest.entries.count) entries from disk")
        }
    }

    /// Returns the cached manifest without any I/O. Never nil after first reconcile.
    var manifest: BackupManifest? { cached?.manifest }

    /// True when the cache is absent or older than the stale threshold.
    var isStale: Bool {
        guard let c = cached else { return true }
        return Date().timeIntervalSince(c.savedAt) > NASManifestCache.staleInterval
    }

    /// Persist a freshly-downloaded manifest. Call after every successful NAS download.
    func update(_ manifest: BackupManifest) {
        let entry = CachedEntry(manifest: manifest, savedAt: Date())
        cached = entry
        log.info("ManifestCache: updated — \(manifest.entries.count) entries")
        Task.detached(priority: .utility) { [entry, url = diskURL] in
            guard let data = try? JSONEncoder.kisani.encode(entry) else { return }
            try? data.write(to: url, options: .atomic)
        }
    }
}

// MARK: — Shared encoder/decoder

extension JSONEncoder {
    static let kisani: JSONEncoder = {
        let e = JSONEncoder()
        e.dateEncodingStrategy = .iso8601
        return e
    }()
}

extension JSONDecoder {
    static let kisani: JSONDecoder = {
        let d = JSONDecoder()
        d.dateDecodingStrategy = .iso8601
        return d
    }()
}