import Foundation
import Photos
import os.log

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

/// On-device index of phone asset metadata — eliminates full PHFetchResult
/// enumeration for status counts after the initial build.
///
/// Build cost: O(N) on first launch, then zero on every subsequent open.
/// Update cost: O(changed) via PHPhotoLibraryChangeObserver.
/// Memory: ~100 bytes × asset count (≈1 MB for 10 k photos).
actor LocalPhotoIndex {
    static let shared = LocalPhotoIndex()

    struct Record: Codable, Sendable {
        let localIdentifier: String
        var filename: String
        var creationDate: Date?
        var mediaType: Int        // PHAssetMediaType.rawValue
        var isScreenshot: Bool
        var isRAW: Bool
    }

    private var records: [String: Record] = [:]
    private let diskURL: URL
    private var needsSave = false

    private init() {
        let caches = FileManager.default.urls(for: .cachesDirectory, in: .userDomainMask)[0]
        diskURL = caches.appendingPathComponent("kisani_local_index.json")
    }

    // MARK: — Lifecycle

    /// Load from disk if available; otherwise build from PHFetchResult in background.
    /// Safe to call multiple times — subsequent calls are no-ops if records are loaded.
    func loadOrBuild() async {
        guard records.isEmpty else { return }
        if let loaded = await Self.loadFromDisk(url: diskURL) {
            records = loaded
            log.info("LocalPhotoIndex: loaded \(self.records.count) records from disk")
            return
        }
        log.info("LocalPhotoIndex: building from scratch (first launch)")
        await buildFromLibrary()
    }

    // MARK: — Queries

    var totalCount: Int { records.count }

    func allRecords() -> [Record] { Array(records.values) }

    func countSafe(against index: ManifestIndex, filter: BackupFilter) -> Int {
        var safe = 0
        for record in records.values {
            guard passes(record, filter: filter) else { continue }
            if index.matches(localIdentifier: record.localIdentifier) {
                safe += 1
            } else if index.isFilenameOnly && index.matches(filename: record.filename) {
                // Pure bootstrap mode: every entry is filename-only, match by name.
                safe += 1
            } else if index.matchesUnclaimed(filename: record.filename) {
                // Mixed mode: this specific entry was uploaded without a localIdentifier
                // (interrupted session / pre-ID backup). Filename match prevents re-upload.
                safe += 1
            }
        }
        return safe
    }

    /// Returns localIdentifiers for assets not yet in the manifest (pending upload).
    func pendingIDs(against index: ManifestIndex, filter: BackupFilter) -> [String] {
        records.values.compactMap { record in
            guard passes(record, filter: filter) else { return nil }
            let alreadyBacked =
                index.matches(localIdentifier: record.localIdentifier) ||
                (index.isFilenameOnly && index.matches(filename: record.filename)) ||
                index.matchesUnclaimed(filename: record.filename)
            return alreadyBacked ? nil : record.localIdentifier
        }
    }

    // MARK: — Incremental updates (from PHPhotoLibraryChangeObserver)

    func apply(inserted: [PHAsset], removed: [PHAsset], changed: [PHAsset]) {
        for asset in removed { records.removeValue(forKey: asset.localIdentifier) }
        for asset in inserted + changed {
            records[asset.localIdentifier] = Self.makeRecord(asset)
        }
        needsSave = true
        log.debug("LocalPhotoIndex: +\(inserted.count) -\(removed.count) ~\(changed.count)")
    }

    func flush() async {
        guard needsSave else { return }
        needsSave = false
        let snapshot = records
        let url = diskURL
        await Task.detached(priority: .utility) {
            guard let data = try? JSONEncoder.kisani.encode(snapshot) else { return }
            try? data.write(to: url, options: .atomic)
        }.value
        log.debug("LocalPhotoIndex: flushed \(snapshot.count) records")
    }

    // MARK: — Private helpers

    private func passes(_ record: Record, filter: BackupFilter) -> Bool {
        let img = PHAssetMediaType.image.rawValue
        let vid = PHAssetMediaType.video.rawValue
        if !filter.includePhotos && record.mediaType == img { return false }
        if !filter.includeVideos && record.mediaType == vid { return false }
        if !filter.includeScreenshots && record.isScreenshot { return false }
        if !filter.includeRAW && record.isRAW { return false }
        return true
    }

    private func buildFromLibrary() async {
        let opts = PHFetchOptions()
        opts.includeAssetSourceTypes = [.typeUserLibrary, .typeCloudShared, .typeiTunesSynced]
        let built: [String: Record] = await Task.detached(priority: .userInitiated) {
            let result = PHAsset.fetchAssets(with: opts)
            var map = [String: Record](minimumCapacity: result.count)
            let batchSize = 200
            var i = 0
            while i < result.count {
                autoreleasepool {
                    let end = min(i + batchSize, result.count)
                    for j in i..<end {
                        let asset = result.object(at: j)
                        let rec = Self.makeRecord(asset)
                        map[rec.localIdentifier] = rec
                    }
                    i = end
                }
            }
            return map
        }.value
        records = built
        needsSave = true
        await flush()
        log.info("LocalPhotoIndex: built \(self.records.count) records")
    }

    private nonisolated static func loadFromDisk(url: URL) async -> [String: Record]? {
        await Task.detached(priority: .utility) {
            guard let data = try? Data(contentsOf: url) else { return nil }
            return try? JSONDecoder.kisani.decode([String: Record].self, from: data)
        }.value
    }

    nonisolated static func makeRecord(_ asset: PHAsset) -> Record {
        let resources = PHAssetResource.assetResources(for: asset)
        let filename = resources.first?.originalFilename
            ?? "\(asset.localIdentifier.prefix(8)).jpg"
        let isRAW = resources.contains { $0.type == .alternatePhoto }
        let isScreenshot = asset.mediaSubtypes.contains(.photoScreenshot)
        return Record(
            localIdentifier: asset.localIdentifier,
            filename: filename,
            creationDate: asset.creationDate,
            mediaType: asset.mediaType.rawValue,
            isScreenshot: isScreenshot,
            isRAW: isRAW
        )
    }
}