frenzy-ios/Persistence/Sources/Persistence/PersistentContainer.swift

//
//  PersistentContainer.swift
//  Reader
//
//  Created by Shadowfacts on 12/24/21.
//

import CoreData
import Fervor
import OSLog

// todo: is this actually sendable?
public class PersistentContainer: NSPersistentContainer, @unchecked Sendable {
    
    private static let managedObjectModel: NSManagedObjectModel = {
        let url = Bundle.module.url(forResource: "Reader", withExtension: "momd")!
        return NSManagedObjectModel(contentsOf: url)!
    }()
    
    public private(set) lazy var backgroundContext: NSManagedObjectContext = {
        let context = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
        // the background context needs to be parented directly to the PSC
        // if it's parented to the viewContext, it blocks the viewContext (and potentially the main thread) when it needs to look things up
        context.persistentStoreCoordinator = self.persistentStoreCoordinator
        context.automaticallyMergesChangesFromParent = true
        context.mergePolicy = NSMergePolicy.mergeByPropertyObjectTrump
        return context
    }()
    
    private let logger = Logger(subsystem: Bundle.main.bundleIdentifier!, category: "PersistentContainer")
    
    public init(account: LocalData.Account) {
        super.init(name: account.persistenceKey, managedObjectModel: PersistentContainer.managedObjectModel)
        
        let groupContainerURL = FileManager.default.containerURL(forSecurityApplicationGroupIdentifier: "group.net.shadowfacts.Reader")!
        let containerAppSupportURL = groupContainerURL
            .appendingPathComponent("Library", isDirectory: true)
            .appendingPathComponent("Application Support", isDirectory: true)
        try! FileManager.default.createDirectory(at: containerAppSupportURL, withIntermediateDirectories: true)
        
        let containerStoreURL = containerAppSupportURL
            .appendingPathComponent(name)
            .appendingPathExtension("sqlite")
        
        if let existingAppSupport = try? FileManager.default.url(for: .applicationSupportDirectory, in: .userDomainMask, appropriateFor: nil, create: false) {
            let existingStore = existingAppSupport
                .appendingPathComponent(name)
                .appendingPathExtension("sqlite")
                .relativePath
                .removingPercentEncoding!
            if FileManager.default.fileExists(atPath: existingStore) {
                for ext in ["", "-shm", "-wal"] {
                    try! FileManager.default.moveItem(atPath: existingStore + ext, toPath: containerStoreURL.relativePath + ext)
                }
            }
        }
        
        let desc = NSPersistentStoreDescription(url: containerStoreURL)
        desc.type = NSSQLiteStoreType
        
        persistentStoreDescriptions = [desc]
        loadPersistentStores { description, error in
            if let error = error {
                fatalError("Unable to load persistent store: \(error)")
            }
        }
        
        viewContext.automaticallyMergesChangesFromParent = true
        viewContext.mergePolicy = NSMergePolicy.mergeByPropertyObjectTrump
    }
    
    @MainActor
    public func saveViewContext() throws {
        if viewContext.hasChanges {
            try viewContext.save()
        }
    }
    
    public func lastSyncDate() async throws -> Date? {
        return try await backgroundContext.perform {
            let state = try self.backgroundContext.fetch(SyncState.fetchRequest()).first
            return state?.lastSync
        }
    }
    
    public func updateLastSyncDate(_ date: Date) async throws {
        try await backgroundContext.perform {
            if let state = try self.backgroundContext.fetch(SyncState.fetchRequest()).first {
                state.lastSync = date
            } else {
                let state = SyncState(context: self.backgroundContext)
                state.lastSync = date
            }
            
            try self.backgroundContext.save()
        }
        try await self.saveViewContext()
    }
    
    public func sync(serverGroups: [Fervor.Group], serverFeeds: [Fervor.Feed]) async throws {
        try await backgroundContext.perform {
            let existingGroups = try self.backgroundContext.fetch(Group.fetchRequest())
            for group in serverGroups {
                if let existing = existingGroups.first(where: { $0.id == group.id }) {
                    existing.updateFromServer(group)
                } else {
                    let mo = Group(context: self.backgroundContext)
                    mo.updateFromServer(group)
                }
            }
            for removed in existingGroups where !serverGroups.contains(where: { $0.id == removed.id }) {
                self.backgroundContext.delete(removed)
            }
            
            let existingFeeds = try self.backgroundContext.fetch(Feed.fetchRequest())
            for feed in serverFeeds {
                if let existing = existingFeeds.first(where: { $0.id == feed.id }) {
                    existing.updateFromServer(feed)
                } else {
                    let mo = Feed(context: self.backgroundContext)
                    mo.updateFromServer(feed)
                }
            }
            for removed in existingFeeds where !serverFeeds.contains(where: { $0.id == removed.id }) {
                self.backgroundContext.delete(removed)
            }
            
            if self.backgroundContext.hasChanges {
                try self.backgroundContext.save()
            }
        }
        try await self.saveViewContext()
    }
    
    public func upsertItems(_ items: [Fervor.Item]) async throws {
        try await backgroundContext.perform {
            try self.doUpsertItems(items, setProgress: nil)
        }
    }
    
    private func doUpsertItems(_ items: [Fervor.Item], setProgress: ((_ current: Int, _ total: Int) -> Void)?) throws {
        let req = Item.fetchRequest()
        req.predicate = NSPredicate(format: "id in %@", items.map(\.id))
        let existing = try self.backgroundContext.fetch(req)
        self.logger.debug("doUpsertItems: updating \(existing.count, privacy: .public) items, inserting \(items.count - existing.count, privacy: .public)")
        // todo: this feels like it'll be slow when there are many items
        for (index, item) in items.enumerated() {
            setProgress?(index, items.count)
            if let existing = existing.first(where: { $0.id == item.id }) {
                existing.updateFromServer(item)
            } else {
                let mo = Item(context: self.backgroundContext)
                mo.updateFromServer(item)
            }
        }
    }
    
    public func syncItems(_ syncUpdate: ItemsSyncUpdate, setProgress: @escaping (_ current: Int, _ total: Int) -> Void) async throws {
        try await backgroundContext.perform {
            self.logger.debug("syncItems: deleting \(syncUpdate.delete.count, privacy: .public) items")
            let deleteReq = Item.fetchRequest()
            deleteReq.predicate = NSPredicate(format: "id in %@", syncUpdate.delete)
            let delete = NSBatchDeleteRequest(fetchRequest: deleteReq as! NSFetchRequest<NSFetchRequestResult>)
            delete.resultType = .resultTypeObjectIDs
            let result = try self.backgroundContext.execute(delete)
            
            if let deleteResult = result as? NSBatchDeleteResult,
               let objectIDs = deleteResult.result as? [NSManagedObjectID] {
                NSManagedObjectContext.mergeChanges(fromRemoteContextSave: [NSDeletedObjectsKey: objectIDs], into: [self.viewContext])
                // todo: does the background/view contexts need to get saved then?
            }
            
            try self.doUpsertItems(syncUpdate.upsert, setProgress: setProgress)
            
            if self.backgroundContext.hasChanges {
                try self.backgroundContext.save()
            }
        }
        try await self.saveViewContext()
    }
    
}