//
//  BackgroundManager.swift
//  Reader
//
//  Created by Shadowfacts on 6/20/22.
//

import Foundation
import BackgroundTasks
import OSLog
import Persistence
import Fervor

private let logger = Logger(subsystem: "net.shadowfacts.Reader", category: "BackgroundManager")

class BackgroundManager: NSObject {
    static let shared = BackgroundManager()
    
    private override init() {}
    
    static let refreshIdentifier = "net.shadowfacts.Reader.refresh"
    
    private var refreshTask: BGAppRefreshTask?
    private var completedRefreshRequests = 0
    
    private var receivedData: [Int: Data] = [:]
    
    func scheduleRefresh() {
        // we schedule refreshes from sceneDidEnterBackground, but there may be multiple scenes, and we only want one refresh task
        BGTaskScheduler.shared.cancel(taskRequestWithIdentifier: BackgroundManager.refreshIdentifier)
        
        logger.debug("Scheduling background refresh task")
        
        let request = BGAppRefreshTaskRequest(identifier: BackgroundManager.refreshIdentifier)
        request.earliestBeginDate = Date(timeIntervalSinceNow: 2 * 60 * 60)
//        request.earliestBeginDate = Date(timeIntervalSinceNow: 60)
        
        do {
            try BGTaskScheduler.shared.submit(request)
        } catch {
            logger.error("Unable to schedule app refresh: \(String(describing: error), privacy: .public)")
        }
    }
    
    func handleBackgroundRefresh(task: BGAppRefreshTask) {
        logger.debug("Handling background refresh task")
        
        scheduleRefresh()
        
        guard !LocalData.accounts.isEmpty else {
            task.setTaskCompleted(success: true)
            return
        }
        
        self.refreshTask = task
        self.completedRefreshRequests = 0
        
        doRefresh()
        
        task.expirationHandler = {
            task.setTaskCompleted(success: self.completedRefreshRequests == LocalData.accounts.count)
        }
    }
    
    func doRefresh() {
        let config = URLSessionConfiguration.background(withIdentifier: BackgroundManager.refreshIdentifier)
        config.sessionSendsLaunchEvents = true
        let backgroundSession = URLSession(configuration: config, delegate: self, delegateQueue: nil)
        
        Task {
            for account in LocalData.accounts {
                let fervorController = await FervorController(account: account, session: backgroundSession)
                let itemsRequest = await fervorController.client.itemsRequest(limit: 32)
                let task = backgroundSession.dataTask(with: itemsRequest)
                task.taskDescription = account.id.base64EncodedString()
                task.resume()
            }
        }
    }
    
    private func receivedData(_ data: Data, for task: URLSessionDataTask) {
        guard let taskDescription = task.taskDescription,
              let id = Data(base64Encoded: taskDescription),
              let account = LocalData.account(with: id) else {
            logger.error("Could not find account to handle request")
            return
        }
        Task { @MainActor in
            defer {
                if let refreshTask {
                    refreshTask.setTaskCompleted(success: completedRefreshRequests == LocalData.accounts.count)
                }
            }
            
            let items: [Fervor.Item]
            do {
                items = try FervorClient.decoder.decode([Fervor.Item].self, from: data)
            } catch {
                logger.error("Unable to decode items: \(String(describing: error), privacy: .public)")
                return
            }
            
            let fervorController = await FervorController(account: account)
            do {
                try await fervorController.persistentContainer.upsertItems(items)
                logger.info("Upserted \(items.count) items during background refresh")
                
                await WidgetHelper.updateWidgetData(fervorController: fervorController)
                
                completedRefreshRequests += 1
            } catch {
                logger.error("Unable to upsert items: \(String(describing: error), privacy: .public)")
            }
        }
    }
}

extension BackgroundManager: URLSessionDataDelegate {
    func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
        if !receivedData.keys.contains(dataTask.taskIdentifier) {
            receivedData[dataTask.taskIdentifier] = Data(capacity: Int(dataTask.countOfBytesExpectedToReceive))
        }
        receivedData[dataTask.taskIdentifier]!.append(data)
        
        logger.debug("Received chunk of \(data.count) bytes")
        
        if dataTask.countOfBytesReceived == dataTask.countOfBytesExpectedToReceive {
            logger.info("Received all data for task, handling update")
            receivedData(receivedData.removeValue(forKey: dataTask.taskIdentifier)!, for: dataTask)
        }
    }
}