// Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import Foundation
import FirebaseCore

/// Possible errors with model downloading.
public enum DownloadError: Error, Equatable {
  /// No model with this name found on server.
  case notFound
  /// Caller does not have necessary permissions for this operation.
  case permissionDenied
  /// Conditions not met to perform download.
  case failedPrecondition
  /// Not enough space for model on device.
  case notEnoughSpace
  /// Malformed model name.
  case invalidArgument
  /// Other errors with description.
  case internalError(description: String)
}

/// Possible errors with locating model on device.
public enum DownloadedModelError: Error, Equatable {
  /// File system error.
  case fileIOError(description: String)
  /// Model not found on device.
  case notFound
}

/// Possible ways to get a custom model.
public enum ModelDownloadType {
  /// Get local model stored on device.
  case localModel
  /// Get local model on device and update to latest model from server in the background.
  case localModelUpdateInBackground
  /// Get latest model from server.
  case latestModel
}

/// Downloader to manage custom model downloads.
public class ModelDownloader {
  /// Name of the app associated with this instance of ModelDownloader.
  private let appName: String

  /// Shared dictionary mapping app name to a specific instance of model downloader.
  // TODO: Switch to using Firebase components.
  private static var modelDownloaderDictionary: [String: ModelDownloader] = [:]

  /// Private init for downloader.
  private init(app: FirebaseApp) {
    appName = app.name
    NotificationCenter.default.addObserver(
      self,
      selector: #selector(deleteModelDownloader),
      name: Notification.Name("FIRAppDeleteNotification"),
      object: nil
    )
  }

  /// Handles app deletion notification.
  @objc private func deleteModelDownloader(notification: Notification) {
    if let userInfo = notification.userInfo,
      let appName = userInfo["FIRAppNameKey"] as? String {
      ModelDownloader.modelDownloaderDictionary.removeValue(forKey: appName)
      // TODO: Clean up user defaults
      // TODO: Clean up local instances of app
    }
  }

  /// Model downloader with default app.
  public static func modelDownloader() -> ModelDownloader {
    guard let defaultApp = FirebaseApp.app() else {
      fatalError("Default Firebase app not configured.")
    }
    return modelDownloader(app: defaultApp)
  }

  /// Model Downloader with custom app.
  public static func modelDownloader(app: FirebaseApp) -> ModelDownloader {
    if let downloader = modelDownloaderDictionary[app.name] {
      return downloader
    } else {
      let downloader = ModelDownloader(app: app)
      modelDownloaderDictionary[app.name] = downloader
      return downloader
    }
  }

  /// Downloads a custom model to device or gets a custom model already on device, w/ optional handler for progress.
  public func getModel(name modelName: String, downloadType: ModelDownloadType,
                       conditions: ModelDownloadConditions,
                       progressHandler: ((Float) -> Void)? = nil,
                       completion: @escaping (Result<CustomModel, DownloadError>) -> Void) {
    // TODO: Model download
    let modelSize = Int()
    let modelPath = String()
    let modelHash = String()

    let customModel = CustomModel(
      name: modelName,
      size: modelSize,
      path: modelPath,
      hash: modelHash
    )
    completion(.success(customModel))
    completion(.failure(.notFound))
  }

  /// Gets all downloaded models.
  public func listDownloadedModels(completion: @escaping (Result<Set<CustomModel>,
    DownloadedModelError>) -> Void) {
    let customModels = Set<CustomModel>()
    // TODO: List downloaded models
    completion(.success(customModels))
    completion(.failure(.notFound))
  }

  /// Deletes a custom model from device.
  public func deleteDownloadedModel(name modelName: String,
                                    completion: @escaping (Result<Void, DownloadedModelError>)
                                      -> Void) {
    // TODO: Delete previously downloaded model
    completion(.success(()))
    completion(.failure(.notFound))
  }
}