//
//  FetchService.swift
//  Loader
//
//  Created by samara on 14.03.2025.
//

import Foundation

// MARK: - Class
public class NBFetchService {
	
	public enum NBFetchServiceError: Error, LocalizedError {
		case invalidURL
		case networkError(Error)
		case httpError(Int)
		case noData
		case parsingError(Error)
		/// The source is private and nobody is signed in on this device.
		///
		/// Its own case rather than an `httpError(403)`, because it is the one
		/// refusal the user can do something about: the app answers it with a
		/// sign-in screen instead of an error row. `stranded` is the operator
		/// having closed the source outright, with nobody who could sign in and
		/// no invitation outstanding, which is the one case the screen has
		/// nothing to offer for.
		case locked(stranded: Bool, note: String?)

		public var errorDescription: String? {
			switch self {
			case .invalidURL: "The URL is invalid."
			case .networkError(let error): "Network error: \(error.localizedDescription)"
			case .httpError(let code): "The server returned HTTP \(code)."
			case .noData: "No data received."
			case .parsingError(let error): "Failed to parse data: \(error.localizedDescription)"
			case .locked: "This source is private."
			}
		}
	}

	/// The shape of the refusal FTStore Admin sends when a source is private.
	private struct _LockedBody: Decodable {
		let code: String?
		/// On, with nobody who can sign in. Omitted when somebody could, so its
		/// absence reads as "there is a form worth showing".
		let stranded: Bool?
		let note: String?
	}
	
	public init() {}

	/// Identifies the app to its repo backend. The server gates its feed on this, so it
	/// must be deterministic and start with `FTStore`/`FTStore-Manager`. Derived from
	/// the running app's bundle so the same code yields the right value in both apps.
	public static let userAgent: String = {
		let info = Bundle.main.infoDictionary
		let version = (info?["CFBundleShortVersionString"] as? String) ?? "1.0"
		let bundleId = Bundle.main.bundleIdentifier ?? "app.ftstore"
		let product = bundleId == "app.ftstore.manager" ? "FTStore-Manager" : "FTStore"
		return "\(product)/\(version) (\(bundleId))"
	}()
}

// MARK: - Class extension: fetch
extension NBFetchService {
	public func fetch<T: Decodable>(
		from urlString: String,
		headers: [String: String] = [:],
		completion: @escaping (Result<T, Error>) -> Void
	) {
		guard let url = URL(string: urlString) else {
			completion(.failure(NBFetchServiceError.invalidURL))
			return
		}
		
		fetch(from: url, headers: headers, completion: completion)
	}
	
	/// - Parameter headers: Anything the *caller* has decided this particular URL
	///   should carry. A plain dictionary and not a hook, because deciding which
	///   hosts may see a credential is the app's business and this package has no
	///   idea what FTStore's server is: it copies what it is handed onto the
	///   request and knows nothing else about it.
	public func fetch<T: Decodable>(
		from url: URL,
		headers: [String: String] = [:],
		completion: @escaping (Result<T, Error>) -> Void
	) {
		DispatchQueue.global(qos: .userInitiated).async {
			// Always pull a fresh copy: source feeds change frequently, and the
			// default protocol cache policy would otherwise serve a stale JSON
			// (showing versions that no longer exist in the live manifest).
			var request = URLRequest(url: url)
			request.cachePolicy = .reloadIgnoringLocalCacheData
			request.setValue(Self.userAgent, forHTTPHeaderField: "User-Agent")
			for (name, value) in headers {
				request.setValue(value, forHTTPHeaderField: name)
			}
			let task = URLSession.shared.dataTask(with: request) { data, response, error in
				if let error = error {
					completion(.failure(NBFetchServiceError.networkError(error)))
					return
				}
				
				// Surface the status code before decoding: an error page (or a
				// JSON error envelope from the repo host) would otherwise fail
				// to decode and be reported as an unhelpful "parsing error".
				if
					let http = response as? HTTPURLResponse,
					!(200..<300).contains(http.statusCode)
				{
					// A private source answers 403 with a code saying so. Told
					// apart from every other refusal because the caller can offer
					// to sign in for this one, and cannot offer anything for the
					// rest.
					if
						http.statusCode == 403,
						let data,
						let body = try? JSONDecoder().decode(_LockedBody.self, from: data),
						body.code == "account_required"
					{
						completion(.failure(NBFetchServiceError.locked(
							stranded: body.stranded ?? false,
							note: body.note
						)))
						return
					}

					completion(.failure(NBFetchServiceError.httpError(http.statusCode)))
					return
				}

				guard let data = data else {
					completion(.failure(NBFetchServiceError.noData))
					return
				}

				do {
					let decoder = JSONDecoder()
					let decodedData = try decoder.decode(T.self, from: data)
					completion(.success(decodedData))
				} catch {
					completion(.failure(NBFetchServiceError.parsingError(error)))
				}
			}
			
			task.resume()
		}
	}
}