import Foundation
import OSLog

/// An error with a message ready to show; `offline` when the server could not be reached.
struct APIError: LocalizedError {
    var message: String
    var offline = false
    var errorDescription: String? { message }
}

/// The schedule server's HTTP API, as the Android client's Api.kt.
struct API: Sendable {
    let base: URL

    /// The server address: Info.plist's PSUAPIURL; Debug builds may set PSU_API_URL instead.
    static let shared: API = {
        #if DEBUG
        if let env = ProcessInfo.processInfo.environment["PSU_API_URL"], let url = URL(string: env) {
            return API(base: url)
        }
        #endif
        let plist = Bundle.main.object(forInfoDictionaryKey: "PSUAPIURL") as? String
        return API(base: URL(string: plist ?? "https://psu.rumblur.by/")!)
    }()

    private static let log = Logger(subsystem: "org.xtimms.psu.schedule", category: "API")
    private static let offline = "Нет связи с сервером расписания."
    private static let retries = 3

    // The server answers "no-cache" with an ETag: URLSession keeps the answer and asks again
    // with If-None-Match, so an unchanged semester comes back as an empty 304.
    private static let session: URLSession = {
        let config = URLSessionConfiguration.default
        config.timeoutIntervalForRequest = 60
        config.urlCache = URLCache(memoryCapacity: 2 << 20, diskCapacity: 10 << 20)
        config.requestCachePolicy = .useProtocolCachePolicy
        return URLSession(configuration: config)
    }()

    private func get<T: Decodable & Sendable>(_ path: String, _ query: [(String, String?)] = []) async throws -> T {
        var components = URLComponents(url: base.appendingPathComponent(path), resolvingAgainstBaseURL: false)!
        let items = query.compactMap { name, value in value.flatMap { $0.isEmpty ? nil : URLQueryItem(name: name, value: $0) } }
        if !items.isEmpty { components.queryItems = items }
        let url = components.url!
        let (data, response) = try await withRetry(url)
        guard let http = response as? HTTPURLResponse else { throw APIError(message: Self.offline, offline: true) }
        guard (200..<300).contains(http.statusCode) else {
            let message = (try? JSONSerialization.jsonObject(with: data) as? [String: Any])?["error"] as? String
            // The server's catch-all 404 means it runs an older version without this endpoint.
            if http.statusCode == 404 && message == "Адрес не найден." {
                throw APIError(message: "Сервер расписания не знает запрос /\(path) — на нём старая версия. Обновите сервер.")
            }
            throw APIError(message: message ?? "Сервер ответил ошибкой \(http.statusCode).")
        }
        do {
            return try JSONDecoder().decode(T.self, from: data)
        } catch {
            Self.log.error("Decoding \(path) failed: \(error)")
            throw APIError(message: "Не удалось прочитать ответ сервера.")
        }
    }

    /// A dropped or timed-out connection is retried twice (after 1 and 3 seconds):
    /// a flaky mobile network often recovers within seconds. Server answers are not retried.
    private func withRetry(_ url: URL) async throws -> (Data, URLResponse) {
        var wait: UInt64 = 1_000_000_000
        for attempt in 1...Self.retries {
            do {
                return try await Self.session.data(from: url)
            } catch let error as URLError where error.code != .cancelled {
                Self.log.warning("GET \(url) failed (attempt \(attempt)): \(error.localizedDescription)")
                if attempt == Self.retries { break }
                try await Task.sleep(nanoseconds: wait)
                wait *= 3
            }
        }
        throw APIError(message: Self.offline, offline: true)
    }

    /// The whole current semester; a server without range=semester answers the from/days window.
    func timetable(doc: String, group: String, from: CivilDate, days: Int) async throws -> Timetable {
        try await get("api/timetable", [("doc", doc), ("group", group), ("from", from.iso), ("days", String(days)), ("range", "semester")])
    }

    func groups(query: String = "", faculty: String = "", course: String = "") async throws -> GroupsResponse {
        try await get("api/groups", [("q", query), ("faculty", faculty), ("course", course)])
    }

    func searchDirectory(_ query: String) async throws -> DirectorySearch {
        try await get("api/directory/search", [("q", query)])
    }

    func teacherDay(_ name: String, date: CivilDate) async throws -> DirectoryDay {
        try await get("api/directory/day", [("teacher", name), ("date", date.iso)])
    }

    func roomDay(_ room: String, date: CivilDate) async throws -> DirectoryDay {
        try await get("api/directory/day", [("room", room), ("date", date.iso)])
    }

    func calendar() async throws -> CalendarInfo { try await get("api/calendar") }

    func exams(group: String, faculty: String) async throws -> ExamsResponse {
        try await get("api/exams", [("group", group), ("faculty", faculty)])
    }
}

extension Timetable: @unchecked Sendable {}
extension GroupsResponse: @unchecked Sendable {}
extension DirectorySearch: @unchecked Sendable {}
extension DirectoryDay: @unchecked Sendable {}
extension CalendarInfo: @unchecked Sendable {}
extension ExamsResponse: @unchecked Sendable {}