import SwiftUI
import Combine

enum URLSessionError: Error {
    case invalidResponse
    case serverErrorMessage(statusCode: Int, data: Data)
    case urlError(URLError)
}

extension URLSession {
    func send(request: URLRequest) -> AnyPublisher<Data, URLSessionError> {

        dataTaskPublisher(for: request)
            .mapError { URLSessionError.urlError($0) }
            .flatMap { data, response -> AnyPublisher<Data, URLSessionError> in
                guard let response = response as? HTTPURLResponse else {
                    return .fail(.invalidResponse)
                }

                guard 200..<300 ~= response.statusCode else {
                    return .fail(.serverErrorMessage(statusCode: response.statusCode,
                                                     data: data))
                }

                return .just(data)
        }.eraseToAnyPublisher()
    }
}

extension JSONDecoder: TopLevelDecoder {}

extension Publisher {

    /// - seealso: https://twitter.com/peres/status/1136132104020881408
    func flatMapLatest<T: Publisher>(_ transform: @escaping (Self.Output) -> T) -> Publishers.SwitchToLatest<T, Publishers.Map<Self, T>> where T.Failure == Self.Failure {
        map(transform).switchToLatest()
    }
}

extension Publisher {

    static func empty() -> AnyPublisher<Output, Failure> {
        return Empty()
            .eraseToAnyPublisher()
    }

    static func just(_ output: Output) -> AnyPublisher<Output, Failure> {
        return Just(output)
            .catch { _ in AnyPublisher<Output, Failure>.empty() }
            .eraseToAnyPublisher()
    }

    static func fail(_ error: Failure) -> AnyPublisher<Output, Failure> {
        return Fail(error: error)
            .eraseToAnyPublisher()
    }
}