enum SignInNavigationStep {
    case signIn
}

final class SignInViewModel: BaseViewModel<
    SignInNavigationStep,
    SignInViewModel.InputFromView,
    SignInViewModel.Output
> {

    private let useCase: AuthenticationUseCase

    public init(useCase: AuthenticationUseCase) {
        self.useCase = useCase
    }

    override func transform(input: Input) -> Output {
        func userDid(_ step: SignInNavigationStep) {
            navigator.navigate(to: .signIn)
        }

        let activityTracker = ActivityTracker()
        let errorTracker = ErrorTracker()

        let isSigningIn = activityTracker.asDriver()

        // Validate input
        let validator = InputValidator()
        let usernameValidation: Driver<Validation<String>> = input.fromView.username.map { validator.validate(email: $0) }
        let validUsername: Driver<String> = usernameValidation.map { $0.asValidated }.filterNil()

        let passwordsValidation: Driver<Validation<String>> = Driver.combineLatest(input.fromView.password, input.fromView.confirmPassword) {
            validator.validate(password: $0, confirmedBy: $1)
        }
        let validPassword: Driver<String> = passwordsValidation.map { $0.asValidated }.filterNil()

        // Make button enabled/disabled based on validation
        let isSignInButtonEnabled = Driver.combineLatest(
            usernameValidation.map { $0.isValid },
            passwordsValidation.map { $0.isValid }
        ) { $0 && $1  }

        // Create request from valid input
        let signInRequest: Driver<SignInRequest> = Driver.combineLatest(validUsername, validPassword) {
            SignInRequest(username: $0, password: $1)
        }

        // Create and dispose of `Disposable`s
        [
            // Make request
            input.fromView.signInTrigger.withLatestFrom(signInRequest) { $1 }
                .flatMapLatest { [unowned useCase] in
                    useCase.signIn(request: $1)
                        .trackError(errorTracker)
                        .trackActivity(activityTracker)
                        .asDriverOnErrorReturnEmpty()
                }.do(onNext: { userDid(.signIn) }) // when successful, notify coordinator
                .drive(),

            // Let Controller display alert for error message (could also be moved to Coordinator)
            errorTracker.asDriver()
                .do(onNext: {
                    // Remember I said ControllerInput was a bit simplified? It has an `alertSubject`
                    // On which we can notify of an `Alert` type, to display errors or short messages.
                    input.fromController.alertSubject.onNext(Alert(error: $0))
                }).drive()

        ].forEach { $0.disposed(by: bag) }

        // Return output to drives UI
        return Output(
            isSignInButtonEnabled: isSignInButtonEnabled,
            isSignInButtonEnabled: isSigningIn,
            usernameValidation: usernameValidation,
            passwordValidation: passwordValidation
        )
    }
}

extension SignInViewModel {

    struct InputFromView {
        let username: Driver<String>
        let password: Driver<String>
        let confirmPassword: Driver<String>
        let signInTrigger: Driver<Void>
    }

    struct Output {
        let isSignInButtonEnabled: Driver<Bool>
        let isSigningIn: Driver<Bool>
        let usernameValidation: Driver<Validation<String>>
        let passwordValidation: Driver<Validation<String>>
    }
}


// MARK: - Field Validation
extension SignInViewModel {
    struct InputValidator {
        // Some simple validator types, implementation omitted.
        private let emailValidator = EmailValidator()
        private let passwordValidator = PasswordValidator()

        func validate(email: String) -> Validation<String> {
            return emailValidator.validate(input: email)
        }

        func validate(password: String, confirmedBy confirmingPassword: String) -> Validation<String> {
            return passwordValidator.validate(password: password, passwordConfirmation: confirmingPassword)
        }
    }
}