// 1. Define `State`s and `Input`s.
enum State {
    case loggedOut, loggingIn, loggedIn, loggingOut
}

enum Input {
    case login, loginOK, logout, logoutOK
    case forceLogout
}

enum EffectQueue: EffectQueueProtocol {
    case `default`
    case request

    var flattenStrategy: FlattenStrategy {
        switch self {
        case .default: return .merge
        case .request: return .latest
        }
    }

    static var defaultEffectQueue: EffectQueue {
        .default
    }
}

// NOTE: `EffectID` is useful for manual effect cancellation, but not used in this example.
typealias EffectID = Never

typealias Harvester = Harvest.Harvester<Input, State>
typealias EffectMapping = Harvester.EffectMapping<EffectQueue, EffectID>
typealias Effect = Harvester.Effect<Input, EffectQueue, EffectID>

// Additional effects while state-transitioning.
let loginOKPublisher = /* show UI, setup DB, request APIs, ..., and send `Input.loginOK` */
let logoutOKPublisher = /* show UI, clear cache, cancel APIs, ..., and send `Input.logoutOK` */
let forceLogoutOKPublisher = /* do something more special, ..., and send `Input.logoutOK` */

let canForceLogout: (State) -> Bool = [.loggingIn, .loggedIn].contains

let mappings: [EffectMapping] = [

  /*  Input   |   fromState => toState     |      Effect       */
  /* ----------------------------------------------------------*/
    .login    | .loggedOut  => .loggingIn  | Effect(loginOKPublisher, queue: .request),
    .loginOK  | .loggingIn  => .loggedIn   | .empty,
    .logout   | .loggedIn   => .loggingOut | Effect(logoutOKPublisher, queue: .request),
    .logoutOK | .loggingOut => .loggedOut  | .empty,

    .forceLogout | canForceLogout => .loggingOut | Effect(forceLogoutOKPublisher, queue: .request)
]

// Prepare input pipe for sending `Input` to `Harvester`.
let inputs = PassthroughSubject<Input, Never>()

var cancellables: [AnyCancellable] = []

// Setup state machine.
let harvester = Harvester(
    state: .loggedOut,
    input: inputs,
    mapping: .reduce(.first, mappings),  // combine mappings using `reduce` helper
    scheduler: DispatchQueue.main
)

// Observe state-transition replies (`.success` or `.failure`).
harvester.replies.sink { reply in
    print("received reply = \(reply)")
}.store(in: &cancellables)

// Observe current state changes.
harvester.state.sink { state in
    print("current state = \(state)")
}.store(in: &cancellables)

let send = inputs.send

expect(harvester.state) == .loggedIn    // already logged in
send(Input.logout)
expect(harvester.state) == .loggingOut  // logging out...
// `logoutOKPublisher` will automatically send `Input.logoutOK` later
// and transit to `State.loggedOut`.

expect(harvester.state) == .loggedOut   // already logged out
send(Input.login)
expect(harvester.state) == .loggingIn   // logging in...
// `loginOKPublisher` will automatically send `Input.loginOK` later
// and transit to `State.loggedIn`.

// 👨🏽 < But wait, there's more!
// Let's send `Input.forceLogout` immediately after `State.loggingIn`.

send(Input.forceLogout)                       // 💥💣💥
expect(harvester.state) == .loggingOut  // logging out...
// `forceLogoutOKublisher` will automatically send `Input.logoutOK` later
// and transit to `State.loggedOut`.