enum State {
    var count: Int = 0
}

enum Action {
    case increment
    case decrement
}

typealias Enviornment = Void

let reducer: Reducer<Action, State, Environment>
reducer = Reducer { action, state, environment in
    switch action {
    case .increment:
        state.count += 1
        return Effect.empty
    case .decrement:
        state.count -= 1
        return Effect.empty
    }
}

let actomaton = Actomaton<Action, State>(
    state: State(),
    reducer: reducer
)

@main
enum Main {
    static func main() async {
        assertEqual(await actomaton.state.count, 0)

        await actomaton.send(.increment)
        assertEqual(await actomaton.state.count, 1)

        await actomaton.send(.increment)
        assertEqual(await actomaton.state.count, 2)

        await actomaton.send(.decrement)
        assertEqual(await actomaton.state.count, 1)

        await actomaton.send(.decrement)
        assertEqual(await actomaton.state.count, 0)
    }
}

reducer = Reducer { action, state, environment in
    switch action {
    case .increment:
        state.count += 1
        return Effect.fireAndForget {
            print("increment")
        }
    case .decrement:
        state.count -= 1
        return Effect.fireAndForget {
            print("decrement and sleep...")
            await Task.sleep(...) // NOTE: We can use `await`!
            print("I'm awake!")
        }
    }
}

enum State {
    case loggedOut, loggingIn, loggedIn, loggingOut
}

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

// NOTE:
// Use same `EffectID` so that if previous effect is still running,
// next effect with same `EffectID` will automatically cancel the previous one.
//
// Note that `EffectID` is also useful for manual cancellation via `Effect.cancel`.
struct LoginFlowEffectID: EffectIDProtocol {}

struct Environment {
    let loginEffect: (userId: String) -> Effect<Action>
    let logoutEffect: Effect<Action>
}

let environment = Environment(
    loginEffect: { userId in
        Effect(id: LoginFlowEffectID()) {
            let loginRequest = ...
            let data = try? await URLSession.shared.data(for: loginRequest)
            if Task.isCancelled { return nil }
            ...
            return Action.loginOK // next action
        }
    },
    logoutEffect: {
        Effect(id: LoginFlowEffectID()) {
            let logoutRequest = ...
            let data = try? await URLSession.shared.data(for: logoutRequest)
            if Task.isCancelled { return nil }
            ...
            return Action.logoutOK // next action
        }
    }
)

let reducer = Reducer { action, state, environment in
    switch (action, state) {
    case (.login, .loggedOut):
        state = .loggingIn
        return environment.login(state.userId)

    case (.loginOK, .loggingIn):
        state = .loggedIn
        return .empty

    case (.logout, .loggedIn),
        (.forceLogout, .loggingIn),
        (.forceLogout, .loggedIn):
        state = .loggingOut
        return environment.logout()

    case (.logoutOK, .loggingOut):
        state = .loggedOut
        return .empty

    default:
        return Effect.fireAndForget {
            print("State transition failed...")
        }
    }
}

let actomaton = Actomaton<Action, State>(
    state: .loggedOut,
    reducer: reducer,
    environment: environment
)

@main
enum Main {
    static func test_login_logout() async {
        var t: Task<(), Never>?

        assertEqual(await actomaton.state, .loggedOut)

        t = await actomaton.send(.login)
        assertEqual(await actomaton.state, .loggingIn)

        await t?.value // wait for previous effect
        assertEqual(await actomaton.state, .loggedIn)

        t = await actomaton.send(.logout)
        assertEqual(await actomaton.state, .loggingOut)

        await t?.value // wait for previous effect
        assertEqual(await actomaton.state, .loggedOut)

        XCTAssertFalse(isLoginCancelled)
    }

    static func test_login_forceLogout() async throws {
        var t: Task<(), Never>?

        assertEqual(await actomaton.state, .loggedOut)

        await actomaton.send(.login)
        assertEqual(await actomaton.state, .loggingIn)

        // Wait for a while and interrupt by `forceLogout`.
        // Login's effect will be automatically cancelled because of same `EffectID.
        await Task.sleep(/* 1 ms */)
        t = await actomaton.send(.forceLogout)

        assertEqual(await actomaton.state, .loggingOut)

        await t?.value // wait for previous effect
        assertEqual(await actomaton.state, .loggedOut)

    }
}

typealias State = Int

enum Action {
    case start, tick, stop
}

struct TimerID: EffectIDProtocol {}

struct Environment {
    let timerEffect: Effect<Action>
}

let environment = Environment(
    timerEffect: { userId in
        Effect(id: TimerID(), sequence: AsyncStream<()> { continuation in
            let task = Task {
                while true {
                    await Task.sleep(/* 1 sec */)
                    continuation.yield(())
                }
            }

            continuation.onTermination = { @Sendable _ in
                task.cancel()
            }
        }
    }
)

let reducer = Reducer { action, state, environment in
    switch action {
    case .start:
        return environment.timerEffect
    case .tick:
        state += 1
        return .empty
    case .stop:
        return Effect.cancel(id: TimerID())
    }
}

let actomaton = Actomaton<Action, State>(
    state: 0,
    reducer: reducer,
    environment: environment
)

@main
enum Main {
    static func test_timer() async {
        assertEqual(await actomaton.state, 0)

        await actomaton.send(.start)

        assertEqual(await actomaton.state, 0)

        await Task.sleep(/* 1 sec */)
        assertEqual(await actomaton.state, 1)

        await Task.sleep(/* 1 sec */)
        assertEqual(await actomaton.state, 2)

        await Task.sleep(/* 1 sec */)
        assertEqual(await actomaton.state, 3)

        await actomaton.send(.stop)

        await Task.sleep(/* long enough */)
        assertEqual(await actomaton.state, 3,
                    "Should not increment because timer is stopped.")
    }
}

enum Root {} // just a namespace

extension Root {
    enum Action {
        case changeCurrent(State.Current?)

        case counter(Counter.Action)
        case stopwatch(Stopwatch.Action)
        case stateDiagram(StateDiagram.Action)
        case todo(Todo.Action)
        case github(GitHub.Action)
    }

    struct State: Equatable {
        var current: Current?

        // Current screen (NOTE: enum, so only 1 screen will appear)
        enum Current: Equatable {
            case counter(Counter.State)
            case stopwatch(Stopwatch.State)
            case stateDiagram(StateDiagram.State)
            case todo(Todo.State)
            case github(GitHub.State)
        }
    }

    // NOTE: `contramap` is also called `pullback` in swift-composable-architecture.
    static var reducer: Reducer<Action, State, Environment> {
        Reducer.combine(
            Counter.reducer
                .contramap(action: /Action.counter)
                .contramap(state: /State.Current.counter)
                .contramap(state: \State.current)
                .contramap(environment: { _ in () }),

            Todo.reducer
                .contramap(action: /Action.todo)
                .contramap(state: /State.Current.todo)
                .contramap(state: \State.current)
                .contramap(environment: { _ in () }),

            StateDiagram.reducer
                .contramap(action: /Action.stateDiagram)
                .contramap(state: /State.Current.stateDiagram)
                .contramap(state: \State.current)
                .contramap(environment: { _ in () }),

            Stopwatch.reducer
                .contramap(action: /Action.stopwatch)
                .contramap(state: /State.Current.stopwatch)
                .contramap(state: \State.current)
                .contramap(environment: { $0.stopwatch }),

            GitHub.reducer
                .contramap(action: /Action.github)
                .contramap(state: /State.Current.github)
                .contramap(state: \State.current)
                .contramap(environment: { $0.github }),

            GitHub.reducer
                .contramap(action: /Action.github)
                .contramap(state: /State.Current.github)
                .contramap(state: \State.current)
                .contramap(environment: { $0.github })
        )
    }
}

struct AppView: View {
    @StateObject
    private var store: Store<Root.Action, Root.State> = .init(
        state: Root.State(...),
        reducer: Root.reducer,
        environment: Root.Environment(...)
    )

    init() {}

    var body: some View {
        // IMPORTANT: Pass `Store.Proxy` to children.
        RootView(store: self.store.proxy)
    }
}

struct RootView: View {
    // IMPORTANT: `Store.Proxy`, not `Store` itself.
    private let store: Store<Root.Action, Root.State>.Proxy

    init(store: Store<Root.Action, Root.State>.Proxy) {
        self.store = store
    }

    var body: some View {
        return VStack {
            NavigationView {
                List {
                    navigationLink(example: allExamples[0])
                    navigationLink(example: allExamples[1])
                    navigationLink(example: allExamples[2])
                    navigationLink(example: allExamples[3])
                    navigationLink(example: allExamples[4])
                }
                .navigationBarTitle(Text("🎭 Actomaton Gallery 🖼️"), displayMode: .large)
            }
        }
    }

    private func navigationLink(example: Example) -> some View {
        NavigationLink(
            destination: example.exampleView(store: self.store)
                .navigationBarTitle(
                    "\(example.exampleTitle)",
                    displayMode: .inline
                ),
            isActive: self.store.current
                .stateBinding(onChange: Root.Action.changeCurrent)
                .transform(
                    get: { $0?.example.exampleTitle == example.exampleTitle },
                    set: { _, isPresenting in
                        isPresenting ? example.exampleInitialState : nil
                    }
                )
        ) {
            HStack(alignment: .firstTextBaseline) {
                example.exampleIcon
                    .frame(width: 44)
                Text(example.exampleTitle)
            }
            .font(.body)
            .padding(5)
        }
    }
}