AppState.swift

func change(_ state:AppState, _ change:[AppState.Change] ) -> AppState { state.alter(change) }

struct AppState: Codable {
    enum Change: Equatable {
        case by(_By); enum _By: Equatable {
            case adding(_Add); enum _Add: Equatable {
                case light(Light)
                case favorite(Favorite) }
            case replacing(_Replace); enum _Replace: Equatable {
                case lights(with:[Light])
                case light (with:Light  )
                case rooms (with:[Room] )
            }
            case removing(_Remove); enum _Remove: Equatable  {
                case favorite(Favorite)
            }
        }
    }
    
    func alter(_ changes: [Change] ) -> AppState { changes.reduce(self) { $0.alter($1) } }
    
    private func alter (_ change:Change) -> Self {
        func sort  (_                    lights:[Light]) -> [Light] { lights.sorted(on:\.id, by:<)     }
        func remove(_ light: Light, from lights:[Light]) -> [Light] { lights.filter{$0.id != light.id} }
        switch change {
        case let .by(.adding   (.light (light ))): return .init(sort(lights                     + [light]), rooms, favorites)
        case let .by(.replacing(.lights(lights))): return .init(     lights                               , rooms, favorites)
        case let .by(.replacing(.light (light ))): return .init(sort(remove(light, from:lights) + [light]), rooms, favorites)
        case let .by(.replacing(.rooms (rooms ))): return .init(     lights                               , rooms, favorites)
        case let .by(.adding   (.favorite(fav ))): return .init(lights, rooms, favorites + [fav])
        case let .by(.removing (.favorite(fav ))): return .init(lights, rooms, favorites.filter({ fav.lightID != $0.lightID }))
        }
    }
    init() {
        self.init([], [], [])
    }
    private
    init(_ lights:[Light],_ rooms:[Room],_ favorites:[Favorite]) {
        self.lights = lights
        self.rooms = rooms
        self.favorites = favorites
    }
    let lights: [Light]
    let rooms : [Room ]
    let favorites: [Favorite]
}