Turnstyle

Turnstyle.swift

struct BrokenTurnstyle {
    private let previousState: WorkingTurnstyle

    func machineRepairDidComplete() -> WorkingTurnstyle {
        return previousState
    }
}

struct WorkingTurnstyle {

    private(set) var state: State = .Locked(credit: 0)
    private static let farePrice: UInt = 50

    enum State {
        case Locked(credit: UInt)
        case Unlocked
    }

    enum Command {
        case SoundAlarm
        case CloseDoors
        case OpenDoors
    }

    mutating func insertCoin(value: UInt) -> Command? {
        switch state {
        case .Locked(let credit):
            let newCredit = credit + value
            if newCredit >= WorkingTurnstyle.farePrice {
                state = .Unlocked
                return .OpenDoors
            } else {
                return nil
            }
        case .Unlocked:
            return nil
        }
    }

    mutating func admitPerson() -> Command? {
        switch state {
        case .Unlocked:
            state = .Locked(credit: 0)
            return .CloseDoors
        case .Locked:
            return .SoundAlarm
        }
    }

    func machineDidFail() -> BrokenTurnstyle {
        return BrokenTurnstyle(previousState: self)
    }
}