State Machine

enum State
{
    case offHook
    case connecting
    case connected
    case onHold
}

enum Trigger
{
    case callDialed
    case hungUp
    case callConnected
    case placedOnHold
    case takenOffHold
    case leftMessage
}

let rules = [
    State.offHook : [
        (Trigger.callDialed, State.connecting)
    ],
    State.connecting : [
        (Trigger.hungUp, State.offHook),
        (Trigger.callConnected, State.connected)
    ],
    State.connected : [
        (Trigger.leftMessage, State.offHook),
        (Trigger.hungUp, State.offHook),
        (Trigger.placedOnHold, State.onHold),
    ],
    State.onHold : [
        (Trigger.takenOffHold, State.connected),
        (Trigger.hungUp, State.offHook),
    ]

]

func main()
{
    var state = State.offHook
    
    while true
    {
        print("The phone is currently \(state)")
        print("Select a trigger : ")
        
        for i in 0..<rules[state]!.count
        {
            let (t, _) = rules[state]![i]
            print("\(i). \(t)")
        }
        if let input = Int(readLine()!)
        {
            let (_, s) = rules[state]![input]
            state = s
        }
    }
}