CountKeeperWithInjection.swift

struct Counter {

    internal var countKeeper: CountKeeper

    init(countKeeper: CountKeeper = RealTimeCounter()) {
        self.countKeeper = countKeeper
    }

    private var start: Double?
    private var totalCount: Double = 0

    var isCounterRunning: Bool {
        return start != nil
    }

    var incrementedCount: Double {
        return totalCount + (start.map {_ in countKeeper.count } ?? 0)
    }

    mutating func startCounter() {
        start = countKeeper.count
    }

    mutating func pauseCounter() {
        totalCount = incrementedCount
        start = nil
    }

    mutating func resetCounter() {
        start = nil
        totalCount = 0
    }
}