NextState.kt

suspend operator fun invoke(action: Action): State {
    val currentState = transitionApi
        .getLastTransition()
        ?.state
        ?: throw IllegalNullStateException

    val nextState = when {
        currentState == State.IDLE && action == Action.START -> State.POMODORO
        currentState == State.POMODORO && action == Action.STOP -> State.IDLE
        currentState == State.POMODORO && action == Action.COMPLETE -> State.DONE
        currentState == State.DONE && action == Action.TAKE -> State.BREAK
        currentState == State.BREAK && action == Action.START -> State.POMODORO
        currentState == State.BREAK && action == Action.COMPLETE -> State.IDLE
        else -> throw IllegalActionException(currentState, action)
    }

    transitionApi.newTransition(timeApi.now(), nextState)

    return nextState
}