traffic-light-with-no-invalid-states.ts

type TrafficLightState =
    { red: true, yellow: false, green: false } |
    { red: true, yellow: true, green: false } |
    { red: false, yellow: false, green: true } |
    { red: false, yellow: true, green: false }

function nextTrafficLightState(
    trafficLight: TrafficLightState): TrafficLightState {

    if (trafficLight.red && trafficLight.yellow) {
        return { red: false, yellow: false, green: true };
    } else if (trafficLight.red) {
        return { red: true, yellow: true, green: false };
    } else if (trafficLight.yellow) {
        return { red: true, yellow: false, green: false };
    } else if (trafficLight.green) {
        return { red: false, yellow: true, green: false };
    }
}