Java 8 streams : How to surface the comparator winning rule to the domain object

java8streamComparatorRulePick.kt

    /**
     * We have a list of cars identified by Make,
     * other attributes are name, rating, mileage
     * The idea is to pick 1 car from each make by applying the below rules in sequence
     *
     * Rule 1 : if there is only one car per make: then mark it for Pick: pick = 'onlyOneItem'
     * Rule 2 : if not Rule 1 then Pick the car with better rating: pick = 'higerRating'
     * Rule 3 : if not Rule 2 then Pick the car with least mileage: pick= 'lowerMileage'
     *
     * acceptence criteria: Rules are to be applied in order and if one rule applies then the rest can be skipped
     */
    val _none = "none"
    val _onlyOneItem = "onlyOneItem";
    val _higherRating = "higherRating";
    val _lowerMiles = "lowerMiles"

    data class Car(val make: String, val name: String, val rating: Int, val mileage: Int, var reason4pick: String = "none", val expectedReason: String = "none"){
        fun negMileage() = mileage * -1
    }

    val cars = listOf<Car>(
            Car("Honda", "AnaKat's new SUV", 5, 2000, expectedReason = _higherRating)
            , Car("Honda", "AnaKat's sedan", 3, 3000)
            // ------------// ------------// ------------// ------------// ------------// ------------
            , Car("Toyota", "AnaKat's HighLander", 5, 1000, expectedReason = _lowerMiles)
            , Car("Toyota", "AnaKat's 4 runner", 5, 2000)
            , Car("Toyota", "AnaKat's Camry", 3, 100)
            // ------------// ------------// ------------// ------------// ------------// ------------
            , Car("Mazda", "Ravi's SUV", 1, 7000, expectedReason = _onlyOneItem)
            // ------------// ------------// ------------// ------------// ------------// ------------
            , Car("Nissan", "Kondal's Nissan Van", 5, 2000, expectedReason = _onlyOneItem)
            // ------------// ------------// ------------// ------------// ------------// ------------
            , Car("LandRover", "Jims old LandRV", 4, 1000)
            , Car("LandRover", "Jims crusr", 5, 1000, expectedReason = _higherRating)
            // ------------// ------------// ------------// ------------// ------------// ------------
    )

    @Test
    fun strmGroupByMarkPrimary() {

        fun applyRules(vl: List<Car>): List<Car> {
            if (vl.size == 1) {vl.get(0).reason4pick = _onlyOneItem; return vl} // ----> marker 1

            val maxRatingOrLowerMileageCar = vl.stream()
                    .max ( Comparator.comparingInt(Car::rating)
                            .thenComparingInt(Car::negMileage)).get()

            // How to update the reason for pick just like we it 5 lines above (marker 1) ???
            return listOf(maxRatingOrLowerMileageCar)
        }

        val grpdBy: MutableMap<String, MutableList<Car>> = cars.stream().collect(Collectors.groupingBy({ e -> e.make }, Collectors.toList<Car>()))
        val f1 = grpdBy.flatMap { (k, vl) -> applyRules(vl) }
        f1.forEach { assertEquals(it.reason4pick, it.expectedReason) }

    }