Kotlin NIF validator - validador de número de contribuinte

NifValidator.kt

object NifValidator {
    
    private val validationSet1 = listOf("1", "2", "3", "5", "6", "8")
    private val validationSet2 = listOf("45", "70", "71", "72", "74", "75", "77", "79", "90", "91", "98", "99")
    
    fun isValid(
        nif: String
    ): Boolean {

        if (nif.length != 9) {
            return false
        }

        if (!validationSet1.contains(nif.substring(0, 1)) && !validationSet2.contains(nif.substring(0, 2))) {
            return false
        }

        val nifNumbers = nif.map{ it.toString().toInt() }
        val total =
            nifNumbers[0] * 9 +
            nifNumbers[1] * 8 +
            nifNumbers[2] * 7 +
            nifNumbers[3] * 6 +
            nifNumbers[4] * 5 +
            nifNumbers[5] * 4 +
            nifNumbers[6] * 3 +
            nifNumbers[7] * 2;

        val module11 = (total % 11)
        val checkDigit = if(module11 < 2) 0 else 11 - module11
        return checkDigit == nifNumbers[8]
    }
}