CPFValidator.kt

fun main(args: Array<String>) {
    if(isValid(args[0])) {
       println("Thanks, the CPF informed is valid")
    } else {
        println("Please, inform a valid CPF.")
    }
}

fun isValid(cpf: String): Boolean {
    val formattedCPF = cpf.replace(".", "").replace("-", "")

    if(basicValidation(formattedCPF)) return false

    val nineDigits = formattedCPF.substring(0,9)
    val digitOne = calcDigit(nineDigits, 10)
    val digitTwo = calcDigit(nineDigits.plus(digitOne), 11)

    return cpf == nineDigits.plus(digitOne).plus(digitTwo)
}

private fun basicValidation(cpf: String) = cpf.length != 11 || !cpf.isNumeric() || cpf.allCharacterIsEquals()

private fun calcDigit(numbers: String, weight: Int): Int {
    var sum = 0

    for(i in 0..(numbers.length-1)) {
        sum += numbers[i].toString().toInt() * (weight-i)
    }

    sum = 11 - sum % 11
    return if (sum > 9) 0 else sum
}

private fun String.allCharacterIsEquals(): Boolean {
    for(char in this) {
        if(char != this[0]) {
            return false
        }
    }

    return true
}

private fun String.isNumeric(): Boolean {
    if (this.isEmpty()) {
        return false
    }

    for (char in this) {
        if (!char.isDigit()) return false
    }

    return true
}