Validador de CNPJ usando Kotlin

cnpj.kt

data class CNPJ(val value: String)


fun CNPJ.isValid(): Boolean {
    val cnpj = value
    return validateCNPJLength(cnpj) && validateCNPJRepeatedNumbers(cnpj)
            && validateCNPJVerificationDigit(true, cnpj)
            && validateCNPJVerificationDigit(false, cnpj)
}

/**
 * Verifies if the CNPJ has 14 digits.
 *
 * @return True if valid.
 */
private fun validateCNPJLength(cnpj: String) = cnpj.length == 14

/**
 * Verifies if the CNPJ is not repeated numbers.
 *
 * A CNPJ with repeated is considered invalid, ex:
 *
 *   '00000000000000'
 *   '11111111111111'
 *   '22222222222222'
 *   ...
 *   '88888888888888'
 *   '99999999999999'
 *
 * @return True if valid.
 */
private fun validateCNPJRepeatedNumbers(cnpj: String): Boolean {
    return (0..9)
            .map { it.toString().repeat(14) }
            .map { cnpj == it }
            .all { !it }
}

/**
 * Verifies the CNPJ verification digit.
 *
 * This algorithm checks the verification digit (dígito verificador) do CNPJ.
 * This was based from: https://www.devmedia.com.br/validando-o-cnpj-em-uma-aplicacao-java/22374
 *
 * @param[firstDigit] True when checking the first digit. False to check the second digit.
 *
 * @return True if valid.
 */
private fun validateCNPJVerificationDigit(firstDigit: Boolean, cnpj: String): Boolean {
    val startPos = when (firstDigit) {
        true -> 11
        else -> 12
    }
    val weightOffset = when (firstDigit) {
        true -> 0
        false -> 1
    }
    val sum = (startPos downTo 0).fold(0) { acc, pos ->
        val weight = 2 + ((11 + weightOffset - pos) % 8)
        val num = cnpj[pos].toString().toInt()
        val sum = acc + (num * weight)
        sum
    }
    val result = sum % 11
    val expectedDigit = when (result) {
        0, 1 -> 0
        else -> 11 - result
    }

    val actualDigit = cnpj[startPos + 1].toString().toInt()

    return expectedDigit == actualDigit
}