CNPJValidator.swift

import Foundation

extension Collection where Element == Int {
    var digitCNPJ: Int {
        let upperBound = 9
        let lowerBound = 2
        let mod = 11
        let secondMod = 10

        let factors = Array((lowerBound...upperBound).reversed())

        let multiplied = self.reversed().enumerated().map {
            return $0.element * factors[$0.offset % factors.count]
        }

        let sum = multiplied.reduce(0, +)

        return (sum % mod) % secondMod
    }
}

extension StringProtocol {
    var isValidCNPJ: Bool {
        let numbers = compactMap(\.wholeNumberValue)
        guard numbers.count == 14 && Set(numbers).count != 1 else { return false }
        let digits = Array(numbers[0..<12])
        let firstDigit = checkDigitCNPJ(for: digits, upperBound: 9, lowerBound: 2, mod: 11)
        let secondDigit = checkDigitCNPJ(for: digits + [firstDigit], upperBound: 9, lowerBound: 2, mod: 11)

        return firstDigit == numbers[12] && secondDigit == numbers[13]
    }

    var isValidCNPJ: Bool {
        let numbers = compactMap(\.wholeNumberValue)
        guard numbers.count == 14 && Set(numbers).count != 1 else { return false }

        return numbers.prefix(12).digitCNPJ == numbers[12] &&
               numbers.prefix(13).digitCNPJ == numbers[13]
    }
}

// Usage

let cnpj = "29.957.575/0001-89".isValidCNPJ // true
//let cnpj = "16.914.610/0001-30".isValidCNPJ // true
//let cnpj = "16914610000130".isValidCNPJ // true
//let cnpj = "66666666666666".isValidCNPJ // false