randomValidCpf.swift

import Foundation

/// 9 radom numbers followed by two verifying digits
var randomValidCpf:  String {
    var cpf = (1...11).map { _ in Int(arc4random_uniform(9)) }
    
    cpf[9] = calcVerifyingDigit(originalValue: cpf, weights: (2...10).map{$0}.reversed())
    
    cpf[10] = calcVerifyingDigit(originalValue: cpf, weights: (2...11).map{$0}.reversed())
    
    return (cpf.map { "\($0)" }).joined(separator:"")
}

/// 1 - Multiply each weight from weights array by a value from originalValue array with the same index
/// 2 - Sums up all the results from previous step
/// 3 - Remainder from the sum divided by 11
/// 4 - If Remainder is less than 2 then digit will be 0, else digit will be (11 - Remainder)
private func calcVerifyingDigit(originalValue: [Int], weights: [Int]) -> Int {
    let valuesWeighted = weights.enumerated().map { i, w in  originalValue[i] * w }
    
    let weightedSum = valuesWeighted.reduce(0, +)
    
    let rest = weightedSum % 11
    
    return (rest < 2) ? 0 : (11 - rest)
}