Validate CPF and Create CPF digits

CPF.swift

//
//  CPF.swift
//
//  Created by dede.exe on 30/12/17.
//  Copyright © 2017 dede.exe. All rights reserved.
//

import Foundation

///
/// Create an CPF Object
///
public struct CPF {
    
    private (set) public var number : String = ""
    
    /// Create a new CPF entity with number
    public init(number:String) {
        self.number = number
    }
    
    /// Get CPF digits verification
    var digits : String {
        return calculateCPFDigits() ?? ""
    }
    
    /// Check if it is a valida CPF
    var isValid : Bool {
        if number.count != 11 { return false }
        if isAnException { return false }
        return self.number.hasSuffix(digits)
    }
    
    // MARK: - Helpers
    private var isAnException : Bool {
        for n in 0...9 {
            let number = String(repeating: String(n), count: 11)
            if self.number == number { return true }
        }
        
        return false
    }
    
    private func calculateCPFDigits() -> String? {
        var result : String = ""
        
        guard canCalculate() else { return nil }
        
        let startIndex = self.number.startIndex
        let endIndex = self.number.index(startIndex, offsetBy: 9)
        let number = String(self.number[startIndex..<endIndex])
        
        result = calculateDigit(for: number)
        result = result + calculateDigit(for: number + result)
        return result
    }
    
    private func calculateDigit(for number:String) -> String {
        let firstDigitSize = 9
        let secondDigitSize = 10
        let numberSize = number.count
        var values : [Int] = []
        
        if numberSize < firstDigitSize || numberSize > secondDigitSize {
            return ""
        }

        let factor = (numberSize == firstDigitSize) ? firstDigitSize + 1 : secondDigitSize + 1
        
        for (index,char) in number.enumerated() {
            let v = (Int(String(char)) ?? 0) * (factor - index)
            values.append(v)
        }
        
        var result = values.reduce(0) { $0 + $1 }
        result = result * 10 % 11
        result = (result < 10) ? result : 0
        
        return String(result)
    }
    
    private func canCalculate() -> Bool {
        return number.count >= 9
    }
}


//Usage
let cpf1 = CPF(number: "123123123").digits               // "87"
let cpf2 = CPF(number: "12312312387").isValid           // true
let cpf3 = CPF(number: "12312312387").isValid           // true