Phone number formatter

PhoneNumberFormattable.swift

import Cocoa

extension String {
    func charAt(at: Int) -> Character {
        let charIndex = index(startIndex, offsetBy: at)
        return self[charIndex]
    }
}

protocol PhoneNumberFormattable {
    
    /// Код страны. Например: "7", "1" или "373"
    var countryCode: String { get }
    
    /// Кол-во цифр в номере телефона, исключая код страны
    var numberCount: Int { get }
    
    /// Маска для форматирования номера телефона исключая код страны. Например "### ###-##-##"
    var mask: String { get }
    
    /// Форматирует номер телефона
    func format(rawNumber: String) -> String?
}

struct RussiaPhoneFormatter: PhoneNumberFormattable {
    let countryCode = "7"
    var numberCount: Int = 10
    let mask: String = "### ###-##-##"
}

struct BelarusPhoneFormatter: PhoneNumberFormattable {
    let countryCode = "375"
    var numberCount: Int = 9
    let mask: String = "## ###-##-##"
}

struct MoldovaPhoneFormatter: PhoneNumberFormattable {
    let countryCode = "373"
    var numberCount: Int = 8
    let mask: String = "# ###-##-##"
}

struct KyrgyzstanPhoneFormatter: PhoneNumberFormattable {
    let countryCode = "996"
    var numberCount: Int = 9
    let mask: String = "## ###-##-##"
}

extension PhoneNumberFormattable {
    
    var countryCodeCount: Int { countryCode.count }
    
    var totalCount: Int { numberCount + countryCodeCount }
    
    func format(rawNumber: String) -> String? {
        let rawNumber = rawNumber.filter { $0.isNumber }
        guard rawNumber.count == totalCount else { return nil }
        guard rawNumber.prefix(countryCodeCount) == countryCode else { return nil }
        
        // numbers without country code
        let numbers = String(rawNumber.dropFirst(countryCodeCount))
        guard numbers.count == numberCount else { return nil }
        
        var index = 0
        var result = ""
        mask.forEach { element in
            if element == "#" {
                result.append(numbers.charAt(at: index))
                index += 1
            } else {
                result.append(element)
            }
        }
        
        return "+\(countryCode) \(result)"
    }
}

func formatPhoneNumber(_ string: String) -> String? {
    let formatters: [PhoneNumberFormattable] = [
        RussiaPhoneFormatter(),
        BelarusPhoneFormatter(),
        MoldovaPhoneFormatter(),
        KyrgyzstanPhoneFormatter()
    ]
    
    for formatter in formatters {
        if let phone = formatter.format(rawNumber: string) {
            return phone
        }
    }
    
    return nil
}
    
print(formatPhoneNumber("71234567890") == "+7 123 456-78-90") // true
print(formatPhoneNumber("+996 12 345-67-89") == "+996 12 345-67-89") // true
print(formatPhoneNumber("+996123456789") == "+996 12 345-67-89") // true
print(formatPhoneNumber("37399999999") == "+373 9 999-99-99") // true