[Simple phone number formatter] Phone number formatter based on a pattern

SimplePhoneFormatter.swift

func formatPhone(phone: String) -> String {
    let cleanPhoneNumber = phone.components(separatedBy: CharacterSet.decimalDigits.inverted).joined()
    let mask = "(XXX) XXX-XXXX"

    var result = ""
    var index = cleanPhoneNumber.startIndex
    for ch in mask where index < cleanPhoneNumber.endIndex {
        if ch == "X" {
            result.append(cleanPhoneNumber[index])
            index = cleanPhoneNumber.index(after: index)
        } else {
            result.append(ch)
        }
    }
    return result
}