1 - Create a extensions for String with a new, wonderfull, superpower method called crop:
extension String {
func crop(from: Int, length: Int) -> String! {
let startIndex = self.index(self.startIndex, offsetBy: from)
var result = self.substring(from: startIndex)
let endIndex = result.index(result.startIndex, offsetBy: length)
result = result.substring(to: endIndex)