How to format a credit card number with an (NS)Formatter

CreditCardFormatter.swift

class CreditCardFormatter: Formatter {
	override func string(for obj: Any?) -> String? {
		guard let input = obj as? String else {
			return nil
		}

		return formattedCardNumber(input: input)
	}

	override func getObjectValue(_ obj: AutoreleasingUnsafeMutablePointer<AnyObject?>?, for string: String, errorDescription error: AutoreleasingUnsafeMutablePointer<NSString?>?) -> Bool {
		obj?.pointee = string as AnyObject?
		return true
	}

	private func formattedCardNumber(input: String?) -> String? {
		guard let input = input else { return nil }
		let stripped = input.replacingOccurrences(of: " ", with: "")
		// Guess which mask we need to use based on what the user has
		// typed so far, without any spacing or formatting.
		let mask = guessFormattingMask(for: stripped)

		var result = ""
		var index = stripped.startIndex

		// Format the stripped input based on the guessed mask
		for mchar in mask where index < stripped.endIndex {
			if mchar == "#" {
				result.append(stripped[index])
				index = stripped.index(after: index)
			} else {
				if stripped[index] != mchar {
					result.append(mchar)
				} else {
					result.append(stripped[index])
					index = stripped.index(after: index)
				}
			}
		}

		//		print("Result: \(result)")
		//		print("Mask: \(mask)")
		//		print("input: \(input.count), result: \(result.count), mask: \(mask.count), stripped: \(stripped.count)")

		// If we exceeded the mask just append additional characters to the end
		// of the result
		if input.count > mask.count && result.endIndex < input.endIndex {
			result.append(String(input[result.endIndex..<input.endIndex]))
		}

		return result
	}

	func guessFormattingMask(for string: String) -> String {
		let defaultMask = "#### #### #### ####"

		switch string.first {
		case "3":
			if string.starts(with: "34") || string.starts(with: "37") {
				return "#### ###### #####"
			} else if string.starts(with: "36") {
				return "#### ##### ####"
			} else {
				return defaultMask
			}
		case "4": return string.count == 13 ? "#### ##### ####" : defaultMask
		default: return defaultMask
		}
	}
}