Generate EAN13 check digit in Swift

EAN13CheckDigitHelper.swift

//
//  EAN13CheckDigitHelper.swift
//

import Foundation

public struct EAN13CheckDigitHelper {
    public enum EAN13Error: Error {
        case mustOnlyContainDigits
        case mustBe12DigitsLong
    }

    /// Generates a check digit from a partial EAN13.
    ///
    /// - Parameter barcode: a 12-digits EAN13 barcode without the check digit
    /// - Returns: the check digit of the partial EAN13 barcode
    public static func getCheckDigit(from barcode: String) throws -> Int {
        guard CharacterSet.decimalDigits.isSuperset(of: CharacterSet(charactersIn: barcode)) else {
            throw EAN13Error.mustOnlyContainDigits
        }

        guard barcode.count == 12 else {
            throw EAN13Error.mustBe12DigitsLong
        }

        let sum = barcode
            .compactMap({ Int(String($0)) })
            .enumerated()
            .map({ (index, digit) in
                digit * ((index % 2 == 0) ? 1 : 3)
            })
            .reduce(0, +)

        return (sum % 10 == 0) ? 0 : (10 - sum % 10)
    }
    
    /// Get the whole EAN13 barcode string from a partial EAN13.
    ///
    /// - Parameter barcode: a 12-digits EAN13 barcode without the check digit
    /// - Returns: the barcode as a string with the check digit at its end
    public static func getEAN13(from barcode: String) throws -> String {
        barcode + String(try Self.getCheckDigit(from: barcode))
    }
}