Swiftly calculates a check-digit for GTIN-8, GTIN-12, GTIN-13, GTIN-14 and SSCC codes.

GTINCalculator.swift

import Foundation

@objc class GTINCalculator: NSObject {
    
    /*!
    An exception thrown when there is a calculation error
    
    - BadDigit: a substring of the input string could not be read as an `Int`
    */
    enum GTINCalculatingError: ErrorType {
        case BadDigit(stringValue: String)
    }
    
    /*!
    Calculate a check digit for a GTIN string
    
    - parameter gtin: the original number - a string of 7, 11, 12 or 13 digits for GTIN-8, GTIN-12, GTIN-13 and GTIN-14 respectively. Will also work for SSCC (17 digits)
    
    - throws: GTINCalculatingError
    
    - returns: a string with one more character than `$0`
    */
    class func calculateCheckDigitForGTIN(gtin: String) throws -> String {
        
        let mod = gtin.characters.count % 2 == 0
        let a = mod ? 1 : 3
        let b = mod ? 3 : 1
        
        do {
            
            let calculated = try gtin.characters.map {String($0)}
                .map(verifyInt)
                .map {Int($0)!}
                .enumerate()
                .map {($0 % 2 == 0 ? a : b) * $1}
                .reduce(0, combine:+)
        
            let checkDigit = Int(ceil(Double(calculated) / 10.0) * 10)
            
            return "\(gtin)\(checkDigit - calculated)"
        }
        catch let e as GTINCalculatingError {
            throw e
        }
    }
    
    /*!
    Verifies that a string can be converted to an `Int`
    
    - parameter c: the string to check
    
    - throws: GTINCalculatingError
    
    - returns: the input string `c`
    */
    private class func verifyInt(c: String) throws -> String {
        
        guard let _ = Int(c) else {
            throw GTINCalculatingError.BadDigit(stringValue: c)
        }
        
        return c
    }
    
    class func test() {
        
        //SSCC
        assert(try! calculateCheckDigitForGTIN("14253165132135354") == "142531651321353546")
        
        //GTIN-14
        assert(try! calculateCheckDigitForGTIN("1456541542542") == "14565415425426")
        
        //GTIN-13
        assert(try! calculateCheckDigitForGTIN("629104150021") == "6291041500213")
        
        //GTIN-12
        assert(try! calculateCheckDigitForGTIN("48564186514") == "485641865148")
        
        //GTIN-8
        assert(try! calculateCheckDigitForGTIN("9638507") == "96385074")
    }
}