fedex_check.swift

//
// Note: This is coded for an XCode Swift playground
//

import Foundation

// 
//    Using the last 15 characters of a FedEx ground "96" barcode, determine the check digit.
// 
//    From page 38 of
//    http:images.fedex.com/us/solutions/ppe/FedEx_Ground_Label_Layout_Specification.pdf
// 
//                              SHIPPER ID         PACKAGE NUMBER   CD
//                         -------------------  ------------------- --
//                         1  1  1  1  1  1
//    Position             5  4  3  2  1  0  9  8  7  6  5  4  3  2  1
//    Odd/Even             O  E  O  E  O  E  O  E  O  E  O  E  O  E  O
//                         -------------------------------------------
//    DATA                 9  8  7  6  5  4  3  1  2  3  4  5  6  7  2
//
//    * Add the even positions starting with pos 2 from the right
//
//        7 + 5 + 3 + 1 + 4 + 6 + 8 = 34
//
//    * Multiply by 3
//
//        34 * 3 = 102
//
//    * Add the odd positions starting with pos 3 from the right
//
//        6 + 4 + 2 + 3 + 5 + 7 + 9 = 36
//
//    * Add two sums together
//
//        102 + 36 = 138
//
//    * Subtract from next highest multiple of 10
//
//        140 - 138 = 2
//
//    * The check digit is 2
// 

func fedex_check_digit(str: String) -> String? {
    var sum = 0
    let reversedCharacters = str.characters.reverse().map { String($0) }
    for (idx, element) in reversedCharacters.enumerate() {
        guard let digit = Int(element) else { return nil }
        switch ((idx % 2 == 1)) {
        case true:  sum += (idx > 0) ? digit * 3 : 0
        case false: sum += (idx > 1) ? digit : 0
        }
    }
    let x = Int(ceil(Double(sum)/10)*10)
    return String(x - sum)
}

// The actual code is 9611020 9876543 12345672

let tracking_number = "987654312345672"

print("Check digit of '\(tracking_number)' is " + fedex_check_digit(tracking_number)!)