Challenge 145 - UPC check digits

challenge_145.md

Challenge 145 - UPC check digits

The Universal Product Code (UPC-A) is a bar code used in many parts of the world. The bars encode a 12-digit number used to identify a product for sale, for example:

042100005264

The 12th digit (4 in this case) is a redundant check digit, used to catch errors. Using some simple calculations, a scanner can determine, given the first 11 digits, what the check digit must be for a valid code. UPC's check digit is calculated as follows (taken from Wikipedia):

• STEP 1: Sum the digits at odd-numbered positions (1st, 3rd, 5th, ..., 11th). If you use 0-based indexing, this is the even-numbered positions (0th, 2nd, 4th, ... 10th).
• STEP 2: Multiply the result from step 1 by 3.
• STEP 3: Take the sum of digits at even-numbered positions (2nd, 4th, 6th, ..., 10th) in the original number, and add this sum to the result from step 2.
• STEP 4: Find the result from step 3 modulo 10 (i.e. the remainder, when divided by 10) and call it M.
• STEP 5: If M is 0, then the check digit is 0; otherwise the check digit is 10 - M.

For example, given the first 11 digits of a UPC 03600029145, you can compute the check digit like this:

Sum the odd-numbered digits (0 + 6 + 0 + 2 + 1 + 5 = 14).

Multiply the result by 3 (14 × 3 = 42).

Add the even-numbered digits (42 + (3 + 0 + 0 + 9 + 4) = 58).

Find the result modulo 10 (58 divided by 10 is 5 remainder 8, so M = 8).

If M is not 0, subtract M from 10 to get the check digit (10 - M = 10 - 8 = 2).

So the check digit is 2, and the complete UPC is 036000291452.

Challenge

Given an 11-digit number, find the 12th digit that would make a valid UPC. You may treat the input as a string if you prefer, whatever is more convenient. If you treat it as a number, you may need to consider the case of leading 0's to get up to 11 digits. That is, an input of 12345 would correspond to a UPC start of 00000012345.

Examples

upc(4210000526) => 4
upc(3600029145) => 2
upc(12345678910) => 4
upc(1234567) => 0

Also, if you live in a country that uses UPCs, you can generate all the examples you want by picking up store-bought items or packages around your house. Find anything with a bar code on it: if it has 12 digits, it's probably a UPC. Enter the first 11 digits into your program and see if you get the 12th.

challenge_tests.py

import unittest


def get_check_digit(upc: int) -> int:
    return 0  # Put your code here!!!


class Test(unittest.TestCase):
    def test_1(self):
        self.assertEqual(4, get_check_digit(4210000526))

    def test_2(self):
        self.assertEqual(2, get_check_digit(3600029145))

    def test_3(self):
        self.assertEqual(4, get_check_digit(12345678910))

    def test_4(self):
        self.assertEqual(0, get_check_digit(1234567))


if __name__ == "__main__":
    unittest.main()

credits.md

Found on /r/dailyprogrammer: [2018-12-17] Challenge #370 [Easy] UPC check digits