A small go program calculating the UMCN (Unique Master Citizen Number) checksum for the numbers of former Yugoslav republics.

umcn_checksum.go

/*
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 * Licensed under the GNU General Public License, Version 3.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at:
 *
 *     https://www.gnu.org/licenses/gpl-3.0.txt
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package main

import (
    "fmt"
    "strings"
    "strconv"
    "os"
)

const (
    EXIT_FAILURE = 1
    UMCN_LENGTH = 13
)

type UMCN struct {
    digits []int
}

func New(number string) (UMCN, error) {
    umcn := UMCN{digits: make([]int, 0, UMCN_LENGTH)}
    digits := strings.Split(number, "")

    for _, digit := range digits {
        digit, err := strconv.ParseInt(digit, 10, 0)

        if err != nil {
            return umcn, err
        }

        umcn.digits = append(umcn.digits, int(digit))
    }

    return umcn, nil
}

func (umcn *UMCN) GetLength() int {
    return len(umcn.digits)
}

func (umcn *UMCN) CalculateChecksum() int {
    checksum := 0

    // Let UMCN DD MM YYY RR BBB C = AB CD EFG HI JKL M
    // The checksum M is calculated by the following formula:
    // 11 - ((7(A+G) + 6(B+H) + 5(C+I) + 4(D+J) + 3(D+K) + 2(E+L)) mod 11)
    for i, j := 0, 7; i < 6; i, j = i + 1, j - 1 {
        checksum += j * (umcn.digits[i] + umcn.digits[i + 6])
    }

    checksum = 11 - (checksum % 11)

    // The cheksum is a single digit value.
    // Sometimes the formula produces either 10 or 11.
    // This is the rule for those cases.
    if (checksum == 10 || checksum == 11) {
        return 0
    }

    return checksum
}

func main() {
    var number string

    fmt.Println("Please enter your UMCN:")

    if _, err := fmt.Scan(&number); err != nil {
        fmt.Fprint(os.Stderr, "Unable to read input.")

        os.Exit(EXIT_FAILURE)
    }

    umcn, err := New(number)

    if err != nil {
        fmt.Fprint(os.Stderr, "Unable to convert digits to integers.")
        
        os.Exit(EXIT_FAILURE)
    }

    if umcn.GetLength() != 13 {
        fmt.Fprint(os.Stderr, "Number must be exactly 13 digits long.")

        os.Exit(EXIT_FAILURE)
    }

    fmt.Printf("Checksum: %d\n", umcn.CalculateChecksum())
}