ICAO 9303 Checksum

checksum.c

int checksum(const char *str)
{
    int weight[] = {7, 3, 1};
    int checksum = 0;

    int len = strlen(str);

    for (int i = 0; i < len; ++i) {
        int mappedNum = 0;
        if (isdigit(str[i])) {
            mappedNum = str[i] - '0';
        } else if (isupper(str[i])) {
            mappedNum = str[i] - 'A' + 10;
        } else if (str[i] == '<') {
            mappedNum = 0;
        }

        checksum += weight[i % 3] * mappedNum;
    }

    checksum = checksum % 10;
    return checksum;
}