luhn.c

luhn.c

#include <stdio.h>
#include <ctype.h>

int checksum(char *str) {
    int cs = 0;
    int length = 0;
    char *c;
    
    /* get sequence length and validate chars in it */
    for(c=str; *c; c++, length++) {
        if(!isdigit(*c)) {
            return -1;
        }
    }

    /* go back one character to ignore the null terminator.
     * c is now equal to the last digit in the sequence. */
    c--;
    for(int i=0; i < length; i++, c--) {
        int v = *c - '0';

        if((i % 2) == 0) {
            cs += v;
        } else {
            v *= 2;
            /* the sum of the digits of a number, n,  between 10 and 19
             * is the same as n-9 */
            if(v > 10) {
                v -= 9;
            }
            cs += v;
        }
    }
    return cs;
}

int is_valid_checksum(int checksum) {
    return (checksum % 10) == 0;
}

int main(int argc, char *argv[]) {
    int cs = checksum("378282246310005");
    int valid = is_valid_checksum(cs);
    printf("cs=%d, valid=%s\n", cs, valid ? "true" : "false");
    return 0;
}