/*
   Name:    upc2.c
   Purpose: Computes UPC check digit.
   Author:  Bill Slough

   Notes:   Loosely based on upc.c (King, Chapter 4, page 57)
*/

#include <stdio.h>
#include <stdbool.h>

#define NDIGITS 11         /* number of digits in a UPC code */
#define EVEN_WT 3          /* weight for digits in even-numbered positions */
#define ODD_WT 1           /* weight for digits in odd-numbered positions */

int checkDigit(long code);

int main(void) {
    long upcCode;          /* Assumes the long data type can hold 11-digits */
    
    printf("Enter the UPC code: ");
    scanf("%ld", &upcCode);

    printf("Check digit: %d\n", checkDigit(upcCode));
    return 0;
}

int checkDigit(long code) {
    /* computes the checksum of a UPC, given its 11-digit code
       code = d_10 d_9 d_8 ... d_2 d_1 d_0                     */
    
    int i;            /* loop index                  */
    bool evenIndex;   /* even or odd digit position? */
    int digit;        /* current digit to process    */
    int sum;          /* running weighted sum        */
    
    /* compute the weighted sum of the digits,
       processing the digits from right to left */
    evenIndex = true;
    sum = 0;
    for (i = 0; i < NDIGITS; i++) {
        /* get the least significant digit */
        digit = code % 10;

        /* process this digit */
        if (evenIndex)
            sum += EVEN_WT * digit;
        else
            sum += ODD_WT * digit;

        /* flip the index position */
        evenIndex = !evenIndex;

        /* right shift the code one digit */
        code /= 10;
    }

    /* return the check digit */
    return 9 - (sum - 1) % 10;
}