upc_checksum.c

#include "postgres.h"
#include <string.h>
#include "fmgr.h"
#include "utils/geo_decls.h"

PG_MODULE_MAGIC;
/* by value */
PG_FUNCTION_INFO_V1(upc_checkdigit);

Datum
upc_checkdigit(PG_FUNCTION_ARGS)
{
    text  *upc = PG_GETARG_TEXT_PP(0);
    int sum = 0;
    int mult = 3;
    char *pstart = VARDATA_ANY(upc);
    char *p = pstart + VARSIZE_ANY_EXHDR(upc);
    while (--p >= pstart) {
        unsigned int digit = *p - '0';
        if (digit < 10) {
            sum += digit * mult;
            mult ^= 2;
        }
    }
    sum = (500 - sum) % 10;
    // sum = ( 10 - ( sum % 10 ) ) % 10;
    PG_RETURN_INT32(sum);
}

// so far untested
PG_FUNCTION_INFO_V1(upc_checksum);

Datum
upc_checksum(PG_FUNCTION_ARGS)
{
    text  *upc = PG_GETARG_TEXT_PP(0);
    int sum = 0;
    int mult = 1;
    char *pstart = VARDATA_ANY(upc);
    char *p = pstart + VARSIZE_ANY_EXHDR(upc);
    while (--p >= pstart) {
        unsigned int digit = *p - '0';
        if (digit < 10) {
            sum += digit * mult;
            mult ^= 2;
        }
    }
    sum %= 10;
    PG_RETURN_INT32(sum);
}