UPC checkdigit calculation as a Postgresql C extension function

upc_checkdigit.txt

~/src/postgresql-12.2/src/tutorial# cat 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_checksum);

Datum
upc_checksum(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);
}

Putting the function into a path that is accessible (I know /tmp is a bad choice, but it worked)

% cat upc_checksum.sh
cc -fPIC -c upc_checksum.c -I/usr/local/pgsql/include/server/
cc -shared -o upc_checksum.so upc_checksum.o
cp upc_checksum.* /tmp/

Installing the function:

% psql postgres
[local] root@streetpulse=# drop FUNCTION upc_checksum;
DROP FUNCTION
Time: 4.181 ms
[local] root@streetpulse=# CREATE FUNCTION upc_checksum(text) RETURNS integer
     AS '/tmp/upc_checksum.so', 'upc_checksum'
     LANGUAGE C STRICT;
CREATE FUNCTION
Time: 5.101 ms
[local] root@streetpulse=# DO $proc$
DECLARE
  StartTime timestamptz;
  EndTime timestamptz;
  Delta double precision;
  rc int;
BEGIN
  StartTime := clock_timestamp();
  SELECT count(*) into rc
  from tbl_superfile
  where right(upc14,1)<>upc_checksum(upc_number||'')::text;
  EndTime := clock_timestamp();
  Delta := ( extract(epoch from EndTime) - extract(epoch from StartTime) );
  RAISE NOTICE 'Duration in secs=%, rc=%', Delta, rc;
END;
$proc$;
NOTICE:  00000: Duration in secs=3.3357880115509033, rc=6037
LOCATION:  exec_stmt_raise, pl_exec.c:3827
DO
Time: 3339.333 ms (00:03.339)

Ref: https://www.postgresql.org/docs/13/xfunc-c.html#XFUNC-C-DYNLOAD