Oracle PL/SQL: Convert ISBN10 to ISBN13 (EAN)

Wrote this because of vendors that still are using ISBN10. Not as optimized as it could be... But should get the job done.

create or replace
FUNCTION "ISBN10TO13" (
  ISBN10 IN VARCHAR2
  ) RETURN VARCHAR2 IS

  v_isbntemp VARCHAR2(13) := '';
  v_isbn13 VARCHAR2(26) := '';
  v_checksum NUMBER := 0;
  v_digit NUMBER := 0; 
  v_pos NUMBER := 1;  

  BEGIN
    v_isbntemp := '978' || substr(ISBN10,0,9);
    
    WHILE (v_pos <= 12) LOOP
      v_digit := substr(v_isbntemp,(v_pos),1);
      IF (MOD(v_pos,2) = 0) 
        THEN v_checksum := v_checksum + (v_digit * 3);
        ELSE v_checksum := v_checksum + (v_digit * 1);
      END IF;
      v_pos := v_pos + 1;
    END LOOP;
    
    v_checksum := 10 - MOD(v_checksum,10);
    IF (v_checksum = 10) 
      THEN v_checksum := 0;
    END IF;
    
    v_isbn13 := v_isbntemp || v_checksum;
    
  RETURN v_isbn13;
END ISBN10TO13;