create or replace function get_luhn_16_check_digit  
        ( p_str in varchar) 
        return pls_integer  
    is 
        b16 sys.dbms_debug_vc2coll := sys.dbms_debug_vc2coll() ; 
        n16 sys.odcinumberlist := sys.odcinumberlist(); 
        tot simple_integer := 0; 
        chk_digit pls_integer; 
    begin 
        -- step 1) 
        select to_number(tkn, 'X') 
        bulk collect into n16 
        from ( select substr(p_str, level, 1) as tkn 
                     from dual 
                     connect by level <= length(p_str)); 
        b16.extend(n16.count()); 
     
        for idx in 1..n16.count() loop 
            if mod(idx, 2) = 0 
            then  
                -- step 2) and 3) 
                b16(idx) := to_char( to_char(n16(idx)) * 2, 'fmXX'); 
                -- step 4) 
                if length( b16(idx)) = 2 then 
                     b16(idx) := to_number(substr(b16(idx),1,1)) + to_number(substr(b16(idx),2,1)); 
                end if;   
            else  
                b16(idx) := trim(to_char(n16(idx))); 
            end if; 
        end loop; 
     
        -- step 5) and 6) [except for mysterious conversion see my comment] 
        for idx in 1..b16.count() loop 
            if  b16(idx) not in ('A','B','C','D','E','F') then 
                tot := tot + to_number(b16(idx)); 
            else 
                tot := tot + n16(idx); 
            end if; 
        end loop; 
         
        -- step 7) and 8) 
        chk_digit := (ceil(tot/16)*16) - tot;  
        return chk_digit; 
    end; 

select get_luhn_16_check_digit('0B012722900021AC35B2') from dual

select get_luhn_16_check_digit('22111111111111111111') from dual