CREATE OR REPLACE FUNCTION gen_barcode36 (i_coupon_number NUMBER) 
RETURN VARCHAR2 IS
     v_number    number;
     v_curval    number;
     v_curinc    number := power(36,14);
     v_true      number := 0 ;
     v_pos       number := 15;
     v_dec_count number := 0 ;
     v_pos_val   number := 0 ;
     v_cur_num   number := 0 ;
     v_test_num  number := 0 ;
     v_new_num   number := 0 ;
     v_cur_digit char := '';
     v_new_val   varchar2(15) := '';
   BEGIN
     v_number := i_coupon_number;
     while ( v_true = 0 ) loop
         v_curinc := v_curinc * 36;
         v_curval := v_number / v_curinc;
         if ( v_curval < 1 ) then
            v_true := 1;
         else
            v_pos := v_pos + 1;
         end if;
     end loop;
     v_dec_count := v_pos;
     v_new_val := NULL;
     v_cur_num := v_number;
     WHILE ( v_dec_count > 0) LOOP
        v_pos_val := power(36,v_dec_count - 1);
        v_test_num := trunc(v_cur_num/v_pos_val);
        select decode(v_test_num,35,'z', 34,'y', 33,'x',
                                 32,'w', 31,'v', 30,'u',
                                 29,'t', 28,'s', 27,'r',
                                 26,'q', 25,'p', 24,'o',
                                 23,'n', 22,'m', 21,'l',
                                 20,'k', 19,'j', 18,'i',
                                 17,'h', 16,'g', 15,'f',
                                 14,'e', 13,'d', 12,'c',
                                 11,'b', 10,'a',  9,'9',
                                  8,'8',  7,'7',  6,'6',
                                  5,'5',  4,'4',  3,'3',
                                  2,'2',  1,'1',    '0')
               INTO v_cur_digit FROM DUAL;
        IF ( v_new_val IS NOT NULL ) THEN
            v_new_val := v_new_val || v_cur_digit;
        ELSE
            v_new_val := v_cur_digit;
        END IF;
        v_cur_num := v_cur_num - (v_pos_val * v_test_num);
        v_dec_count := v_dec_count - 1;
     END LOOP;
     RETURN v_new_val;
   END gen_barcode36;
/

SQL> select gen_barcode36(7700000000000000000000) from dual;
GEN_BARCODE36(7700000000000000000000)
-----------------------------------------------------
1950zn8fqxjygow
Starting with 7700000000000000000000
Position: 6140942214464815497216 current number: 1
Position 14 current value: 1
Position: 170581728179578208256 current number: 9
Position 13 current value: 19
Position: 4738381338321616896 current number: 5
Position 12 current value: 195
Position: 131621703842267136 current number: 0
Position 11 current value: 1950
Position: 3656158440062976 current number: 35
Position 10 current value: 1950z
Position: 101559956668416 current number: 23
Position 9 current value: 1950zn
Position: 2821109907456 current number: 8
Position 8 current value: 1950zn8
Position: 78364164096 current number: 15
Position 7 current value: 1950zn8f
Position: 2176782336 current number: 26
Position 6 current value: 1950zn8fq
Position: 60466176 current number: 33
Position 5 current value: 1950zn8fqx
Position: 1679616 current number: 19
Position 4 current value: 1950zn8fqxj
Position: 46656 current number: 34
Position 3 current value: 1950zn8fqxjy
Position: 1296 current number: 16
Position 2 current value: 1950zn8fqxjyg
Position: 36 current number: 24
Position 1 current value: 1950zn8fqxjygo
Position: 1 current number: 32
Position 0 current value: 1950zn8fqxjygow
Final value: 1950zn8fqxjygow