Optimistic locking and HTML DB checksum calculation

Question

I create a form for fetching and updating row from a database table. I use Automated Row Fetch for fetching row. But for updating row I must use a procedure from a package, not Automatic Row Processing (DML).
And my question is: how I can implement optimistic locking in this way?
Automated Row Fetch already calculated checksum and stored it in p_md5_checksum hidden field, can I get calculated value of checksum from p_md5_checksum?
How I can calculate checksum by myself for compare with value from p_md5_checksum?
What algorithm uses HTML DB for calculate checksum in Automated Row Fetch and Automatic Row Processing (DML) components?

Answer 1

I have read this topic already. It's recommended to build own functionality for calculate checksum, but Automated Row Fetch already calculated checksum and stored it in p_md5_checksum hidden field.
I also created my own function, for example for table dept:
create or replace function BUILD_DEPT_MD5 (
P_DEPTNO in number,
P_DNAME in varchar2 default null,
P_LOC in varchar2 default null,
P_COL_SEP in varchar2 default '|') return varchar2 is
begin
return utl_raw.cast_to_raw(dbms_obfuscation_toolkit.md5(input_string => P_DEPTNO || P_COL_SEP || P_DNAME || P_COL_SEP || P_LOC));
end BUILD_DEPT_MD5;
but checksum values what calculated with this function is not equal with checksum value calculated by Automated Row Fetch.
I shall specify:
I need to use Automated Row Fetch for get row from table.
I can not use Automatic Row Processing (DML) for update row, I need use PL/SQL procedure for this.
I want implement optimistic locking and I want use for this a checksum value, what already calculated by Automated Row Fetch. I need to know how I can get this checksum value and I need to know algorithm, which Automated Row Fetch uses for checksum calculation.