need help in udf logic

Hi XI / java Experts

every day we are getting some idoc xmls which included the idoc number in the payload. at the end of all individual idoc xmls a final idoc xml triggers which includes all the above Idoc numbers that are processed indivuadally. This is the routine process.

but, sometimes in this final Idoc xml there some additional idoc numbers were included, those were not processed individually.

My requirement is to send a mail to the sender telling that, In the final Idoc xml some additional idoc numbers are included for those additional idoc xml, individually are not received at my end.

for this i need to prepare a file to list all the idoc numbers which were received at my end and once the final idoc comes it should compare with the file which contains the individual idoc numbers in the file. if any mismatching in the number of idocs in the file, a error file should be generate telling that mismatch of idoc numbers.

Pls tell me how to write UDF(code) to achieve this requirement.

Thanks

Robin