Writing a PK2 Editor from scratch

[sjkdfnasjdfh]

Hi all,
I'm trying to write a pk2 editor from scratch, This is how i do it,

First I'll get the corresponding entry of the file to be edited,
then Append the file content at the end of the pk2 file,
get the new size and position,
update the entry.

This kinda works but there is a weird issue i'm facing.
when i update a file, the changes wont show up.
I tried updating 2 files and only the changes in first file applied!(and the ones from before!!)

so after so hopeless retries i figured i should add some garbage at the end of the pk2 file and for some reason it worked and for some other reason the number of those bytes is not constant.

I can't figure this out,
do i really need to append those redundant bytes at the end of the pk2 file?
if yes then what size?

Thanks