bbs bump ordered index

I'm making a BBS thing without using sql and I'm wondering what's the best way to save the bump order
I'm thinking of having a array-like index that each line has the thread-id but if someone bumps the last thread it has to move everything one line down and insert the last line into the first line
do you have any other way to save the index? how does shiichan/tablecat/non-sql forums/4chan do it?

>>12
*Random access in sorted order, I should have said.

I would either go with a tree or use an array and settle for the shuffle swap. Most threads that are bumped are already near the top, so the array shuffling isn't that much on average. But then you could be ddosed with necro spam.