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?

Tablecat has an index page that it builds every time a post is made. The page you see on the front page of this board is actually a static page. Nothing is executed when you access the board's front page, the static html is simply served. If I make changes to the board files manually, I need to run the "Rebuild index pages" function from the admin side for it to be reflected on the board's front page / in the thread list.

4chan uses SQL, so does shiichan. I don't know about any other textboards other than tablecat that don't use SQL.

There are ways to make an structured index file such that you don't have to rewrite the entire file if an old thread is bumped, by just having fixed size lines and the ability to handle ``gaps'' in the list, but I don't know if it's really worth the implementation complexity.