You don't even strictly need it in the WM, as you can resize windows from other programs, even shell script with xdotool etc, although xdotool/shell quickly becomes somewhat unwieldy I once wrote a "tiling WM" like this. It actually worked fairly well, and I kind of liked the modularity of it.

At any rate, I dug up the old thing: https://pastebin.com/i91yD96F – I also had some other scripts to control window placement, movement, etc. I once planned to rewrite it to a "real" language and combine it with a "slightly less tiny wm" as a full release, but never really got around to it, and I also don't really use tiling any more (I just have one full-screen window all the time).

Not very hard as long as you don't care about drawing window frames.