I think you're going to want to be a little more specific...
Very well. A couple of days ago I started putting together what was supposed to be a quick and dirty platform engine. Things haven't really worked out, and while I probably ought to move back to what I know, I don't really want to be beat!
It's of the style where no horizontal collisions are necessary at all, except where the player moves up or down (single pixel max. gradient) slopes - we just stop him falling when he lands on some ground.
My first version used no detectors, just single-pixel collision masking (quick and dirty, remember?). I had it bug-free in about a half hour, fairly versatile too, could easily make up for the limited collision width by adding ghost edges to obstacles. (To elaborate: I'm just testing player X,Y and X,Y+1 or something under a range of conditions.)
I want moving platforms though and this seems to be really hard to implement without starting from scratch. I can, of course, start from scratch. I've actually started from scratch many times but haven't arrived at a solution I particularly like. I've made platform engines before, of the detector-based, head-bumping, sides-bumping, no-slopes kind, with platforms that float all over the show, but I'm rusty as hell and I just can't get things right at the moment.
Anyone got any bright ideas? I'm just talking the general theory of this kind of engine in TGF. I'm using the Newgrounds edition, so no extensions.