I need to name an interface in a program I'm writing as being able to collide, but I've seen use of both collideable and collidable in projects with a similar type. Both of them look right in some ways, and wrong in others. Which spelling is more correct?
Short answer: There's no clear choice; take your pick.
Long answer: Neither collideable nor collidable is a word you're likely to find in a dictionary, but in your context using it (one of them) may be exactly the right choice. As for the spelling preference, Wikipedia's detailed article on American and British English spelling differences says:
The "polysyllabic" rule would point towards collidable, but elsewhere, a search brings up the following poly-syllabic words ending in eable (other than soft c, ch, g instances, of which there are many) (I haven't checked their provenance):
Or you may want to look specifically at -able words formed from verbs ending in -de, and decide whether -dable or -deable is preferable:
It seems that analogy with dividable and decidable (the closest?) would suggest collidable. (But if you still prefer collideable, it's fine to use it…)
-able forms adjectives that mean:
Collidable would mean able to be collided, not able to collide. Instead of collidable, you can use hittable, which would mean able to be hit.
As per choosing between collidable and collideable, the adjective that derives from cite is citable; in the same way, you should write collidable, not collideable.