Rules engine for Codex by Sirlin Games
Still very much in development. For now, to get started, look at the documentation for board-state.
As far as I'm aware everything should work according to the official rules.
Master Midori and Behind the Ferns: the rulings page doesn't seem to give a clear explanation of why Ferns beats Midori but only for units that arrive later than both; it's possible we can get away with picking a different implementation here.
Smoker and Dreamscape: this is the only case where multiple effects can occur as a result of something becoming a target. Rather than add a whole separate subsystem just for that, we may wish to just have the Illusion effect of Dreamscape take priority automatically (which is probably what AP wants most of the time.)
Focus Master: if multiple units or heroes would take exactly lethal damage at the same time and Focus Master doesn't have enough focus runes to save them all, we may wish to determine which ones to save at random (it's very strange for AP to get to decide this).