EDIT 2026-04-24 the proof is wrong! As pointed out by Andy Tockman (see his comment below), there’s no rule that prevents the ball from crossing the same stop cell multiple times or coming to rest against a wall in the same location (and therefore it could cross the same gadget multiple times if it’s part of a loop). Probably the only way to “save” the NP-completeness is to require a gem to also act as a stop for the player-controlled ball (and replace the stop walls near the ball with mines); in this case, when the gem in a gadget is taken, the ball could no longer pass through the same gadget.
A quick proof that the puzze game Inertia (see this version on Simon Tatham’s Portable Puzzle Collection) is NP-complete.
