Sokoban deadlocks: four patterns to recognize
A deadlock is a position from which no legal sequence can place every crate on a target. Crate Bloom deliberately makes a narrower claim: it calls a position Deadlocked only when a static proof applies, and calls uncertain wall contact Danger.
Non-target corners
A crate in a corner made by two perpendicular walls cannot leave, because the player can never stand beyond either wall. A target corner is the essential counterexample: a crate there may already be correctly placed, so it must not be reported as deadlocked.
Proven wall regions
Some wall squares cannot reach any target even when all other crates are ignored. Crate Bloom computes those squares by reverse pull reachability. Ordinary wall contact is not enough: if a route along the wall reaches a target, the position remains reversible.
Frozen 2×2 blocks
A 2×2 area filled by walls and crates is frozen. If at least one frozen crate is off target, the room cannot finish. The check is local and explicit; a loose group of crates is not labeled deadlocked merely because it looks crowded.
What to do
Undo to the push before the proven trap, then identify the standing square required for the next useful push. If the interface says Danger, pause and inspect the route rather than assuming the puzzle is lost.