While thinking about simple “puzzles” that seem hard at a first glance, but have not enough rules and structural constraints that make it easy to prove that they are NP-complete, I designed the following game (but perhaps it has already a name … let me know if you know it ðŸ™‚ ):

- a $N \times N$ grid contains $S \times S$ blue boxes in the upper left area, the
*home area*; each box occupies a cell; - a column (or row) that contains at least one box is picked at random and it is
*shifted*downward (or rightward). If a box exits from one border it re-enters on the opposite site; - the random shift is repeated
*maxmoves*times (e.g. $maxmoves = S^2$); - the aim of the game is to repack the boxes in the upper left home area, using at most
*maxmoves*upward column shifts or leftward row shifts.

A simple javascript version of the game can be played here.

The rules are simple, but even in a small game with a 4×4 home area it’s hard to find the correct shift sequence …

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