The basic idea is simple: take a puzzle based on group theory, like Rubik's Cube, but easier; and make the main goal of the game not just to reach the solved position, but to do it in the optimal number of moves.
The playing board of Spinpossible is a 3x3 grid, containing numbers. A move consists in rotating a rectangle of any size (including 1x1) by 180 degrees.
For example here is the starting position of one of the tutorial levels:
commutator to reach the solved state—you should have no problem with this new set of rules. If you don't, don't despair: the first few levels of the game do an excellent job of explaining the basic concepts.
Attempting to solve every puzzle in the optimal number of moves, however, changes things dramatically. For example, here is one of the early puzzles:
The authors have written a paper about the mathematics of Spinpossible, and also verified through exhaustive search that 9 moves are sufficient and sometimes necessary to solve every position. However, you can be sure that finding those 9 moves can be extremely hard for a human. 5 moves, like in the example above, is already pretty difficult.
The game has 4 different playing modes:
- puzzle: the main mode, which contains a tutorial and about 100 selected levels of increasing difficulty;
- arcade: you need to solve puzzles within a time limit; after each puzzle, you get a time bonus depending on how quick you are and how many moves you use;
- random: solve randomly generated puzzles at your own pace;
- multiplayer: compete online with other players.
Spinpossible is a must-have for every logic puzzle enthusiast. It is sure to push your brain to its limits, because, as the FAQ says, "no one has beaten all the levels without computer assistance".
The only criticism I have is for the user interface, which is a bit confused, and sluggish on some devices. This is a byproduct of the game being a port of a Flash game rather than a native iPhone app. Hopefully this will be improved in future updates.
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