Sokoban

A Sokoban puzzle.

Sokoban 「倉庫番」 (warehouse keeper^?) is a type of transport puzzle, in which the player pushes boxes or crates around in a warehouse, trying to get them to storage locations. The puzzle is usually implemented as a video game.

Sokoban was created in 1981 by Hiroyuki Imabayashi, and published in 1982 by Thinking Rabbit, a software house based in Takarazuka, Japan.

Rules

• Only one box can be pushed at a time.
• A box cannot be pulled.
• The player cannot walk through boxes or walls.
• The puzzle is solved when all boxes are located at storage locations.

Selected official Sokoban releases

Sokoban published by Thinking Rabbit

• Sokoban (1982) (FM-7) with 20 levels.
• Sokoban 2 (1984) (NEC PC-8801) with 50 levels.
• Sokoban Perfect (1989) (NEC PC-9801) with 306 levels.
• Sokoban Revenge (1991) (NEC PC-9801) with 306 levels.

Although the first 10 puzzles of the original 1982 Sokoban game followed the rules described above, the remaining 10 puzzles could only be solved after transforming several walls into floors by pushing them from a particular side.

Sokoban published by Spectrum Holobyte

• Soko-Ban (1988) (DOS) with 50 levels.

In 1988 Sokoban was published in US by Spectrum HoloByte for the Commodore 64, IBM-PC and Apple II series as Soko-Ban. A 1988 review in Computer Gaming World praised the game for being "pure and simple, very playable and mentally challenging", citing its addictive qualities.^[1] It was also reviewed in 1988 in Dragon #132 by Hartley, Patricia, and Kirk Lesser in "The Role of Computers" column. The reviewers gave the game 4¹⁄₂ out of 5 stars.^[2]

Implementations of Sokoban

Implementations of Sokoban have been written for numerous computer platforms, including almost all home computer and personal computer systems. Versions also exist for several hand held and video game consoles, mobile phones, graphic calculators, and Canon PowerShot digital cameras.

Scientific research on Sokoban

Sokoban can be studied using the theory of computational complexity. The problem of solving Sokoban puzzles has been proven to be NP-hard.^[3] This is also interesting for artificial intelligence researchers, because solving Sokoban can be compared to designing a robot which moves boxes in a warehouse. Further work has shown that solving Sokoban problems is also PSPACE-complete.^[4]

Sokoban is difficult not only due to its branching factor (which is comparable to chess), but also its enormous search tree depth; some levels require more than 1000 "pushes". Skilled human players rely mostly on heuristics; they are usually able to quickly discard futile or redundant lines of play, and recognize patterns and subgoals, drastically cutting down on the amount of search.

Some Sokoban puzzles can be solved automatically by using a single-agent search algorithm, such as IDA*, enhanced by several techniques which make use of domain-specific knowledge.^[5] This is the method used by Rolling Stone, a Sokoban solver developed by the University of Alberta GAMES Group. The more complex Sokoban levels are, however, out of reach even for the best automated solvers.

See also

• Logic puzzle
• Sliding puzzle
• Chip's Challenge
• Kwirk
• Lasertank

References

1. ^ Wagner, Roy (May 1988). "Puzzling Encounters". Computer Gaming World: 42–43. 
2. ^ Lesser, Hartley, Patricia, and Kirk (April 1988). "The Role of Computers". Dragon (132): 80–85. 
3. ^ M. Fryers and M.T. Greene (1995). "Sokoban". Eureka (54). 
4. ^ Joseph C. Culberson, Sokoban is PSPACE-complete. Technical Report TR 97-02, Dept. of Computing Science, University of Alberta, 1997. Also: http://web.cs.ualberta.ca/~joe/Preprints/Sokoban
5. ^ Andreas Junghanns, Jonathan Schaeffer (2001) Sokoban: Enhancing general single-agent search methods using domain knowledge, Artificial Intelligence 129(1-2):219-251 (Special issue on heuristic search in artificial intelligence)

External links

• Official Sokoban site (in Japanese)
• The University of Alberta Sokoban page
• Erim Sever Sokoban pages