Sokoban

Sokoban takes place in a warehouse viewed from above and composed of walls and floor squares. A floor square may be empty, occupied by the player, or occupied by a box. Some floor squares are storage locations. The number of storage locations equals the number of boxes. The objective of the puzzle is to push all boxes onto storage locations.^[1]

The player can move one square at a time, either horizontally or vertically, onto an empty floor square.^[2] Boxes and walls block the player's movement, but the player can walk up to a box and push it to an empty square directly beyond it. If a box is pushed against a wall or another box, it does not move. Pulling boxes is not possible.^[3]

Sokoban requires players to plan several moves ahead and consider all possible outcomes.^[4] Careless moves could leave a box permanently trapped against a wall or another box,^[2] stuck in a dead end,^[5] or permanently prevent access to boxes that still need to be moved.^[6]: 38 Any such situation creates a deadlock that makes the puzzle unsolvable, regardless of future moves,^[7] unless the player undoes enough steps.^[8]

In 1981, Hiroyuki Imabayashi created the first Sokoban game for the NEC PC-8001 as a hobby, featuring text-based graphics and five original levels.^{[9]: 136–137} The core mechanic was inspired by Hudson Soft's 1980 game Aldebaran #1 for the MZ-80K,^[10] where players pushed luggage to block radiation.^[11] Imabayashi envisioned a warehouse setting where incorrect box placement could make further progress difficult or even impossible, and designed levels that proved challenging even for his friends.^[12] At the time, his wife's parents owned a record store with a small computer section. A salesman who saw the game suggested it had commercial potential.^{[9]: 137 [13][12]}

Imabayashi then began preparation for Sokoban as a product; using the machine from the computer section, he ported the game to the NEC PC-8801, enhancing the graphics and expanding it to twenty levels.^{[9]: 137 [12]} He founded Thinking Rabbit, and Sokoban was released as its first commercial title. Sources differ on the timeline: official records report a 1982 founding^[12] and a December 1982 release,^[14]: 3 while a book states a June 1983 debut for both company and game;^[15]: 128 separately, a magazine states that Sokoban was released in May 1983 and the company was incorporated in June 1983.^[9]: 136

In August 1983, the Japanese magazine PC Magazine published Sokoban Extra Edition as a type-in program with ten new puzzles, developed by Thinking Rabbit at the magazine's request.^[16] In 1984, Thinking Rabbit released Sokoban 2, which included a puzzle editor.^[17] Throughout the 1980s, new titles appeared on various Japanese platforms, including home computers such as the MSX and PC-9801, and consoles like the Famicom, Sega SG-1000, Sega Mega Drive, and Game Boy.^{[14]: 112 [18]} These releases were developed either by Thinking Rabbit or by other companies under license agreements.^[12] In 1987, Spectrum HoloByte in California licensed Sokoban from Japan's ASCII, adapted the MSX version for IBM PC, Apple II, and Commodore 64, added features for the U.S. market, and released it as Soko-Ban in early 1988.^[19][20] Falcon, a Japanese software company, trademarked the game's kanji name "倉庫番" in 1988.^[21] New titles in the official series^[22] were released in Japan throughout the 1990s for platforms including the Super Famicom, Windows, Macintosh, and PlayStation.^[18]

Around 2000, Thinking Rabbit became inactive but remained a legal entity.^[11] In 2001, Falcon acquired the copyrights for Sokoban,^[23] and trademarked the Latin-script name "Sokoban" in 2003^[24] and the name "Thinking Rabbit" in 2011.^[25] From 2004 to 2007, Falcon developed several titles for Japanese mobile phones.^[26][27] Between 2015 and 2018, Falcon developed five Sokoban titles for Windows and the smartphone game Sokoban Touch (2016), publishing each under the Thinking Rabbit brand.^[18][28] Notably, Falcon bundled four of these Windows titles into the Sokoban Complete Pack (倉庫番 コンプリートパック) (2016) to commemorate the series' 35th anniversary, which was published by Skill Information [ja].^[29][18] Falcon developed three Sokoban titles for Japanese digital terrestrial television broadcasters in 2018.^[30] Unbalance developed and published an official title in 2021, The Sokoban, for the Nintendo Switch^[31] and PlayStation 4.^[32]

Sokoban has a fixed set of gameplay rules;^[3] however, certain official releases have expanded the rules or introduced variations on the core mechanics. For example, the PlayStation version of Ultimate Sokoban (究極の倉庫番) features a themed stage where boxes must be stored in an arrangement that completes an electrical circuit.^[33]: 14 Sokoban Special of Tears (涙の倉庫番スペシャル) includes a game mode allowing the use of tools such as ropes,^[34]: 98 and Sokoban Legend: Land of Light and Darkness (倉庫番伝説 光と闇の国) offers a mode in which the player must push enemies into holes and push puppets onto magic circles.^[35] Conversely, Power Sokoban (Power倉庫番) is an action-puzzle game that deviates from the warehouse keeper mechanic; the player shoots orbs and fills holes with boulders.^[36][37]

More than 20 official Sokoban titles have been released across over 30 platforms.^[38] The following table lists titles in the Sokoban series; it is not exhaustive.^{[18][14]: 112}

The first Sokoban title became a bestseller in Japan, with over 25,000 copies sold by July 1984.^[85][39][86] Junji Tanaka wrote that the first level of the PC-8801 version perplexed puzzle enthusiasts upon release.^[14]: 4 Titles for Japanese home computers, such as the NEC PC-9801 and Sharp X1, were said to have sold more than 100,000 copies combined.^[87] The MSX version alone, published by ASCII, sold over 400,000 copies and was considered a commercial success.^[19][88] The U.S. release, Soko-Ban, sold over 50,000 copies by mid-September 1988.^[19] By c. 1996, after 13 years of availability, the series had sold over one million copies;^[89] By c. 1998, after 15 years, total sales, including Ultimate Sokoban (究極の倉庫番) for PlayStation exceeded 1.4 million units across all platforms, including game consoles, word processors, and electronic organizers.^[90] By 2018, Chukyo Television Broadcasting reported that the series had sold over 4.1 million copies worldwide since its 1982 release.^[91]

Reviewers often emphasized the game's addictive nature. In 1983, Micomgames staff remarked that players would find it difficult to stop playing Sokoban.^[4] In 1988, Roy Wagner of Computer Gaming World suggested that anyone trying the US version, Soko-Ban, would likely remain absorbed for an extended period.^[8] The Computer Entertainer newsletter described the game as "fascinating" and "almost impossible to stop playing".^[92] In its console reviews, Computer and Video Games magazine called Sokoban for Game Boy "an infuriatingly addictive little title" and said its appeal was "not far off" from Tetris.^[93] In 1990, Famicom Winning Guide recognized Sokoban as a staple puzzle game, citing its difficulty, depth, and continued presence across multiple platforms.^[94]

Commentators often highlighted one or more aspects of the game: its simplicity, the level of thought it required of players, or its challenging nature. Micomgames staff described the first Sokoban title as simple yet requiring deep thought comparable to playing Go or Shogi.^[4] The 1988 book Video Games: Complete Collection of TV Games described Sokoban as a pure computer-based puzzle game with a logic akin to Japanese disentanglement puzzles.^[15]: 189 It stated that while the game appears simple, the boxes are "cleverly" arranged; some levels require moving other boxes for twenty moves just to shift a single target box one space.^[15]: 189

In 1989, writing for MSX Magazine [ja], a reviewer cited Sokoban as the representative example of a purely logic-based puzzle, distinguishing it from puzzles that incorporate action elements such as enemies or time pressure, and those involving elements of chance.^[95] Family Computer magazine's All Catalog supplement described Sokoban for Game Boy as great due to the simplicity of its gameplay,^[34]: 199 and Computer and Video Games magazine staff described it as one of the Game Boy's "simple but effective puzzle games".^[93]

Reviewers for the German magazine Happy Computer praised Soko-Ban as a brilliant logic puzzle that kept players thinking without pressure and recommended that players carefully observe a level before moving a box,^[96] and in Computer Gaming World, Wagner summarized it as "very playable and mentally challenging".^[8] In Game Player's magazine, Tom R. Halfhill reviewed Shove It! for the Sega Genesis, noting it was challenging and would require players to plan their moves carefully,^[97] and reviewing Boxxle for Game Boy, he stated that it required careful planning or plenty of trial and error (usually both).^[98] He later commented on Boxyboy for the TurboGrafx-16 that while the initial rooms were not difficult, players would eventually encounter one that "seems impossible".^[99]

The series faced occasional criticism for a lack of variety. Tom R. Halfhill wrote that the puzzles in Shove It! were "essentially the same",^[97] and noted that in Boxxle, variation was limited to crate count, placement, and room shape.^[98] Reviewing Boxyboy, he described it as "virtually identical" to the others, concluding that all three games required players to enjoy solving the same type of puzzle repeatedly.^[99]

The 1988 book Video Games: Complete Collection of TV Games credits Sokoban with sparking a "puzzle game boom" in the Japanese personal computer market starting in 1983 and continuing for a time.^[15]: 189 In the decades since, the series has attracted many enthusiasts in Japan and overseas.^[38] The game's core mechanics have been replicated in numerous clones across a wide variety of platforms,^[100] including the web.^[101] The Sokoban community has created thousands of puzzles^[102] distinct from the official releases,^[103] freely available online and ranging in difficulty.^[100] Puzzles resembling Sokoban, involving pushing boxes or similar obstacles to the correct targets, have been present in gaming,^[104] particularly in 1980s and 1990s action-adventure games with grid-based movement.^[105] The Legend of Zelda series and titles such as Adventures of Lolo (1989) and LIT (2009) incorporate Sokoban-style elements into their gameplay;^[104] for example, The Legend of Zelda: A Link to the Past (1991) has a puzzle in which blocks must be pushed to clear a path to a treasure chest.^[105] Resident Evil 2 (1998) includes a puzzle similar to Sokoban in which two statues must be pushed onto corresponding marked floor areas, with the correct matching inferred.^[105] Additionally, Sokoban-like games such as Sokomania 2 (2014) introduce new mechanics, including switches and conveyor belts.^[104]

Research

The computational problem of solving Sokoban puzzles has been studied using computational complexity theory, and is known to be NP-hard^[106][107] and PSPACE-complete.^[108][109] Solving non-trivial Sokoban puzzles is difficult for computers because of the high branching factor (many legal pushes at each turn) and the considerable search depth (many pushes needed to reach a solution).^[110][111] Even small puzzles can require lengthy solutions.^[112]

Sokoban puzzles provide a challenging testbed for developing and evaluating automated planning techniques.^[113] The first documented automated solver, Rolling Stone, was developed at the University of Alberta. It employed a conventional search algorithm enhanced with domain-specific techniques such as deadlock detection.^{[6]: 139–140 [114]} A later solver, Festival, introduced the FESS search algorithm and became the first automatic system to solve the standard XSokoban benchmark, a suite of ninety puzzles with a ten-minute per-puzzle time limit that had eluded a complete solution for more than twenty years.^[115][116] Despite these advances, Sokoban puzzles that have been solved by humans are beyond the reach of state-of-the-art solvers.^{[103][117][118]} Humans solve such instances by breaking down puzzles into subproblems,^[6]: 40 recognizing patterns and exceptions, and drawing on learning from prior puzzles.^[6]: 62

• Logic puzzle
• Sliding puzzle
• Transport puzzle
• Motion planning