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Solver plug-ins:

YASS (Yet Another Sokoban Solver) plug-in
(version 2.141)
This is a solver made by Brian Damgaard, who is also author of the YASC Sokoban clone. The plug-in is distributed under GPL licence. See his home page for the source code. You can also download Sokoban YASC, and a command-line version of YASS there.
Sokolution plug-in
(download is at Sokolution website) This is a solver made by Florent Diedler. You can download the solver on the Sokolution website. To use the solver in Sokoban++, copy the downloaded solver DLL file to C:\Program Files (x86)\Sokoban++\solvers\. The tbb.dll file should not be placed in the solvers folder, but in the main Sokoban++ directory (C:\Program Files (x86)\Sokoban++\).
PocoSolv plug-in (1.0d)
(also available in .zip file)
PocoSolv is a solver made by David White. This version is now fully integrated with Sokoban++, so it works out-of-the-box. For more information and a command-line version of this solver you can see the PocoSolv homepage. This solver can also be downloaded in a .zip file, which is useful if you don't use Sokoban++.
BoxSearch plug-in
(download is at Ge Yong's site) The solver BoxSearch is made by Ge Yong. He has included a plug-in with his program. You can download his solver from Ge Yong's homepage (internet archive).
Takaken 7.2.2 Solver plug-in
Download The Takaken plugin included with Sokoban++ only supports Takaken Solver 7.0, this plugin allows you to use the newest version.
Tse's Sokoban Solver plug-in
Download This is the solver that was included in previous versions of Sokoban++. For this to work you need to have Tse's Sokoban installed. Click here to download Tse's Sokoban.
Prolog Solver Generator
Download This is a solver for programmers only. It generates a solver for a level in the Prolog language. The solver is very bad, more than 2 boxes is taking way too long to solve. You need a Prolog compiler to run the generated solver, for example SWI-Prolog. From the prolog command prompt, simply type "solve." (including the dot) to start the solving.
Takaken Solver
Download This is a plug-in based on an older version of the Takaken solver (version 2.7). You probably don't need this, because the Takaken Solver included with Sokoban++ can use version 7.0 of Takaken's solver.

If you want to make your own solver plug-in, download the development kit from the Download page on this website. With this kit an example with source code is included.