Overview

Prisoners' Chess (PC for short) is a free, powerful UCI chess engine forked from Stockfish. It aims to implement the Prisoners' Chess variant outlined here, but, as of today, it's a work in progress. Like Stockfish, it is not a complete chess program and requires a UCI-compatible GUI (e.g. XBoard with PolyGlot, Scid, Cute Chess, eboard, Arena, Sigma Chess, Shredder, Chess Partner or Fritz) in order to be used comfortably. Read the documentation for your GUI of choice for information about how to use Stockfish with it: the same instructions are valid for PC too.

Files

This distribution of PC consists of the following files:

• Readme.md, the file you are currently reading.

• Copying.txt, a text file containing the GNU General Public License version 3.

• src, a subdirectory containing the full source code, including a Makefile that can be used to compile PC on Unix-like systems.

UCI parameters

Currently, PC has the following UCI options:

• Debug Log File

  Write all communication to and from the engine into a text file.

• Contempt

  A positive value for contempt favors middle game positions and avoids draws.

• Analysis Contempt

  By default, contempt is set to prefer the side to move. Set this option to "White" or "Black" to analyse with contempt for that side, or "Off" to disable contempt.

• Threads

  The number of CPU threads used for searching a position. For best performance, set this equal to the number of CPU cores available.

• Hash

  The size of the hash table in MB.

• Clear Hash

  Clear the hash table.

• Ponder

  Let PC ponder its next move while the opponent is thinking.

• MultiPV

  Output the N best lines (principal variations, PVs) when searching. Leave at 1 for best performance.

• Skill Level

  Lower the Skill Level in order to make PC play weaker.

• Move Overhead

  Assume a time delay of x ms due to network and GUI overheads. This is useful to avoid losses on time in those cases.

• Minimum Thinking Time

  Search for at least x ms per move.

• Slow Mover

  Lower values will make PC take less time in games, higher values will make it think longer.

• nodestime

  Tells the engine to use nodes searched instead of wall time to account for elapsed time. Useful for engine testing.

• UCI_Chess960

  An option handled by your GUI. If true, PC will play Chess960.

• UCI_AnalyseMode

  An option handled by your GUI.

• SyzygyPath

  Path to the folders/directories storing the Syzygy tablebase files. Multiple directories are to be separated by ";" on Windows and by ":" on Unix-based operating systems. Do not use spaces around the ";" or ":".

  Example: C:\tablebases\wdl345;C:\tablebases\wdl6;D:\tablebases\dtz345;D:\tablebases\dtz6

  It is recommended to store .rtbw files on an SSD. There is no loss in storing the .rtbz files on a regular HD. It is recommended to verify all md5 checksums of the downloaded tablebase files (md5sum -c checksum.md5) as corruption will lead to engine crashes.

• SyzygyProbeDepth

  Minimum remaining search depth for which a position is probed. Set this option to a higher value to probe less agressively if you experience too much slowdown (in terms of nps) due to TB probing.

• Syzygy50MoveRule

  Disable to let fifty-move rule draws detected by Syzygy tablebase probes count as wins or losses. This is useful for ICCF correspondence games.

• SyzygyProbeLimit

  Limit Syzygy tablebase probing to positions with at most this many pieces left (including kings and pawns).

What to expect from Syzygybases?

If the engine is searching a position that is not in the tablebases (e.g. a position with 8 pieces), it will access the tablebases during the search. If the engine reports a very large score (typically 153.xx), this means that it has found a winning line into a tablebase position.

If the engine is given a position to search that is in the tablebases, it will use the tablebases at the beginning of the search to preselect all good moves, i.e. all moves that preserve the win or preserve the draw while taking into account the 50-move rule. It will then perform a search only on those moves. The engine will not move immediately, unless there is only a single good move. The engine likely will not report a mate score even if the position is known to be won.

It is therefore clear that this behaviour is not identical to what one might be used to with Nalimov tablebases. There are technical reasons for this difference, the main technical reason being that Nalimov tablebases use the DTM metric (distance-to-mate), while Syzygybases use a variation of the DTZ metric (distance-to-zero, zero meaning any move that resets the 50-move counter). This special metric is one of the reasons that Syzygybases are more compact than Nalimov tablebases, while still storing all information needed for optimal play and in addition being able to take into account the 50-move rule.

Compiling PC yourself from the sources

On Unix-like systems, it should be possible to compile PC directly from the source code with the included Makefile.

PC has support for 32 or 64-bit CPUs, the hardware POPCNT instruction, big-endian machines such as Power PC, and other platforms.

In general it is recommended to run make help to see a list of make targets with corresponding descriptions. When not using the Makefile to compile (for instance with Microsoft MSVC) you need to manually set/unset some switches in the compiler command line; see file types.h for a quick reference.

Donating hardware

Please don't. If you ever feel like that, please donate it to Stockfish instead, thanks.

Improving the code

If you want to help improve the code, please do so only if you want to make it better implement the specific PC variant rules.

Any other FIDE chess improvement, or Chess960 improvement, or any improvement of anything Stockfish does, should be based on Stockfish and not on PC.

• The latest source can always be found on GitHub.

Terms of use

Prisoners' Chess is free, and distributed under the GNU General Public License version 3 (GPL v3). Essentially, this means that you are free to do almost exactly what you want with the program, including distributing it among your friends, making it available for download from your web site, selling it (either by itself or as part of some bigger software package), or using it as the starting point for a software project of your own.

The only real limitation is that whenever you distribute Prisoners' Chess in some way, you must always include the full source code, or a pointer to where the source code can be found. If you make any changes to the source code, these changes must also be made available under the GPL.

For full details, read the copy of the GPL v3 found in the file named Copying.txt.