Monchester

Happy Halloween 2020! The origins of this little chess engine date back to 16th October 2002 and it has just recently risen from the grave. It plays strictly on scholastic chess level in 1.0 version and demonstrates that often the way to win is not making really good moves, but just avoiding bad ones :)

Repository structure

Downloadable source code release archives created so far are all listed in release section.

In GitHub, the master branch is strictly for README and pre-compiled binaries. Code is in branches named X.Y-branch and releases are created from suitable tags, e.g. for 1.0 there is 1.0-branch where there is 1.0 tag from which source code release is created -- and when there is bugfix release in 1.0 then there will be tag like 1.0.1 created on the same branch.

About

This is a basic chess engine (in C) that is able to play game from console and via CECP (XBoard) protocol. It should perform on the level suitable mostly for scholastic chess -- always looking only 4 plys ahead and without extensions it can be beaten in the opening, middle game and end-game :). It supports randomization (e.g. CECP random that is sent by default when new game starts) for varying play.

It is monstrous only so far as it has risen from the grave, not with regard to its monstrous playing "quality". Its origins date back to 16th October 2002 when I started writing it -- however it was thrown out after about a half-month of messing with it as my C-code ran into obscure crashes and memory errors that I was not able to trace back at the time. I was reminded of that program by a friend and I managed to locate its source code and went in to make a "quick bug fix". First errors I truly located easily with better tools of 2020, but one mysterious crash took long to track down and then I was already hooked in trying to make "simple" fixes to the program, which contained pretty much every error and oversight that can occur when writing chess engine from scratch:

• It strongly preferred stalemating to checkmating -- as the king capture was initially allowed and stalemate situations resulted in better material position.
• When it was ahead by too much and pretty much nothing threatened it, it never moved towards actual win -- plan was totally missing and principal variation remembrance badly desired.
• Since it was totally deterministic, it managed to end up in positions with 4 queens and 2 rooks vs single king and still not being able to checkmate.
• While king capture was allowed, it managed to do totally mysterious move choices due to the fact that apparently it internally quite often managed to find situations where both kings were taken off the board and then the rest of the material battled on.

So all in all much more time was spent on this than planned :), and now the Monchester is unleashed upon the world.

Usage

If used from console, new command starts new game, resign resigns and moves must be entered in full algebraic coordinates, e.g. e2e4, promotions must be denoted by following lowercase letter, e.g. e7e8q.

If used as engine via CECP (XBoard) protocol, the standard engine installation procedures should apply. Or to set active engine from the command line, specify the first chess program location, e.g. for XBoard, with Monchester binary located in current folder: xboard -fcp ./monchester.

Compilation

The program should compile under Linux and Mac OS X and has been reported to compile under Windows using MSYS2. Precondition is presence of make and gcc C-compiler or compatible compiler symlinked as gcc (clang) -- in all my computers clang produces MUCH faster binary than gcc. Compiler can also be changed by passing make CC=some_compiler argument to make.

Build (likely) fastest binary

• make CC=clang clean release

Build with simplest command line using available gcc

• make clean release
• or simply make if there are no old files to clean

Features

• Full FIDE rules for standard chess (castling, en passant, check and stalemate, draw by repetition, 50-move draw recognition, insufficient material detection.
• Minimax search with constant 4-ply search depth.
• Material evaluation only, with knight placement bonuses :)
• Nearly instant movement on modern hardware.

Non-Features

Since reading around, I get a feel that every chess engine must have a long list of features, so here I present a long list of non-features:

This engine does not use, feature or support: Alpha-Beta, Attack Tables, Bitboards (rotated, magic, black magic or any other kind), Iterative Deepening, Null-Move Heuristics / Pruning, Hash Tables, Late Move Reductions, MVV-LVA, Neural Networks, Opening Books, ProbCut, Razoring, Static Exchange Evaluation, Texel Tuning, Threads, Tablebases, Transposition Tables, UCI and many other things that I have either forgotten or never known :)

Compilation options

Defines

Features

In features.h there is a list of toggleable features with user-visible effects that can be toggled or configured at compile time. Currently there are just two, these are both off (and probably should remain off for casual user).

• FEATURE_KEEP_ALL_PVS (OFF) Causes engine to remember PVs for all move branches and is mostly useful for engine debugging and development. If this and FEATURE_KEEP_GAMESCORES are both ON, then written gamescores will include PV variations in gamescore PGN.

• FEATURE_KEEP_GAMESCORES (OFF) Causes engine to write finished game scores to file .monchester.pgn in user's home folder. It is not generally very useful, as CECP and adapter interfaces for CECP provide their own game score keeping facilities. This feature also uses flock() file locking call that is not available on Windows, so enabling it should only be done if wanting to do some development or testing on Mac/Linux or other compatible systems.

Other

Randomization of play can be disabled by defining:

• DISABLE_SCORE_RANDOMIZATION

at build time, e.g.

make EXTFLAGS="-DDISABLE_SCORE_RANDOMIZATION"

but this is not recommended, as non-deterministic play actually gives Monchester much better chances in converting totally won endgames with major pieces, that it otherwise would draw with deterministic play.

There are couple more possible defines, those interested can probably easily find out their purpose:

• SCORE_RANDOM_BITS
• EXTRA_VERSION

Thanks

• Henri Lakk for reminders and MAC compilation for 0.99 preview version.
• Juhan and Ingrid "Daddy, it is gonna take your horsie!" Aasaru for playtesting.
• Günther Simon for first Windows compile and computer chess record-keeping.
• Roland Chastain for problem reports running on CuteChess.

Acknowledgements

This engine would not have been reanimated, if it weren't for XBoard CECP protocol implementation, a GNU project currently managed by Harm Geert Muller.

For game related bug-squashing, following engines proved to be nice sparring partners, as they are relatively stable, relatively beatable and relativily compileable:

• N.E.G. ("Niet Erg Goed") 1.3 chess engine by Harm Geert Muller
• ChessPuter by Miles Bright
• GiuChess by Giuliano Ippoliti aka JSorel