Overview

Arcanum is a free UCI chess engine under the GPL-3.0 license. Arcanum 2.0 has a rating of ~2450 elo in blitz on CCRL.

Building

Arcanum requires c++17 and has only been tested using g++. It can be compiled on Windows (Tested for Windows 11 using MinGW), and Linux (Tested for Ubuntu 22.04). Arcanum takes advantage of and requires some x86 intrinsics: AVX2, FMA, BMI1, BMI2 POPCNT and LZCNT.

Development build

To build a development build, run:

make -j

This will create a build directory and build a staticly linked executable. The development build allows Arcanum to log information to the terminal. This can however interfere with the UCI protocol, thus it is not recomended to use this build when connecting to a UCI gui.

To delete the build directory and its contents all the contents, run:

make clean

UCI build

To build a UCI build, meant for connecting to a UCI GUI such as Cute Chess or Lucas Chess, run:

make -j CFLAGS="-DPRINT_TO_FILE"

This redirects all logging to a file. It is also possible to add -DDISABLE_DEBUG, -DDISABLE_LOG, -DDISABLE_WARNING and -DDISABLE_ERROR to the CFLAGS in this command to disable these logs.

To add a version name, which shows up in the id section of the UCI protocol, -DARCANUM_VERSION=<version> can be added to the CFLAGS. If not included, the version will be set to dev_build.

Optionally, a UCI build can be built by running:

snapshot.bat <executable-name> <version-name>

This will create a fresh UCI build named <executable-name> and copy it to the snapshots directory.

NNUE

Arcanum has a floating point NNUE, which is created from selfplay, starting from the HCE from Arcanum v1.12. The architecture is 768->256->1, where the feature set is 'flipped' based on the perspective rather than having two feature transformers.

Both the inference and backpropagation is written from scratch and requires AVX2.

The path to the NNUE file can be set by the UCI command:

setoption name nnuepath value <path>

where <path> is the path relative to the Arcanum executable file.

Syzygy

Arcanum has an option to enable the use of Syzygy, an endgame table base. This is implemented using an adaptation of Pyrrhic by AndyGrant. To enable Syzygy, use the UCI command:

setoption name syzygypath value <folder>

where <folder> is the absolute path to the folder containing the tablebase.

Testing

Arcanum has a number of arguments to validate the engine and test the performance:

• --capture-test Test that move generation for capture moves works.
• --zobrist-test Testing that zobrist hashing works.
• --perft-test Runs perft on a number of predefined positions with known results.
• --search-perf Runs a 40 ply game searching at depth 10 with quiescence search. This is a performance test, checking the speed of the search.
• --engine-perf Runs a search for 5 seconds on a number of difficult test-positions (Bratko-Kopec Test). This is a performance test, checking the strength of the search.