Texel is a free software (GPL v3) chess engine written in C++11.
More information about the program is available in the chess programming wiki:
https://www.chessprogramming.org/TexelThe Texel distribution contains the following pre-compiled executables located in the bin directory:
texel64-avx512.exe For Windows 7 or later x86-64 systems using AVX-512, VNNI, POPCOUNT and BMI2 instructions.
texel64-avx2-bmi.exe: For Windows 7 or later x86-64 systems using AVX2, POPCOUNT and BMI2 instructions.
texel64-avx2-pop.exe: For Windows 7 or later x86-64 systems using AVX2 and POPCOUNT instructions.
texel64-avx2.exe: For Windows 7 or later x86-64 systems using AVX2 instructions.
texel64-ssse3.exe For Windows x86-64 systems with SSSE3 support.
texel64-old.exe For Windows x86-64 systems.
texel64cl.exe: Cluster version of texel64-avx2-pop.exe. Requires Microsoft MPI to be installed.
texel-arm64: For the ARMv8-A 64-bit architecture. Should work on most modern Android devices.
texel-arm64-dot: For the ARMv8-A 64-bit architecture with dot product CPU instruction. Should work on many modern Android devices.
texel64: For Linux x86-64 systems with AVX2 and POPCOUNT support.
If you need an executable for a different system, see the "Compiling" section below.
Hash
Controls the size of the main (transposition) hash table. Texel supports up to 512GiB for transposition tables. Other hash tables are also used by the program, such as a pawn hash table. These secondary tables are quite small and their sizes are not configurable.
OwnBook
When set to true, Texel uses its own opening book. When set to false, Texel relies on the GUI to handle the opening book.
BookFile
If set to the file name of an existing polyglot opening book file, Texel uses this book when OwnBook is set to true. If set to an empty string, Texel uses its own small built in book when OwnBook is true. BookFile is not used when OwnBook is false. An opening book called texelbook.bin is included in this distribution.
Ponder
Texel supports pondering mode, also called permanent brain. In this mode the engine calculates also when waiting for the other side to make a move. This option changes the time allocation logic to better suit pondering mode. The option is normally handled automatically by the GUI.
UCI_AnalyseMode
This option is normally set automatically by the GUI when in analysis mode. In analysis mode, Texel does not use its opening book.
Strength
Strength can be smoothly adjusted between playing random legal moves (0) and playing at full strength (1000).
UCI_LimitStrength, UCI_Elo
These are standard UCI options to reduce the playing strength and should be handled by the GUI. Internally the Elo value is converted to a Strength value, so the Strength option has no effect when UCI_LimitStrength is enabled.
MaxNPS
If set to a value larger than 0, Texel will not search faster than this many nodes per second. This can be used as an alternative to or in combination with the Strength option to reduce the playing strength.
Threads
Texel can use multiple CPUs and cores using threads. For best performance, don't set this value higher than the number of cores or hyperthreads in the computer.
MultiPV
Set to a value larger than 1 to find the N best moves when analyzing a position. This setting has no effect when playing games. The GUI normally handles this option so the user does not have to set it manually.
UseNullMove
When set to true, the null move search heuristic is disabled. This can be beneficial when analyzing positions where zugzwang is an important factor.
Contempt
When playing a game this value specifies how big an advantage, measured in centipawns, the engine thinks it has over its opponent, because of differences in playing strengths. A positive value means the engine sees itself as stronger than the opponent and therefore tries to avoid draws by repetition and simplifying piece trades. A negative value has the opposite effect, causing the engine to actively look for draws and simplifying exchanges.
AnalyzeContempt
When analyzing a position the AnalyzeContempt value is used instead of the Contempt value. AnalyzeContempt is always specified from the white player's point of view, even when it is black's turn to move. For example, if you are analyzing an adjourned game where you are playing white and are happy with a draw, you can set AnalyzeContempt to a negative value, such as -50. The analysis will then take into account that white "wants" a draw, even when you are analyzing a position where it is black's turn to move.
AutoContempt
When AutoContempt is set to true, the Contempt option is ignored and the contempt value during game play is instead determined based on the opponent as defined by the ContemptFile option.
ContemptFile
Used when AutoContempt is set to true. Specifies the path to the text file
that defines what contempt value to use for a given opponent. Each line in the
file has the format regex
GaviotaTbPath
Semicolon separated list of directories that will be searched for Gaviota tablebase files (*.gtb.cp4).
GaviotaTbCache
Gaviota tablebase cache size in megabytes.
SyzygyPath
Semicolon (Windows) or colon (Linux, Android) separated list of directories that will be searched for Syzygy tablebase files.
MinProbeDepth
Minimum remaining search depth required to probe tablebases. If tablebase probing slows down the engine too much, try making this value larger. If all tablebase files are on fast SSD drives or cached in RAM, a value of 0 or 1 can probably be used without much slowdown.
MinProbeDepth6, MinProbeDepth7
Minimum remaining search depth required to probe 6-men and 7-men tablebases. Values smaller than MinProbeDepth are ignored. Values larger than MinProbeDepth can be useful if the larger tablebases are on slower disks than the smaller tablebases.
MinProbeDepth6dtz, MinProbeDepth7dtz
Minimum remaining search depth required to probe 6-men and 7-men DTZ tablebases. Values smaller than MinProbeDepth6, MinProbeDepth7 are ignored. Values larger than MinProbeDepth6/7 can be useful if DTZ tables are on slower disks than WDL tables.
Clear Hash
When activated, clears the hash table and the history heuristic table, so that the next search behaves as if the engine had just been started.
AnalysisAgeHash
When set to false the transposition table is not "aged" when starting a new search in analysis mode. This helps keeping older but deeper entries around in the transposition table, which is useful when analysing a position and making and un-making moves to explore the position.
Texel can use endgame tablebases to improve game play and analysis in the endgame. Both Gaviota tablebases (only .cp4 compression) and Syzygy tablebases are supported, and both tablebase types can be used simultaneously.
For game play Syzygy tablebases are recommended because the Syzygy probing code scales better when the engine uses multiple cores.
For analysis, using both Syzygy and Gaviota tablebases at the same time is recommended. This gives accurate mate scores and PVs when the search can reach a 5-men position (thanks to Gaviota tablebases), and game theoretically correct results (also taking 50-move draws into account) when the search can reach a 6-men position (thanks to Syzygy tablebases).
Syzygy tablebases contain distance to zeroing move (DTZ) information instead of distance to mate (DTM) information. DTZ values are converted internally to upper or lower DTM bounds before being presented to the user. This means that there is no separate score range for known tablebase wins, but it also means that the shortest possible mate can be much shorter than the reported mate score indicates.
For syzygy tablebases it is recommended to have the corresponding DTZ table for each WDL table. Texel tries to handle missing tablebase files gracefully, but in some situations missing DTZ tables may lead to trouble converting a won tablebase position. This can happen even in relatively simple endgames that Texel could have won without using tablebases at all.
Because of technicalities in the Syzygy probing code, 6-men tablebases are only supported for the 64-bit versions of Texel.
The 6-men Syzygy tablebases will likely only increase the playing strength of Texel if at least the WDL tables are stored on SSD.
Non-uniform memory access (NUMA) is a computer memory design common in computers that have more than one CPU. Texel can take advantage of NUMA hardware when running on Windows or Linux.
The pre-compiled 64-bit Windows executables are compiled with NUMA awareness. The pre-compiled Linux executable is not NUMA aware because it adds a dependency on the libnuma library which may not be installed on all Linux systems.
When running a NUMA aware executable, NUMA awareness can be disabled at runtime by giving the -nonuma argument when starting Texel.
When NUMA awareness is enabled and Texel runs on NUMA hardware, Texel binds its search threads to suitable NUMA nodes and tries to allocate thread-local memory on the same nodes as the threads run on. If Texel uses fewer search threads than there are cores in the computer, the threads will be bound to NUMA nodes such that there are no more than one thread per core and such that as few NUMA nodes as possible are used. This arrangement speeds up memory accesses.
Texel can run on computer clusters by using the MPI system. It has only been tested using MPICH in Linux and MS-MPI in Windows but should work with other MPI implementations as well.
The pre-compiled windows executable texel64cl.exe is compiled and linked against MS-MPI version 8.1. It requires the MS-MPI redistributable package to be installed and configured on all computers in the cluster.
Running on a cluster is an advanced functionality and probably requires some knowledge of cluster systems to set up.
Texel uses a so called hybrid MPI design. This means that it uses a single MPI process per computer. On each computer it uses threads and shared memory, and optionally NUMA awareness.
After Texel has been started, use the "Threads" UCI option to control the total number of search threads to use. Texel automatically decides how many threads to use for each computer, and can also handle the case where different computers have different number of CPUs and cores.
If there are 4 Linux computers called host1, host2, host3, host4 and MPICH is installed on all computers, start Texel like this:
mpiexec -hosts host1,host2,host3,host4 /path/to/texel
Note that /path/to/texel must be valid for all computers in the cluster, so either install Texel on all computers or install it on a network disk that is mounted on all computers.
Note that it must be possible to ssh from host1 to the other hosts without specifying a password. Use for example ssh-agent and ssh-add to achieve this.
If there are two computers called host1 and host2 and MS-MPI is installed on both computers, proceed as follows:
To run the cluster version of Texel in a GUI, the engine should be defined as the mpiexec command with all parameters as given in the examples above. If the GUI does not support adding command line parameters to the engine you can use a wrapper program that passes the required parameters to mpiexec. If you are using Linux, creating a one-line shell script should be enough. If you are using Windows, you can use the included runcmd.exe program:
Texel uses the CMake build system. To compile Texel follow these steps, assuming you have access to a terminal environment:
The following options can be enabled to activate CPU, OS and compiler specific features. Not all options are available for all operating systems and compilers:
USE_SSSE3
Use SSSE3 instructions to speed up neural network evaluation.
USE_AVX2
Use AVX2 instructions to speed up neural network evaluation.
USE_AVX512
Use AVX-512 instructions to speed up neural network evaluation.
USE_BMI2
Use BMI2 instructions to speed up move generation.
USE_POPCNT
Use CPU popcount instructions to speed up counting of number of 1 bits in a bitboard object.
USE_NEON
Use NEON instructions (for ARM CPUs) to speed up neural network evaluation.
USE_NEON_DOT
Use NEON and dot product instructions (for ARM CPUs) to speed up neural network evaluation.
USE_CTZ
Use a special CPU instruction to find the first 1 bit in a bitboard object.
USE_PREFETCH
Use CPU prefetch instructions to speed up hash table access.
USE_NUMA
Optimize thread affinity and memory allocations when running on NUMA hardware.
USE_LARGE_PAGES
Prefer large pages when allocating memory for the transposition table. Only effective if large page support is enabled in the operating system. For Linux systems an alternative to using this option is to execute the following command as root: echo always >/sys/kernel/mm/transparent_hugepage/enabled
USE_CLUSTER
Use MPI to distribute the search to several computers connected in a cluster.
USE_WIN7
Compile for Windows 7 and later versions. This is required to be able to take advantage of large computers that have more than 64 hardware threads.
Source code for Texel's automatic test suite is provided in the test directory.
Source code for various tools used during Texel development is provided in the app/texelutil directory. This program depends on the libraries Armadillo and GSL for full functionality.
Source code for neural network training is provided in the app/torchutil directory. This program depends on LibTorch, which is the C++ part of PyTorch.
Source code for an interactive interface to the Texel book building algorithm is provided in the app/bookgui directory. It depends on gtkmm-3.0 and probably only works in Linux.
Some utilities require access to tablebases. Set the environment variables GTBPATH and RTBPATH to specify where the tablebase files are located.
The core Texel chess engine is developed by Peter Österlund, but Texel also contains auxiliary code written by other people:
Gaviota Tablebases Probing Code, Copyright 2010 Miguel A. Ballicora. See lib/texellib/gtb/readme.txt for more information.
LZMA compression by Igor Pavlov, used by the Gaviota Tablebases Probing code.
Syzygy tablebases probing code, Copyright 2011-2013 Ronald de Man.
Chess Cases font by Matthieu Leschemelle, used by the opening book builder graphical user interface.
Google Test framework, Copyright 2008, Google Inc.