texel

Texel is a UCI Chess Engine written by Peter Österlund

Introduction

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:

http://chessprogramming.wikispaces.com/Texel

UCI options

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).

Threads

Texel can use multiple CPUs and cores using threads. Up to 64 threads are supported.

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.

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.

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.

Tablebases

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.

NUMA

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. To compile a NUMA-aware linux version, uncomment the FLAGS and LDFLAGS lines in the Makefile and run "make texel64".

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.

Compiling

The distribution contains a Makefile set up to compile the program using the GCC compiler.

To build a generic executable that does not require any special CPU instructions, type "make" in a terminal window.

To build a 64-bit linux executable with POPCNT support, type "make texel64" in a terminal window.

To build a 32-bit linux executable with POPCNT support, type "make texel32" in a terminal window.

There are other targets in the Makefile that can be used to build versions optimized for Intel CPUs and versions using the POPCNT CPU instruction. See the Makefile for details.

Additional source code

Source code for Texel's automatic test suite is provided in the test directory. The test program can be compiled by typing "make texeltest" in a terminal window.

Source code for various tools used during Texel development is provided in the util directory. The utility program can be compiled by typing "make texelutil" in a terminal window. Note that this program uses OpenMP and depends on the libraries armadillo and gsl. It may not work unmodified in Windows.

Source code for an interactive interface to the texel book building algorithm is provided in the bookgui directory. It depends on gtkmm and can probably only be compiled in linux.

Copyright

The core Texel chess engine is developed by Peter Österlund, but Texel also contains auxiliary code written by other people:

LZMA compression by Igor Pavlov, used by the Gaviota Tablebases Probing code.

Chess Cases font by Matthieu Leschemelle, used by the opening book builder graphical user interface.

CUTE unit testing framework, Copyright Peter Sommerlad and Emanuel Graf.

B4dT0bi / texel

readme