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Source Update syzygy.cpp Nov 6, 2018

Monolith 1.0

This program is distributed under the GNU General Public License. Please read LICENSE for more information.

Monolith is an open source UCI-compliant chess engine written in C++11 by a not too busy medical student who prefers to spend his time messing with computer chess rather than to seek better grades. It's simply much more fun.

Monolith is not a standalone chess program, but needs a GUI in order to be used properly, for example the freely available Arena, Cute Chess or Tarrash. Big thanks go to the Chess Programming Wiki and to the equally fantastic which offer an endless source of chess-programming-wisdom and inspiration, to the CCRL group and the CEGT team for including the engine in their rating lists, and to all the open-source chess-engines out there which never fail to offer a helpful insight when all seems lost. Special thanks also go to Tom Kerrigan who provided a lot of information about his simplified ABDADA SMP-algorithm, to Ronald de Man for providing his syzygy ETB probing code, and to grzegoszwu for the picture.

Main features

  • move-generation: staged, pseudo-legal, using fancy-magic or PEXT bitboards
  • search-algorithm: alpha-beta- & principal-variation-search
  • evaluation: hand crafted, tuned automatically with the Texel-tuning-method
  • support for:
    • Universal Chess Interface (UCI) protocol
    • multiple CPU-cores through simplified-ABDADA
    • PolyGlot opening-books
    • Syzygy endgame-tablebases
    • Fischer Random Chess / Chess960


Even for the best human Grandmasters it should be very difficult to beat Monolith running even on slow hardware. Monolith 1.0 achieved about 2800 Elo on both CCRL 40/4 and CCRL 40/40.

Main changes to the previous version

  • added support for multi-processors with simplified-ABDADA
  • added support for syzygy endgame-tablebases
  • added support for PEXT instruction
  • added new search enhancements (SEE pruning, singular extension, LMR, aspiration window)
  • added new evaluation terms
  • tuned the evaluation automatically with Texel's tuning method
  • simplified the code
  • fixed a bug that caused crashes while moving backwards through the game

Which pre-compiled executable to use

  • x64_pext is the fastest, making use of the PEXT instruction which only works with recent CPUs (Intel Haswell and newer).
  • x64_popcnt is almost as fast, making use of the POPCNT instruction which is not supported by older CPUs.
  • x64 does not need modern CPU instruction sets and therefore runs a bit slower, but works also on older computers (requiring only a 64-bit architecture).
  • x86 runs also on 32-bit systems, but is considerably slower because there is no dedicated code for 32-bit instruction-sets.

Compile it yourself

Simply run make if you don't want to be bothered with optimized processor instructions.

For more options, run make [target] [ARCH=architecture] [COMP=compiler]

  • supported targets:
    • release (default): standard optimized build.
    • release_log: same build but with all engine output redirected to a log-file.
    • tune: build with enabled ability to tune the evaluation.
  • supported architectures:
    • x64 (default)
    • x64-pext
    • x64-popcnt
    • x86
  • supported compilers:
    • clang (default): Clang C++ compiler
    • gcc: GNU C++ compiler
    • icc: Intel C++ compiler

Running the Monolith bench command should result in a total of 33143855 nodes if compiled correctly.

UCI options overview

  • Threads: Specifying the number of CPU cores that can be used in parallel. Default is set to 1.
  • Ponder: Searching also during the opponents turn. Default is set to false.
  • OwnBook: Giving the engine access to a PolyGlot opening book. Default is set to true, i.e. if the book specified by Book File (see below) is found, it will be used.
  • Book File: Resetting the location in which the PolyGlot opening book is placed. Default is set to the same location as the engine, the default book name is 'monolith.bin'.
  • Hash: Specifying the size of the transposition hash table. Default is set to 128 MB.
  • Clear Hash: Clearing the hash table (to be able to start searching without being affected by previous hash entries).
  • UCI_Chess960: Playing the chess variant Fischer Random Chess / Chess960. Default is set to false.
  • MultiPV: Specifying the number of the engines best variations to be displayed in detail. Default is set to 1.
  • Contempt: Resetting the level of the internal draw score. A positive contempt value means that draws are being avoided in order to play more aggressively and risky. A negative contempt value means that draws are being pursued more likely in order to play safely and less risky. Default is set to 0.
  • Move Overhead: Adding a time buffer for every move if communications between GUI and engine are delayed, in order to avoid time losses. Default is set to 0 milliseconds.
  • SyzygyPath: Specifying the location of the folders containing the Syzygy tablebases. Default is set to . Multiple paths should be separated with a semicolon (;) on Windows and with a colon (:) on Linux.
  • SyzygyProbeDepth: Limiting the use of the tablebases to nodes which remaining depth is higher than SyzygyProbeDepth. Default is set to 5, i.e. the tablebases are used until the remaining depth of a node is 5 or smaller. A higher value is more cache-friendly and should be used if nps drop a lot because of tablebase-probing, a smaller value might lead to better play on computers where tablebases can be accessed very fast.
  • SyzygyProbeLimit: Limiting the number of pieces that have to be on the board before the tablebases are probed. Default is set to 6 which is the upper limit of pieces that Syzygy tablebases currently cover.
  • Syzygy50MoveRule: Considering the 50-move-rule while probing Syzygy tablebases. Default is set to true. Setting it to false can be useful to analyze cursed wins or blessed losses.

Additional unofficial commands

  • bench: Running an internal benchmark consisting of a fixed set of positions.
  • bench [positions.epd] [time]: Running a benchmark with positions in EPD format from an external file for the time specified in milliseconds.
  • perft: Running a performance test on some positions using legal move-generation.
  • perft pseudo: Running the same performance test using pseudo-legal move-generation.
  • eval: Outputting the detailed static evaluation of the current position.
  • board: Displaying the current chess-board.
  • summary: Displaying some statistics about the previous search.
  • tune [quiet-positions.epd]: Tuning evaluation parameters using quiet positions if compilation-target tune was enabled.

Have fun

That's what it's all about.

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