Fathom is a stand-alone Syzygy tablebase probing tool. The aims of Fathom are:

• To make it easy to integrate the Syzygy tablebases into existing chess engines
• To make it easy to create stand-alone applications that use the Syzygy tablebases

Fathom is compilable under either C99 or C and supports a variety of platforms, including at least Windows, Linux, and MacOS.

It is possible to build and install Fathom as a shared library on Unix-like systems. Simply run the commands

make
make install

in the top level directory.

Fathom includes a stand-alone command-line syzygy probing tool fathom. To probe a position, simply run the command:

fathom --path=<path-to-TB-files> "FEN-string"

The tool will print out a PGN representation of the probe result, including:

• Result: "1-0" (white wins), "1/2-1/2" (draw), or "0-1" (black wins)
• The Win-Draw-Loss (WDL) value for the next move: "Win", "Draw", "Loss", "CursedWin" (win but 50-move draw) or "BlessedLoss" (loss but 50-move draw)
• The Distance-To-Zero (DTZ) value (in plys) for the next move
• WinningMoves: The list of all winning moves
• DrawingMoves: The list of all drawing moves
• LosingMoves: The list of all losing moves
• A pseudo "principal variation" of Syzygy vs. Syzygy for the input position.

For more information, run the following command:

fathom --help

Fathom provides a simple API. Following are the main function calls:

• tb_init initializes the tablebases.
• tb_free releases any resources allocated by Fathom.
• tb_probe_wdl probes the Win-Draw-Loss (WDL) table for a given position.
• tb_probe_root probes the Distance-To-Zero (DTZ) table for the given position. It returns a recommended move, and also a list of unsigned integers, each one encoding a possible move and its DTZ and WDL values.
• tb_probe_root_dtz probes the Distance-To-Zero (DTZ) at the root position. It returns a score and a rank for each possible move.
• tb_probe_root_wdl probes the Win-Draw-Loss (WDL) at the root position. it returns a score and a rank for each possible move.

Fathom does not require the callee to provide any additional functionality (e.g. move generation). A simple set of chess-related functions including move generation is provided in file tbchess.c. However, chess engines can opt to replace some of this functionality for better performance (see below).

Chess engines can use tb_probe_wdl to get the WDL value during search. This function is thread safe (unless TB_NO_THREADS is set). The various "probe_root" functions are intended for probing only at the root node, but are also theread-safe.

Chess engines and other clients can modify some features of Fathom and override some of its internal functions by configuring tbconfig.h. tbconfig.h is included in Fathom's code with angle brackets. This allows a client of Fathom to override tbconfig.h by placing its own modified copy in its include path before the Fathom source directory.

One option provided by tbconfig.h is to define macros that replace some aspects of Fathom's functionality, such as calculating piece attacks, avoiding duplication of functionality. If doing this, however, be careful with including typedefs or defines from your own code into tbconfig.h, since these may clash with internal definitions used by Fathom. I recommend instead interfacing to external functions via a small module, with an interface something like this:

#ifndef _TB_ATTACK_INTERFACE
#define _TB_ATTACK_INTERFACE

#ifdef __cplusplus
#include <cstdint>
#else
#include <stdint.h>
#endif

extern tb_knight_attacks(unsigned square);
extern tb_king_attacks(unsigned square);
extern tb_root_attacks(unsigned square, uint64_t occ);
extern tb_bishop_attacks(unsigned square, uint64_t occ);
extern tb_queen_attacks(unsigned square, uint64_t occ);
extern tb_pawn_attacks(unsigned square, uint64_t occ);

#endif

You can add if wanted other function definitions such as a popcnt function based on the chess engine's native popcnt support.

tbconfig.h can then reference these functions safety because the interface depends only on types defined in standard headers. The implementation, however, can use any types from the chess engine or other client that are necessary. (A good optimizer with link-time optimization will inline the implementation code even though it is not visible in the interface).

The Syzygy tablebases were created by Ronald de Man. The original version of Fathom (https://github.com/basil00/Fathom) combined probing code from Ronald de Man, originally written for Stockfish, with chess-related functions and other support code from Basil Falcinelli. That codebase is no longer being maintained. This repository was originaly a fork of that codebase, with additional modifications by Jon Dart.

However, the current Fathom code in this repository is no longer derived directly from the probing code written for Stockfish, but instead derives from tbprobe.c, which is a component of the Cfish chess engine (https://github.com/syzygy1/Cfish), a Stockfish derivative. tbprobe.c includes 7-man tablebase support. It was written by Ronald de Man and released for unrestricted distribution and use.

This fork of Fathom replaces the Cfish board representation and move generation code used in tbprobe.c with simpler, MIT-licensed code from the original Fathom source by Basil. The code has been reorganized so that tbchess.c contains all move generation and most chess-related typedefs and functions, while tbprobe.c contains all the tablebase probing code. The code replacement and reorganization was done by Jon Dart.

This version of Fathom is released under the MIT License. See the LICENSE file for details.