Syzygy TableBase

Syzygybases
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Syzygy structure tablebases

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Configuration

Syzygybases are configured using the UCI options "SyzygyProbeLimit" and "SyzygyPath".

The option "SyzygyPath" should be set to the directory or directories where the .rtbw and .rtbz files can be found. Multiple directories should be separated by ";" on Windows and by ":" on Unix-based operating systems.

Example:

Code:

C:\tablebases\wdl345;C:\tablebases\wdl6;D:\tablebases\dtz345;D:\tablebases\dtz6

It is recommended to store .rtbw files on an SSD. There is no loss in storing the .rtbz files on a regular HD.

Note: At the moment, the "SyzygyPath" option can only be set once. If you want to change it, you need to restart the engine.

If you have the 6-piece tables, set the value of "SyzygyProbeLimit" to 6 (the default). If you only have the 5-piece table, set it to 5. Set the value of this option to 0 if you want to temporarily disable tablebase probing.

Working mode
If the engine is searching a position that is not in the tablebases (e.g. a position with 7 pieces), it will access the tablebases during the search. If the engine reports a large mate score, this means that it has found a winning line into a tablebase position. Example: mate in 60 means 10 moves into a winning tablebase position.

If the engine is given a position to search that is in the tablebases, it will use the tablebases at the beginning of the search to preselect all good moves, i.e. all moves that preserve the win or preserve the draw while taking into account the 50-move rule. It wil then perform a search only on those moves. The engine will not move immediately, unless there is only a single good move. The engine might not report a mate score even when the position is won. Instead, it reports the score that is returned by the search.

It is therefore clear that behaviour is not identical to what one might be used to with Nalimov tablebases. There are technical reasons for this difference, the main technical reason being that Nalimov tablebases use the DTM metric (distance-to-mate), while Syzygy bases use a variation of the DTZ metric (distance-to-zero, zero meaning any move that resets the 50-move counter). This special metric is one of the reasons that Syzygybases are more compact than Nalimov tablebases, while still storing all information needed for optimal play and in addition being able to take into account the 50-move rule.

In the near future an option will be added to switch between the current behaviour and a mode in which Stockfish will immediately play one of the good moves. This new mode will have the problem that it leads to unnatural play once the engine has reached a tablebase position. Example: the engine will then prefer any winning pawn move (even those that lose material and complicate the win) over moves that lead to a quick mate but have a higher "distance-to-zero" value.