Games - Endgame tablebase

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	Games - Endgame tablebase

In chess, a tablebase is a database containing the win/loss status of every possible position of pieces in the endgame. Such a database acts as an oracle for the remainder of the game, providing perfect play. On the winning side, a tablebase will show how to force a win in the shortest possible way; on the losing side, a tablebase will show how to prolong loss as long as possible. Any position that is not a win or loss is a guaranteed draw.

Tablebases were generated for 6 pieces (including both kings) on the board and below. There are some 7 pieces already generated - for example KNNNN-KQ (win in 85 moves) . The 6-piece tablebases factored heavily into the analysis of Kasparov versus The World.

There are various kinds of tablebase formats widely in use named by author, including Ken Thompson, Steven J. Edwards, Eugene Nalimov tablebases. Most chess programs can use tablebases.

Using tablebases

Some computer chess operators have suggested that endgame tablebases will actually weaken performance strength in chess computers. Because some positions are analyzed as forced wins for one side, the program will avoid the losing side of positions at all costs. However, many endgames are forced wins only with flawless play, where an even slight error would produce a different result. Consequently, most modern engines will play many endgames well enough on their own. A symptom of this problem is that computers may resign too early because they see that they are being forced into a position that is theoretically dead lost (although they may be thirty or more moves away from the end of the game, and most human opponents would find it hard to win in that time).

The Nalimov tablebases do not consider the fifty move rule, under which a game where fifty moves pass without a capture or pawn move can be claimed to be a draw by either player. This results in the tablebase returning results such as "Forced mate in 66 moves" in some positions which are actually a dead draw because of the fifty move rule.

One reason for this is that if the rules of chess were to be changed once more, giving more time to win such positions, it will not be necessary to regenerate all the tablebases. It is also very easy for the program using the tablebases to notice and take account of this 'feature'.

The Nalimov tablebases, which use state-of-the-art compression techniques, require 7.05 GB of hard disk space for all five-piece endings. It is estimated that to cover all the six-piece endings will require at least 1 terabyte. It is estimated that seven-piece tablebases will require more storage capacity than will be available in the foreseeable future.

Since endgame positions are typically very simple, and with the power of desktop computers growing exponentially, most engines can calculate in endgames very effectively, making the usefulness of endgame databases questionable in light of these serious shortcomings.

Algorithm

The basic algorithm for creating a tablebase is relatively simple. Optimization and handling the "stateful" cases of en passant and castling are more complicated. (Castling is usually ignored in a tablebase, because games in practice rarely reach the endgame without a king or rook moving.)

[ Visit the complete Wikipedia entry for Endgame tablebase ]

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