Author: Ed Trice
Date: 22:56:42 11/18/05
I am wondering if any programs incorporate the concept of fractional ply search extensions as leaf nodes are encountered that are "near" the tablebases. For example, in my own program, Gothic Vortex, I have the complete 5-piece tablebase available for probing, and about 128 MB of buffers set aside for this (overkill is my middle name.) As an experiment, I sampled some random positions encountered during the search that had 6 and 7 pieces of the form: 1. Piece + Pawn vs. Same Piece + Pawn (+ kings of course) 2. Piece + Pawn vs. Same Piece + 2 Pawns 3. Piece + Pawn vs. Same Piece + Minor 4. Piece + Minor vs. Same Piece + Same Minor + 1 Pawn These leaf nodes (6 or 7 pieces only) were hit from a distance, with about 12-13 pieces on the board total (all counts including kings.) I had the program print out the position, and the score, for about 1000 random positions in each of the 4 categories shown above. I was amazed how often it would show a score of about +/- 1.3 pawns when one side was literally just a few plies from swapping into the tablebases. Even more shocking, in the 6-piece positions, the scores were +/- 0.2 pawns, and sometimes dead wrong (showing close to a draw in R + P vs. R + P when one side could win decisively!) But, since these were leaf nodes, the search was stopped, and the less accurate score was returned, and sent back down the tree. In one extreme case, a score of -1.28 was returned as a leaf node is a Pice + Pawn vs. Piece position, but when I set up that position and did a search, it announced mate in 61 in a fraction of a second (because it searched and hit the tablebases.) So, I have been instructing the program to extend the search in certain leaf nodes with 6 or 7 pieces, for the purpose of finding ways to hit the tablebases. The results so far are mixed: much more accurate principal variations are displayed in the endgames "near" the tablebases, but the tree tends to bloat by a factor of 3 (too many extensions are not a positive payoff). Even with this bloating, the time-to-depth, on average, is a 30% improvement over the non-extended searches the closer you get to the tablebases (8- and 9-piece positions, for example, are much better overall.) I was wondering if any other programs employ search extensions when in close proximity to tablebases? My tablebases page: http://www.gothicchess.com/javascript_endings.html
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