Author: Zappa
Date: 09:51:10 11/22/05
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On November 22, 2005 at 12:40:02, Joshua Shriver wrote: >I did some searching to find out about bitbases and tablebases. >BB's seem really nice, though my understanding of TB's is now fuzzy. > >I thought the whole purpose of a TB was to give you the most optimal moves to >mate/draw/loss given any possible position. From what I've read, TB's just tell >you the # of moves it takes to get there. > >If you have a tablebase that says you can win in 100 moves, it's pointless >unless you know what those moves are. > >If anyone can clear this up for me I'd greatly appreciate it. > >It would seem BB + TB would make a good combination. You could load the BB into >memory for quick probing, then if you find a mate you can pull the move list off >of the TB's. > >Josh You just try all child positions and choose the one with the smallest DTM. The advantages of bitbases is that they are MUCH smaller. First, many 6-men tables have DTMs > 127, which means they need two full bytes per position. Bitbases need only 0.25 bytes. Secondly, bitbases compress MUCH better. This is easy to see, as a tablebase will look like this: +11 +9 +25 +4 0 +9 +7 -> mostly wins, but all with different distances as compared to a bitbase 11 11 11 11 01 11 11 -> and all the wins can be compressed very neatly using run-length/Huffman etc. Furthermore all the wins will tend to lie together if you order your indexing right. The complete set of 6-man Nalimov TBs are >= 2TB I think, whereas I think Vincent said his Diep bitbases were 100GB or something like that. anthony
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