Author: Dann Corbit
Date: 11:44:04 04/05/04
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On April 05, 2004 at 12:54:30, rasjid chan wrote: > >I think most chess programmers implement hash tables my way, ie >by thinking it thru themselves. All the internet articles I found >about hashing only mention the the bare idea of hashing. I am not sure >whether any book on chess programming devote a full chapter on hashing >and if they do, what details do they provide. > >I remember a certain professor (Marsland) who wrote something like... >the bulk of chess programming bugs come from hash-tables.... >alpha-beta interacts with hash-tables in strange ways... > >At least they interact strangely for me. Every a little while, it seems >something new turn up and I just have to post one example, a recent >observation,to this forum to be safe that I am not way wrong ! > >I do fail soft and fails / hash outside window bounds. >If on a hash probe and the hash is EXACT and fails high, >I return hash value ( > beta). The ply below fails low with >this "exact score". > >If after seaching all moves of a node and it fails low with a best-score >< alpha, the normal hash is hash as best-score as upper bound. >But if this best-score happen to be "exact", I fail low but hash as >EXACT. > >Hope I am not missing something big ? Here are some hashing safeguards: 1. If you are in debug mode, do a full recompute of the hash and compare it to the incremental hash (both after makemove and unmakemove). If something is different you have a bug. 2. Check reversing colors -- is the hash different? 3. Check all castle states -- is every hash different? 4. Check all 16 e.p. values -- is every hash different? Check what happens when you do a null move both make and unmake
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