Author: J. Wesley Cleveland
Date: 12:06:50 06/20/03
Go up one level in this thread
On June 19, 2003 at 17:10:54, Robert Hyatt wrote: >On June 19, 2003 at 08:11:07, Steve Coladonato wrote: > >>On June 18, 2003 at 20:12:32, Robert Hyatt wrote: >> >>>On June 18, 2003 at 11:30:30, Steve Coladonato wrote: >>> >>>>Dr. Hyatt, >>>> >>>>Any chance of Crafty doing backward analysis "instead of/in addition to" forward >>>>analysis? I use the chess engines mainly for game analysis, and overall Crafty >>>>tends to give lines that are not sound compared to other engines. I understand >>>>the reasoning behind forward analysis in that it tends to give more consistent >>>>results. But starting at the end of the game, where the outcome is generally >>>>known, seems to make sense if you want to cache favorable vs. unfavorable lines. >>>> >>>>Thanks. >>>> >>>>Steve >>> >> >>> >>>I started out doing it backward. But I (and others) didn't like the >>>results as it would inconsistently find good moves when the hash didn't >>>get clobbered, but there is no guarantee. >>> >>>I'll look to see if the old code is still around anywhere but I doubt >>>it. >> >>Thanks. >> >>Steve Lim's suggestion of making it one of the options doesn't sound bad. >> >>Has the technology improved to the point where the original inconsistencies >>would no longer be an issue? >> >>Steve > > >No... here is the problem. > >With the current approach (front to back) if you give it (crafty) a minute >per move, you will get consistent N-ply evaluations. IE if it can go 12 >plies deep, it will see what it should see within 12 plies, on every move, >and the comments it gives you will be from that perspective. > >If (as it did early on) it goes back to front, then you carry info back >from the endpoints toward the beginning, and you can see some things earlier >in the analysis as a result. Until the hash table can't hold such important >stuff, then suddenly the score drops or rises, and it has _nothing_ to do >with the search engine and the tree, but it changes due to an important hash >entry getting overwritten. Now when you go over the annotated output, you >see a comment that doesn't make sense within a N ply search, because it is >based on a much deeper search due to the hash entry, but at some random point >that hash information gets clobbered, and the score drops (or rises) as a >result. > >It generated lots of questions and complaints, even though I originally thought >it was an interesting approach. But now, if a 12 ply search should see >something, it does. And it won't randomly act like a 20+ ply search due to >old hash values... One simple change could eliminate nearly all the table overwrites. On a hash probe, if the value and depth gives a cutoff and the age is not current, reset the age to the current age. The advantage of backwards analysis is that you get almost twice the effictive nodes for a given amount time, as you never need to analyze the move made in the game after analyzing the end position.
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