Author: Vasik Rajlich
Date: 10:42:56 06/20/05
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On June 20, 2005 at 08:22:17, Ricardo Gibert wrote: >On June 20, 2005 at 05:31:17, Vasik Rajlich wrote: > >... >> >>I don't understand why that would be the case. Can you explain? > >To understand the reason, it is necessary to explain no progress pruning. Here >is an illustration of no progress pruning: > >a - b - c - d - ... - e - f - g > \ \ > h i > >Let the letters a thru g represent successive moves in a line being analyzed. No >progress pruning says that if the position after move i instead of move f leads >to the same position as move instead d, then move i can be pruned from >consideration, because it could have been reached earlier. > >Where this gets you in trouble is if move h turns out to be quite a good move. >The positions after moves from d to e will not reflect this fact, since move i >was pruned. The positions after moves from d to e will get stored in the HT with >the wrong score and if a subsequent line should get a HT hit that uses these >scores, the search will get fooled as to the true value of the positions after >the moves from d to e. This is not good. > >One solution to this is to store the hash moves but not the hash scores for >positions dependent (d thru e) on a no progress prune. Another idea is to keep >the hash scores, but associate a reduced draft for the scores. Yet another idea >is to chuck no progress pruning as being more trouble than it is worth. The >latter idea is attractive, since it is not a big win. > Aaaah. Yeah, never mind what I said before, I understand now - those HT scores will be _really_ not correct. Vas >> >>The interaction of no progress pruning with HT will cause you some problems >>because the pruning is path-dependent, while the positions in the HT are not. >>You'll store in the HT a position which was later pruned (actually reduced) >>because of lack of progress, and then you'll come to that same position via a >>different path where this pruning (actually reducing) shouldn't be done. >> >>The question not everybody agrees about is how of much of a problem this is. >> >>Vas >> >...
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