Author: Sune Fischer
Date: 08:28:45 09/18/04
Go up one level in this thread
On September 18, 2004 at 11:09:26, Robert Hyatt wrote: >On September 18, 2004 at 04:05:38, Sune Fischer wrote: > >> >>I decided to try out the triangular PV thing Bob >>speaks so highly of, to see if it improves move ordering... >> >>I was careful to terminate the PV on all exact scores - of course. >>Still I was getting illegal moves in the PV. >> >>It turned out to be a hash/nullmove problem. >> >>See, in the hash I adjust the window down on UPPER bounds, ie. >>something like: >> if (flag==UPPER && score<beta) >> beta = score; > >Are you doing PVS? If so you won't do this a dozen times during a long search, >most likely. But I do this in Crafty as well, and it doesn't _ever_ get illegal >moves in the PV. I haven't looked but I guess you are doing something equivalent to what Richard suggested with beta = score+1. Small but important difference. :) >>The idea is that if we know the score is in the interval [x,y] >>then there is no reason to search with a window of [x,y+z]. >>It's a common trick I believe. >> >>Unfortunately, the search will occasionally fail high on the new beta, in my >>case it was a nullmove that failed-high. >> >>That's clearly contradictive with what the hash just told us so >>it doesn't actually make any sense, but apparently it happens anyway >>(instability?). >> >>Now the score at the previous node might be an exact value (or a fail high later >>on), and the PV will now be concatenated from an old search branch. >> > >That can't happen. You don't update the PV on a fail high, ever. Only if score >is > alpha and < beta... The PV is not updated on that node either, it's updated above at the parent node because here the FH-score will fall within the window. And of course that will result in a random clipping of the PV. This doesn't seem possible to prevent a 100%, because how do we guarantee that the new search will produce the same score as the old one when the hash has been changed meanwhile? -S.
This page took 0 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.