Author: jean-christophe WEILL
Date: 04:46:07 10/23/97
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On October 23, 1997 at 02:43:12, Reinhold Gellner wrote: >Doing a zero-window search, fail high moves have to be re-searched. >Usually I do it with > >actual_value = -re_search( -beta, -actual_value ); > >However, as extensions or just move ordering changes, the re_search is >not neccessarily the same and may result in a fail low! That is, >actual_value is now below the former actual_value but >= alpha. > >I think this is a common problem. I have three solutions: > >1. ignore this fail low at all >2. do another re_search: actual_value = -re_search( -actual_value, >-alpha) >3. do the initial re_search an open window: > actual_value = - re_search( -beta, -alpha ); > >ad 1) this version may be dangerous in blitz games and of no importance >in tournament games, resulting in a speed up. .Sometimes it may help you to lose games . Not only blitz one ! if the fail low occurs just near the root. This behaviour is described in my thesis and much of the time this is a result of a heuristic cut-off(or extensions) which depends on the alpha-beta values. > >ad 2) the second search may now fail high again, or even oscillate, >resulting in an eternal loop. > >ad 3) this safe approach seems to significantly increase the effective >branching factor. > When you extend the window, normally the transposition table value should reduce it at the childs node ! Giving you no benefits for window extension. But that is the good thing to do with some special flag preventing the transposition table effect. This saves games even if this also slow down your program a little bit. BTW in 1), everyone should check its code to know which of the move is chosen as the best move, especially if you are at the root. JC
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