Author: Roberto Waldteufel
Date: 14:46:59 10/27/98
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On October 27, 1998 at 14:31:02, Ernst A. Heinz wrote: >On October 27, 1998 at 14:18:45, Roberto Waldteufel wrote: >> >> [...] >> >>If your code detects that one side has an unstoppable passed pawn when the >>opponent has only king and pawns left, how do you establish that the opponent >>can't do something nasty like capture one of your pawns on the 2nd rank thereby >>creating a more advanced passed pawn on the 6th rank, or even worse trapping >and >>mating your king with an onslaught of king and several pawns, aided perhaps by >>some squares being blocked by your own pawns? I use an array of bitboards >>indexed by side to move and king position (ie 128 bitmaps) which tells me which >>squares would be "unstoppable" squares for pawns. However, I only use this to >>determine trivial wins when one side has only a bare king, and when prior tests >>have determined that I do not have mating material unless I promote a pawn. I >>think the same quick "unstoppable passed pawn test" might be useful when the >>losing side has some pawns as well, but I never figured out a satisfactory way >>to tell whether the unstoppable passed pawn was enough to be sure of victory or >>not in this more complicated situation. I would be interested to know how you >>handled this. > >That's exactly why I said I deem the "square of the pawn" rule as being >too simplistic. :-) > >Actually, you only need to account for the Pawn races in your evaluation >function but *not* for possible mating scenarios which should be resolved >by the search in general. > >=Ernst= I think it get's even more complicated because even if you have an unstoppable passed pawn that can queen two or three moves before an opponent could possible promote a pawn, you could still end up with a lost position if the opponent has a mass of advanced passed pawns where you can only delay one of them queening by a series of checks that ultimately end, wherupon the opponent may have achieved a winning Q+P ending with a remaining advanced passed pawn. Nonetheless, I think maybe the old "square of the pawn" rule might make a good pruning heuristic at nodes in the main search close to the frontier, like you discuss in your excellent paper on pruning techniques at frontier, pre-frontier and pre-pre-frontier nodes. I can't help thinking that I am wasting the potential of this easily computed test by only usig it when the opponent has a bare king. Even if the side to move has many pawns and its king opposing a single enemy (unpassed) pawn as well as having the unstoppable passed pawn, my search continues to explore more plies instead of simply terminating the search and scoring it as a win, which is what I think it ought to do, if only I could think of a good way to weed out the exceptions. I once tried to score it as a win only if the opponent had no passed pawn, but this also proved inadequate because sometimes a passed pawn can be created by a breakthrough combination or simply by a king gobbling up pawns. You said you use a sophisticated K+P evaluation. I assume this understands about passed pawns and the "square of the pawn". How does it recognise when such a "trivial" win really is trivial, and when it is an exception? Best wishes, Roberto
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