Author: Tom Likens
Date: 12:43:29 06/16/04
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On June 16, 2004 at 10:54:18, Jaime Benito de Valle Ruiz wrote: >I'm re-writting the eval code to give consider different weights at different >stages of the game... which is something I should have tried earlier. I've even >considered having a late-midgame stage. > >I've tried a few methods for differenciating these stages of the game, all of >them based on counting pieces on the board, but I suspect it has more little to >do with the number of pawns, and it might not be so simple. > >Can anyone give me some suggestions on this? >Thanks, > > Jaime Hello Jaime, In Djinn I scale a number of the evaluation terms based on the amount of material left, excluding pawns. If you give the pieces pawn equivalent values of 3, 3, 5 and 9 (used only for indexing purposes of course) then the index can range from 0 to 31. I keep track of this number incrementally during the search so it is always available. To use it I have a number of 32 entry floating point arrays with names like "Scale3to1", "Scale1to6" etc., (I wrote a C program to generate all the arrays and the floating point values). These arrays scale linearly from 3.00 to 1.00 across 32 values or 1.00 to 6.00 and so forth. I also scale the values for white and black independently depending on the material for each side. One caveat, with using this method- often a score will be scaled depending on the amount of material the *opponent* has (in fact, this is often the case). A good example is king safety, if your opponent has little material left then it may be okay to bring your king into the fray, regardless of how much material you have left. Once you start down this road, it's amazing how many places it can be applied. I use this idea to scale passed pawns, king attacks, center control etc. Another option, which I don't currently use, is to blend (i.e. interpolate) the final score from the middlegame evaluation and the endgame evaluation. I believe Phanlanx, Gothmog and perhaps Fruit all use this technique so you might inquire from them how they apply it. I've always shied away from this idea because it seemed computationally expense, but perhaps not. regards, --tom
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