Author: Heiner Marxen
Date: 06:43:10 11/27/99
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On November 27, 1999 at 09:22:23, blass uri wrote: >On November 27, 1999 at 08:51:43, Heiner Marxen wrote: > >>>How many nodes(legal moves) does your program search per second? >> >>Not even 20K NPS on a P/166, so CHEST is not a fast searcher. >>But at the same time it can do 1500 "mate in 2" per second. > >Simple calculation shows that it needs only 20000/1500=13.333 nodes to find if >there is a mate in 2. > >I do not understand it. >only generating all the legal moves in the first ply is more than it and I do >not understand how can you avoid it in cases when there is no mate in 2. > >Even if there is a mate in 2 and you are lucky to guess the right move by first >try if you check all the legal moves of the opponent you usually have more than >13.333 legal moves. > >I can understand saving nodes in the last ply when you need only to generate >moves that threat the king but how do you do it before the last ply? For the last ply, where a mate move is needed, I have a special move generator, which not only restricts to checking moves, but also estimates the attacks around the king, and omits moves which leave obvious escapes for the king. Then, generally the defender is eager to say check, before there is left a "mate in 2". Therefore the attacker himself is more often in check, than not. This obviously reduces the number of legal moves to consider. Then, for the defenders move inside the mate in 2 (after which we search a direct mate move), there is a tricky logic to detect a "fatal anti check". I.e. if the defender here can check, in most cases we can easily detect that no mate move can follow, since the attacker must go out of check, which severely restricts his chances to deliver a mate. As a consequence, the defender move need not be executed. I´ll stop here (there is even more). Such considerations make CHEST fast. Most of them are not exactly trivial. >Uri
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