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<h4>Definition: Quiescent Search</h4> <p>This is a continuation of a discussion from the Angels and Devils page. ChessV, and all commercial Chess programs, use a trick called a quiescent search. I will attempt to describe what this is...</p> <p>Chess programs search to a given depth, but you <i>cannot</i> just stop and evaluate the position when you reach that depth. Consider this situation: the program searches to a depth of 9, and the 9th move is Queen takes Pawn. If you stop right there and evaluate, you will think you are up by a pawn. However, if you were to look one move further, you would see that the opponent's response is Pawn takes Queen! So, obviously it is bad to look at a position and conclude that you are up by a Pawn, when actually you are down by a Queen! Chess programmers refer to this as the horizon problem, and this is where the quiescent search comes in. The standard move-search considers all legal moves and counter-moves to the given depth. Then, rather than evaluating the position at that point, it enters a quiescent search which continues to search deeper. But the quiescent search only considers moves which are captures, and a good quiescent search only considers captures which win material. In this way, the quiescent search continues to play out all exchanges in progress before evaluating the position. Although the technical details of Zillions aren't publically available, I'm 99% sure that it has no capability of quiescent search. This is why sometimes, after searching deeply, it thinks it is way ahead, but after you make your response, it suddenly thinks it is way down. It has overlooked important moves beyond the search horizon.</p> <p><b>Note:</b> Consider games with drops, like Shogi and Chessgi. Quiescent search is pointless in these games because there are no quiescent (quiet) positions! You would have to consider not only captures, but also drops, and if you did that, the quiescent search would never end! You can capture and drop forever! This is why ChessV doesn't support Shogi. This is also why Zillions doesn't support quiescent search ... because if it did, it would not be able to play Shogi, Chessgi, Go, etc ... Professional Shogi programs need to use some really advanced tricks to make it work. What they generally do is this: rather than considering all possible moves, as Chess programs do, they consider only some moves, and they deeper they get, the less and less moves they consider. Which moves they consider is based on game-specific knowledge. For example, in Shogi, dropping a pawn on the square immediately in front of a Bishop is very often good, so that specific move is considered, even when the search is very deep. The problem with this approach is that it require a lot of game-specific knowledge, and tweeks of this kind which work for Shogi would not work for Chessgi (for example.) So, each game becomes a problem all of its own. When adding new games to ChessV, I try to make sure that the intelligence features I'm adding are useful for many games, not just for one game. Or, in other words, I'm picking only the low-hanging fruit.</p>