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This item is a computer program
It belongs to categories: Orthodox chess, 
It was last modified on: 2020-01-23
 Author: Greg  Strong. ChessVThis item is a computer program
It belongs to categories: Orthodox chess, 
It was last modified on: 2020-01-23
 Author: Greg  Strong.. Missing description[All Comments] [Add Comment or Rating]
H. G. Muller wrote on 2020-11-26 UTC

I found the following search scheme to work quite well in games with drops: you distingush check-drops from other drops, and make a special move generator to selectively generate those (by generating retro moves for all piece types, starting from the King). At d=1 the only drops searched are the check drops; at d>=2 LMR reduces non-captures by 1 ply, and (non-check) drops by 2 ply.

In CrazyWa I used a King Safety evaluation that seemed to work quite well generically across many drop variants. It was kind of unusual by being asymmetric. It awardeds a bonus to the side to move which is the product of the hand value and the number of squares attacked next to the enemy King (the possible drop locations). The latter could be cheaply calculated as side effect of the move generation, while the hand value can be kept track of incrementally. The idea was that in games with drops 'initiative' is all important, and the side that has the initiative typically checks the opponent all the time (mostly through drops, and captures with the slider it just dropped when evasion was an interposition). The side that is in check will never evaluate, but extend for the evasion even at d=0. So the leaves typically have the side on move that has the initiative, and this player usually doesn't have to worry much about his own King Safety.

Xiangqi chasing rules indeed are very annoying; especially the fact that they are based on legal attacks and protects means that you have to search 3 ply ahead to judge them: one to play the attacks that are  chase candidates as a capture, then the possible pseudo-legal recaptures, and finally the potential King captures resulting from these. And you have to do that for every position in the repeat loop. Always for both sides, because they could be mutually chasing, and then it is a draw again.

It can be acceptable to only detect chases for repeats that have a certain minimum search depth left (say 4 ply).