Depth reductions Pruning in general is a way of deciding to reduce the search depth of the current line. There are many ways in which you can do this, and some of them are game specific (see below). Others are of a more general nature. The whole idea comes from the fact that most of the lines your program is looking at are absolutely ridiculous. Now, if we could only get rid of these ridiculous lines and look at the important ones instead... One very general pruning technique that is nearly guaranteed to work in most games is Michael Buro's ProbCut algorithm (and variants thereof): In ProbCut, you decide to search less deeply at some fixed remaining depth, e.g. when there are 8 ply remaining. Instead, you decide to search only 4 ply. Now, if this search returns a value far outside of your alpha-beta window you decide to believe the shallow search and return the result. If it's not too far outside of the alpha-beta window, or even inside it, you have to re-search to the full depth. There are a lot of constants you can tune in this algorithm, and you will have to experiment with it to get it up to full strength. It was used by Buro himself in his world-class Othello program Logistello, and I once implemented a version of Multi-ProbCut in my checkers program Cake. It worked much better than plain alphabeta, but not as well as my game-specific pruning algorithm. Nevertheless, ProbCut is highly interesting since it doesn't depend on the game you are playing. To me it's no surprise that it is outperformed by reduction algorithms which know something about the game - after all, these have some knowledge advantage!

See also