A Faster Alternative to SSS* with Extension to Variable Memory

The two best known and most frequently referenced minimax search methods for game trees are Alpha-Beta [2] and SSS* [6]. The algorithms are quite dissimilar in structure and properties. Alpha-Beta is a depth-first recursive procedure that needs little memory to execute. Although it evaluates more terminal nodes than SSS*, it generally runs considerably faster owing to its low overhead. In contrast, SSS* is a non-recursive procedure similar in many ways to the best-first search algorithm A*. Although SSS* is superior to Alpha-Beta in number of terminals evaluated, SSS* is seldom used in practice. This is because the improved pruning power of SSS” is more than offset by (see [4]) (a> the enormous storage requirement for the global OPEN list, which is brd/‘l units for a (6, d) uniform tree; (b) the overhead of maintaining OPEN sorted on h-values; (c) the overhead of occasionally purging nodes from OPEN belonging to provably suboptimal solution trees.