Transpositions and move groups in Monte Carlo tree search

Monte Carlo search, and specifically the UCT (Upper Confidence Bounds applied to Trees) algorithm, has contributed to a significant improvement in the game of Go and has received considerable attention in other applications. This article investigates two enhancements to the UCT algorithm. First, we consider the possible adjustments to UCT when the search tree is treated as a graph (and information amongst transpositions are shared). The second modification introduces move groupings, which may reduce the effective branching factor. Experiments with both enhancements were performed using artificial trees and in the game of Go. From the experimental results we conclude that both exploiting the graph structure and grouping moves may contribute to an increase in the playing strength of game programs using UCT.