On some improvements to Unbounded Minimax

TL;DR

The paper presents the first systematic empirical evaluation of four modifications to Unbounded Best-First Minimax (UBFM). It conducts 22-deterministic game tests, comparing variants with transposition tables, full backpropagation, completion, and learned versus exact terminal evaluation under a fixed time budget. The results show that transposition tables, full backpropagation, and completion yield clear performance gains, while replacing exact terminal evaluation with a learned heuristic generally harms performance unless exact evaluations are costly. The findings offer practical guidance on tuning UBMF-based solvers in real-time game-playing scenarios and situate the work within the broader evolution of non-depth-limited search methods such as Athénan.

Abstract

This paper presents the first experimental evaluation of four previously untested modifications of Unbounded Best-First Minimax algorithm. This algorithm explores the game tree by iteratively expanding the most promising sequences of actions based on the current partial game tree. We first evaluate the use of transposition tables, which convert the game tree into a directed acyclic graph by merging duplicate states. Second, we compare the original algorithm by Korf & Chickering with the variant proposed by Cohen-Solal, which differs in its backpropagation strategy: instead of stopping when a stable value is encountered, it updates values up to the root. This change slightly improves performance when value ties or transposition tables are involved. Third, we assess replacing the exact terminal evaluation function with the learned heuristic function. While beneficial when exact evaluations are costly, this modification reduces performance in inexpensive settings. Finally, we examine the impact of the completion technique that prioritizes resolved winning states and avoids resolved losing states. This technique also improves performance. Overall, our findings highlight how targeted modifications can enhance the efficiency of Unbounded Best-First Minimax.