Verified Null-Move Pruning

TL;DR

Verified null-move pruning enhances standard null-move pruning by reducing search tree size and improving tactical strength, especially in zugzwang positions, with minimal implementation effort.

Contribution

The paper introduces verified null-move pruning, an extension that improves search accuracy and efficiency over standard null-move pruning, particularly in zugzwang scenarios.

Findings

Constructs smaller search trees on average.

Achieves greater tactical strength.

Detects and correctly handles zugzwang positions.

Abstract

In this article we review standard null-move pruning and introduce our extended version of it, which we call verified null-move pruning. In verified null-move pruning, whenever the shallow null-move search indicates a fail-high, instead of cutting off the search from the current node, the search is continued with reduced depth. Our experiments with verified null-move pruning show that on average, it constructs a smaller search tree with greater tactical strength in comparison to standard null-move pruning. Moreover, unlike standard null-move pruning, which fails badly in zugzwang positions, verified null-move pruning manages to detect most zugzwangs and in such cases conducts a re-search to obtain the correct result. In addition, verified null-move pruning is very easy to implement, and any standard null-move pruning program can use verified null-move pruning by modifying only a few lines of code.