Sublinear-Time Reconfiguration of Programmable Matter with Joint Movements

TL;DR

This paper presents sublinear-time algorithms for reconfiguring geometric amoebot structures using joint movements, demonstrating efficient parallel reconfiguration without additional assumptions.

Contribution

It proves the possibility of sublinear-time universal reconfiguration in the joint movement model, including a $O(\sqrt{n}\log n)$ algorithm and a constant-time spiral-to-line reconfiguration.

Findings

Reconfiguration into a line segment can be achieved in $O(\sqrt{n}\log n)$ rounds.

A constant-time reconfiguration algorithm exists for spirals to lines.

Joint movement enables sublinear reconfiguration without extra assumptions.

Abstract

We study centralized reconfiguration problems for geometric amoebot structures. A set of $n$ amoebots occupy nodes on the triangular grid and can reconfigure via expansion and contraction operations. We focus on the joint movement extension, where amoebots may expand and contract in parallel, enabling coordinated motion of larger substructures. Prior work introduced this extension and analyzed reconfiguration under additional assumptions such as metamodules. In contrast, we investigate the intrinsic dynamics of reconfiguration without such assumptions by restricting attention to centralized algorithms, leaving distributed solutions for future work. We study the reconfiguration problem between two classes of amoebot structures $A$ and $B$: For every structure $S\in A$, the goal is to compute a schedule that reconfigures $S$ into some structure $S'\in B$. Our focus is on sublinear-time algorithms. We affirmatively answer the open problem by Padalkin et al. (Auton. Robots, 2025) whether a within-the-model sublinear-time universal reconfiguration algorithm is possible, by proving that any structure can be reconfigured into a canonical line-segment structure in $O(\sqrt{n}\log n)$ rounds. Additionally, we give a constant-time algorithm for reconfiguring any spiral structure into a line segment. These results are enabled by new constant-time primitives that facilitate efficient parallel movement. Our findings demonstrate that the joint movement model supports sublinear reconfiguration without auxiliary assumptions. A central open question is whether universal reconfiguration within this model can be achieved in polylogarithmic or even constant time.