Average Case - Worst Case Tradeoffs for Evacuating 2 Robots from the Disk in the Face-to-Face Model

TL;DR

This paper studies the tradeoff between average and worst case evacuation times for two robots on a disk, introducing new algorithms that optimize average case performance under worst case constraints.

Contribution

It introduces a new constrained optimization framework and develops parameterized algorithms to balance average and worst case evacuation costs.

Findings

Algorithm B2 has an average case cost of 5.1172.

Simple algorithm B1 has a very low average case cost of 1+π.

New families of algorithms are designed to optimize average case given worst case thresholds.

Abstract

The problem of evacuating two robots from the disk in the face-to-face model was first introduced in [Czyzowicz et al., DISC'14], and extensively studied (along with many variations) ever since with respect to worst case analysis. We initiate the study of the same problem with respect to average case analysis, which is also equivalent to designing randomized algorithms for the problem. First we observe that algorithm $B_{2}$ of~[Czyzowicz et al., DISC'14] with worst case cost $WRS(B_{2}):=5.73906$ has average case cost $AVG(B_{2}):=5.1172$. Then we verify that none of the algorithms that induced worst case cost improvements in subsequent publications has better average case cost, hence concluding that our problem requires the invention of new algorithms. Then, we observe that a remarkable simple algorithm, $B_{1}$, has very small average case cost $AVG(B_{1}):=1+\pi$, but very high worst case cost $WRS(B_{1}):=1+2\pi$. Motivated by the above, we introduce constrained optimization problem $_2Evac_{F2F}^w$, in which one is trying to minimize the average case cost of the evacuation algorithm given that the worst case cost does not exceed $w$. The problem is of special interest with respect to practical applications, since a common objective in search-and-rescue operations is to minimize the average completion time, given that a certain worst case threshold is not exceeded, e.g. for safety or limited energy reasons. Our main contribution is the design and analysis of families of new evacuation parameterized algorithms $A(p)$ which can solve $_2Evac_{F2F}^w$, for every $w \in [WRS(B_{1}),WRS(B_{2})]$.