Simpler and Improved Replacement Path Coverings

2026-04-30

AI Summary

This work addresses the problem of efficiently restoring shortest paths of length at most $L$ under up to $f$ edge failures. It proposes a deterministic $(L,f)$-replacement path cover (RPC) construction based on the method of conditional expectations, eschewing complex algebraic tools such as error-correcting codes in favor of a streamlined derandomization strategy. The approach achieves a cover size of $\widetilde{O}(f L^{f+o(1)})$ and reduces query time to $\widetilde{O}(f^{5/2} L^{o(1)})$. Notably, when $f = o(\log L)$, the cover size improves to $\widetilde{O}((L/f)^f L^{o(1)})$, nearly matching the theoretical lower bound and significantly outperforming existing deterministic constructions.

Abstract

An important tool in the design of fault-tolerant graph data structures are $(L,f)$-replacement path coverings (RPCs). An RPC is a family $\mathcal{G}$ of subgraphs of a given graph $G$ such that, for every set $F$ of at most $f$ edges, there is a subfamily $\mathcal{G}_F \,{\subseteq}\, \mathcal{G}$ with the following properties. (1) No subgraph in $\mathcal{G}_F$ contains an edge of $F$. (2) For each pair of vertices $s,t$ that have a shortest path in $G-F$ with at most $L$ edges, one such path also exists in some subgraph in $\mathcal{G}_F$. The covering value of the RPC is the total number $|\mathcal{G}|$ of subgraphs. The query time is the time needed to compute the subfamily $\mathcal{G}_F$ given the set $F$. Weimann and Yuster [TALG'13] devised a randomized RPC with covering value $\widetilde{O}(fL^f)$ and query time $\widetilde{O}(f^2 L^f)$. This was derandomized by Karthik and Parter [TALG'24], who also reduced the query time to $\widetilde{O}(f^2 L)$. Their approach uses some heavy algebraic machinery involving error-correcting codes and an increased covering value of $O((cfL \log n)^{f+1})$ for some constant $c > 1$. We instead devise a much simpler derandomization via conditional expectations that lowers the covering value back to $\widetilde{O}(fL^{f+o(1)})$ and decreases the query time to $\widetilde{O}(f^{5/2}L^{o(1)})$, assuming $f = o(\log L)$. We also investigate the optimal covering value of any $(L,f)$-replacement path covering (deterministic or randomized) for different parameter ranges. We provide a new randomized construction as well as improving a known lower bound, also by Karthik and Parter. For example, for $f = o(\log L)$, we give an RPC with $\widetilde{O}( (L/f)^f L^{o(1)})$ subgraphs and show that this is tight up to the $L^{o(1)}$ term.

Problem

• replacement path covering
• fault-tolerant
• graph data structures
• derandomization
• shortest paths

Innovation

• replacement path covering
• derandomization
• conditional expectations
• fault-tolerant graph algorithms
• covering value