The Single-Face Ideal Orientation Problem in Planar Graphs

TL;DR

This paper studies the ideal orientation problem in planar graphs, proving NP-hardness in general but providing polynomial-time solutions under specific conditions, and introduces algorithms for serial instances and a distance minimization variant.

Contribution

It establishes complexity results for the ideal orientation problem in planar graphs and offers efficient algorithms for fixed, serial, and minimized-distance cases.

Findings

NP-hardness in general planar graphs

Polynomial-time solvability for fixed, non-crossing terminal pairs on the same face

Efficient algorithms for serial instances and distance minimization

Abstract

We consider the ideal orientation problem in planar graphs. In this problem, we are given an undirected graph $G$ with positive edge lengths and $k$ pairs of distinct vertices $(s_1, t_1), \dots, (s_k, t_k)$ called terminals, and we want to assign an orientation to each edge such that for all $i$ the distance from $s_i$ to $t_i$ is preserved or report that no such orientation exists. We show that the problem is NP-hard in planar graphs. On the other hand, we show that the problem is polynomial-time solvable in planar graphs when $k$ is fixed, the vertices $s_1, t_1, \dots, s_k, t_k$ are all on the same face, and no two of terminal pairs cross (a pair $(s_i, t_i)$ crosses $(s_j, t_j)$ if the cyclic order of the vertices is $s_i,s_j,t_i,t_j$). For serial instances, we give a simpler and faster algorithm running in $O(n \log n)$ time, even if $k$ is part of the input. (An instance is serial if the terminals appear in cyclic order $u_1, v_1, \dots, u_k, v_k$, where for each $i$ we have either $(u_i, v_i) = (s_i, t_i)$ or $(u_i, v_i) = (t_i, s_i)$.) Finally, we consider a generalization of the problem in which the sum of the distances from $s_i$ to $t_i$ is to be minimized; in this case we give an algorithm for serial instances running in $O(kn^5)$ time.