Approximation Algorithms for Regret-Bounded Vehicle Routing
This research explores regret-bounded vehicle routing problems (VRPs) where the focus is on minimizing client delays based on their distances from the starting depot. The study introduces a client-centric view to measure regret and devises algorithms for both additive and multiplicative regret-based VRPs, with a particular emphasis on additive regret, showcasing an O(1)-approximation algorithm for additive RVRP using LP-rounding techniques.
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Approximation Algorithms for Regret-Bounded Vehicle Routing and Applications Chaitanya Swamy University of Waterloo Joint work with Zachary Friggstad University of Alberta
Vehicle routing problems (VRPs) Metric space starting depot r client Typical setup: visit all clients via route(s) starting from depot so as to minimize client delays: e.g., max client delay (TSP) But this does not differentiate between clients close to the depot and those far away from it Nearer clients may face more delay than further-away clients source of dissatisfaction
Regret-bounded VRP Metric space starting depot r client Adopt a more client-centric view: seek bounded client regret client regret measure of waiting time of a client relative to its shortest-path distance from depot
Regret-bounded VRP cP(v) Metric space r starting depot P client v Dv cP(v) = time to reach v along P Adopt more client-centric view: ensure bounded client regret client regret measure of waiting time of a client relative to its shortest-path distance from depot Two natural ways to measure regret: additive regret of v = cP(v) Dv multiplicative regret of v = cP(v)/Dv Dv (min. possible waiting time of v)
Regret-bounded VRP cP(v) Metric space r starting depot P client v Dv Two natural ways to measure regret: additive regret of v = cP(v) Dv multiplicative regret of v = cP(v)/Dv Two problems: Given regret bound R, find minimum no. of paths rooted at r that cover all clients such that: 1. additive regret of each node R 2. multiplicative regret of each node R multiplicative RVRP additive RVRP
Both additive- and multiplicative- RVRP are NP-hard to approximate to a factor better than 2. Additive RVRP turns out to be more fundamental. (RECALL: Cover all nodes with the minimum no. of rooted paths such that additive regret of each node v R.) cP(v) Dv (where v lies on path P) Algorithms and techniques developed for additive RVRP also yield algorithms for: multiplicative RVRP and other regret-based VRPs other classical vehicle routing problems In the rest of the talk: regret additive regret, regret-related VRP VRP under additive regret RVRP additive-RVRP
Main result 30 We devise an O(1)-approx. algorithm for (additive) RVRP. Our algorithm is based on LP-rounding: contrasts with our limited understanding of LPs for VRPs (with TSP being the exception) We write a set-cover style configuration LP (with path variables): Previously only O(log n)-approximation and integrality gap was known follows easily from set-cover analysis + orienteering Our main contribution: we show how to exploit LP-structure and round an LP-solution losing only a constant factor One of the few results showing how to leverage configuration LPs (other such results are known for bin-packing, Santa Claus problem, min-makespan scheduling, combinatorial auctions) Near-optimal LP solution can be efficiently obtained: orienteering yields approximate separation oracle for dual LP
Other results Using our algorithm for RVRP and/or our techniques, we obtain: O(log(R/(R-1))-approx. for multiplicative-RVRP O(min{log D/log log D,OPTLP, log n})-approx. for distance-constrained VRP: cover all nodes with minimum no. of rooted paths s.t. waiting time cP(v) of each node v is D; improves upon the previous-best O(min{log D, log n})-guarantee (Nagarajan-Ravi) O(k2)-approx. for k-RVRP: use k paths to cover nodes and minimize max-regret; previous guarantees were only for k=1 via min-excess path (Blum et al.) also show that integrality gap of configuration LP is k
Other results (contd.) Also consider directed graphs. Observe that one can replace regret (in objective or constraint) by cost (in a different asymmetric metric) Hence, known results give: (a) O(log n)-approx. for RVRP; (b) O(k2 log n)-approx. for k-RVRP c-approx. for RVRP 2c-approx. for ATSP
Related work Additive-RVRP proposed in Operations Research literature under the generic name schoolbus problem Bock et al. studied RVRP and k-RVRP, and design: an O(log n)-approximation using set cover + orienteering a 3-approximation algorithm in tree metrics a 12.5-approximation for k-RVRP in tree metrics Additive regret also studied by Blum et al., who used excess to denote regret. They used the min-excess path problem to approximate orienteering. Nagarajan-Ravi studied distance-constrained VRP: give an O(min{log D, log n})-approx. (D=distance bound) 2-approximation in tree metrics
A useful transformation Define cregu,v := Du + cuv Dv for all u, v. creg is an asymmetric metric call this the regret metric cregr,v = 0 for all v For any path P and any v P, cregP(v) = cP(v) Dv = regret of v along P So RVRP minimize no. of rooted paths of creg-length at most R that cover all nodes distance-constrained VRP in asymmetric creg-metric c-length and creg-length of any cycle are equal
Building some intuition Suppose that all nodes were at the same distance from r V = nodes other than r r
Building some intuition Suppose that all nodes were at the same distance from r V = nodes other than r r V can be grouped into k paths, each of creg-cost = c-cost R can partition V into k components of total cost kR
Building some intuition Suppose that all nodes were at the same distance from r V = nodes other than r r V can be grouped into k paths, each of creg-cost = c-cost R can partition V into k components of total cost kR by doubling + shortcutting, get k paths of total cost 2kR Attach each path to r (pick an end-node v, add rv edge) to get a rooted path. Total creg-length of resulting paths 2kR
Building some intuition Suppose that all nodes were at the same distance from r V = nodes other than r r V can be grouped into k paths, each of creg-cost = c-cost R can partition V into k components of total cost kR by doubling + shortcutting, get k paths of total cost 2kR Attach each path to r (pick an end-node v, add rv edge) to get a rooted path. Total creg-length of resulting paths 2kR Break each resulting rooted path into segments of creg-length R and attach each segment to r (this does not increase regret) This gives 3k rooted paths covering V, each of creg-length R.
Lemma: Given at most ak paths of total creg-cost at most bkR, we can efficiently find at most (a+b)k paths, each of creg-cost R.
Lemma: Given at most ak paths of total creg-cost at most bkR, we can efficiently find at most (a+b)k paths, each of creg-cost R. So, suffices to find O(OPT) paths of total creg-length O(R.OPT). path P Dv Lemma (Blum et al.): total c-cost of red edges on P 1.5creg(P).
Configuration LP Let C(R) = {rooted paths P: cregP(v) = cP(v) Dv R for all v P} Minimize P C(R) xP s.t. P C(R): v P xP 1 v V, x 0 Dual separation problem is an orienteering problem There is a (2+ )-approximation for orienteering (Chekuri et al.) This yields an approximate separation oracle, so a (2+ )-approx. solution x* to the configuration LP can be computed efficiently. Let k* = P C(R) x*P .
Rounding the LP solution x* Easy case: suppose that directing edges of all paths P such that x*P>0 away from r gives an acyclic graph Then x* is (the path-decomposition of) an acyclic flow of value k* and creg-cost k*R that covers every node Integrality of flows + acyclicity can find k* paths of total creg-cost k*R that cover all nodes
Rounding the LP solution x* Of course, x* need not yield an acyclic flow. Form a set W of witness nodes, partition V\W into components: 1. Suitably shortcutting paths P with x*P>0 yields an acyclic flow that covers every node in W to an extent of at least 0.5 has value at most k* and creg-cost at most k*R can obtain O(k*) paths covering W of total creg-cost O(k*R) 2. The total cost of all components is O(k*R), and every component contains r or some witness node we can attach the V\W to the paths found in step 1 incurring O(k*R) total additional regret (and cost) So overall, obtain O(k*) paths with total regret O(k*R)
Rounding the LP solution x* Of course, x* need not yield an acyclic flow. Form a set W of witness nodes, partition V\W into components: 1. Shortcutting paths P with x*P>0 yields a suitable acyclic flow that covers each node in W to an extent of at least 0.5 2. The total cost of all components is O(k*R), and every component contains r or some witness node Form components whose cost can be charged to the red edges of the paths in the support of x* Can be done cleanly by setting up a network design problem with a downwards-monotone cut-requirement function Also ensures that each component contains a node with large incoming flow on blue edges, which becomes a witness node
Conclusions and open questions We systematically study regret-bounded VRPs devise a constant-approx. for additive-RVRP novel rounding method for the configuration LP; new ideas to deal with regret this yields bounds for various other RVRPs, as well as distance-constrained VRP (DVRP) Is there an O(1)-approx. for DVRP? our work is a promising step: improves upon the usual log- guarantee given by set cover, but we use additive-RVRP as a black box; better way of leveraging underlying ideas Improve upon the O(k2)-approximation for k-RVRP. What about other regret-based objectives: e.g., minimizing sum of regrets (much stronger than minimizing latency)?
