New Bounds for Old Algorithms: On the Average-Case Behavior of Classic Single-Source Shortest-Paths Approaches

Despite disillusioning worst-case behavior, classic algorithms for single-source shortest-paths (SSSP) like Bellman-Ford are still being used in practice, especially due to their simple data structures. However, surprisingly little is known about the average-case complexity of these approaches. We provide new theoretical and experimental results for the performance of classic label-correcting SSSP algorithms on graph classes with non-negative random edge weights. In particular, we prove a tight lower bound of Omega(n(2)) for the running times of Bellman-Ford on a class of sparse graphs with O(n) nodes and edges; the best previous bound was Omega(n(4/3-epsilon)). The same improvements are shown for Pallottino's algorithm. We also lift a lower bound for the approximate bucket implementation of Dijkstra's algorithm from Omega(n log n/log log n) to Omega(n(1.2-epsilon)). Furthermore, we provide an experimental evaluation of our new graph classes in comparison with previously used test inputs.