Functional Correctness of C implementations of Dijkstra's, Kruskal's, and Prim's Algorithms

Anshuman Mohan, Wei Xiang Leow, Aquinas Hobor

Keywords: separation logic, graph algorithms, VST, CompCert, Coq

Abstract: We develop machine-checked verifications of the full functional correctness of C implementations of the eponymous graph algorithms of Dijkstra, Kruskal, and Prim. We extend Wang et al.'s CertiGraph platform to reason about labels on edges, undirected graphs, and common spatial representations of edge-labeled graphs such as adjacency matrices and edge lists. We certify binary heaps, including Floyd's bottom-up heap construction, heapsort, and increase/decrease priority. Our verifications uncover subtle overflows implicit in standard textbook code, including a nontrivial bound on edge weights necessary to execute Dijkstra's algorithm; we show that the intuitive guess fails and provide a workable refinement. We observe that the common notion that Prim's algorithm requires a connected graph is wrong: we verify that a standard textbook implementation of Prim's algorithm can compute minimum spanning forests without finding components first. Our verification of Kruskal's algorithm reasons about two graphs simultaneously: the undirected graph undergoing MSF construction, and the directed graph representing the forest inside union-find. Our binary heap verification exposes precise bounds for the heap to operate correctly, avoids a subtle overflow error, and shows how to recycle keys to avoid overflow.