Using Hypersafety Verification for Proving Correctness of Programming Assignments

We look at the problem of deciding correctness of a programming assignment submitted by a student, with respect to a reference implementation provided by the teacher, the correctness property being output equivalence of the two programs. Typically, programming assignments are evaluated against a set of test-inputs. This checks for output equivalence, but is limited to the cases that have been tested for. One may compose the programs sequentially and assert at the end that the outputs must match. But verifying such programs is not easy; the proofs often require that the functionality of every component program be captured fully, making invariant inference a challenge. One may discover mismatches (i.e., bugs) using a bounded model checker, but their absence brings us back to the question of verification. In this paper, we show how a hypersafety verification technique can effectively be used for verifying correctness of programming assignments. This connects two seemingly unrelated problems, and opens up the possibility of employing tools and techniques being developed for the former to efficiently address the latter. We demonstrate the practicability of this approach by using a hypersafety verification tool named weaver and several sample assignment problems.