In this work, we verify the mutable LongMap from the Scala standard library, a hash table using open addressing within a single array, using the Stainless program verifier. As a reference implementation, we write an immutable map based on a list of tuples. We then show that LongMap's operations correspond to operations of this association list. To express the resizing of the hash table array, we introduce a new reference swapping construct in Stainless. This allows us to apply the decorator pattern without introducing aliasing. Our verification effort led us to find and fix a bug in the original implementation that manifests for large hash tables. Our performance analysis shows the verified version to be within a 1.5 factor of the original data structure.
Cite
@article{arxiv.2107.08824,
title = {Verifying a Realistic Mutable Hash Table},
author = {Samuel Chassot and Viktor Kunčak},
journal= {arXiv preprint arXiv:2107.08824},
year = {2024}
}