Verifying a Realistic Mutable Hash Table: Case Study (Short Paper)

In this work, we verify, using the Stainless program verifier, the mutable LongMap from the Scala standard library, a hash table using open addressing within a single array. As an executable specification, we write an immutable map based on a list of tuples and verify it against the mathematical definition of a map. 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 design 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.