140 Bugs Were Hiding in One Function, and My Tests Couldn't See Any of Them

Anyone can port a library. Point a translator at the source, clean up the output, get it to compile, and you have something that looks like a port. The actual engineering problem is different and much harder: proving that the new code means the same thing as the old code, across thirty algorithms, hundreds of edge cases, and a test suite written by people who were not thinking about you. This is the story of porting textdistance , a Python library for measuring string similarity, to Rust. The result is textdistance-rs . The porting took a fraction of the time. Everything else: the differential fuzzing, the 140 divergences, the 35 year old threshold I violated, the floating-point drift at the 15th decimal place, is what this writeup is actually about. Thirty algorithms and one architectural bet The original textdistance covers a lot of ground: edit-based distances (Levenshtein, Damerau-Levenshtein, Hamming, Jaro-Winkler), token-based measures (Jaccard, Sørensen-Dice, cosine, Tversky), sequence-based methods (LCS, Ratcliff-Obershelp), phonetic algorithms (MRA, Editex), and compression-based distances built on normalized compression distance. Over thirty algorithms in total, all reimplemented in Rust. But before writing a single algorithm, I had to make the decision that shaped everything downstream: how does the existing Python test suite (397 tests I did not write) talk to the Rust code? The obvious answer is PyO3: wrap every algorithm in a #[pyclass] , build a native extension, and the Python tests import Rust directly. The answer I chose instead was a subprocess CLI. The Rust core is a standalone binary that speaks JSON over stdin/stdout, and a thin Python adapter shells out to it: // The entire cross-language surface is one struct. #[derive(Deserialize)] struct Request { algorithm : String , s1 : String , s2 : String , qval : Option < usize > , external : Option < bool > , } fn dispatch ( req : & Request ) -> Response { match req .algorithm .as_str () { "hamming"