Episode 65 of Software Testing with Fexingo explores differential testing (a.k.a. metamorphic or diff testing) as a way to catch regressions when no oracle exists. Lucas explains how the technique works by comparing outputs of two program versions on the same input, using real-world examples like JPEG encoders. Luna pushes back on the cost of generating valid inputs and the challenge of flaky diffs. They discuss tools like AFL, LibFuzzer, and Cargo-diff, and when this method beats traditional unit or integration tests. The episode lands on a practical take: use diff testing for core algorithms where a simple transformation (re-encode, re-serialize) should yield identical results. No prior episode has covered this specific technique.

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