Differential Testing to Compare Implementations
Replacing an implementation creates an uncomfortable testing problem. You may know that the new code should preserve existing behavior, but writing an expected result for every possible input can be expensive. A parser may accept thousands of valid forms. A pricing engine may combine many rules. A rewritten library may have years of accumulated edge cases. In these situations, an implementation you already trust can help test another implementation. Differential testing runs the same input through two or more implementations that are expected to behave equivalently, then compares their observable results. A disagreement does not automatically prove which implementation is wrong. It does something more basic and extremely useful: it gives you a concrete case that requires explanation.