The semantic distinction is important but the end result is very similar. "Fuzzing" as it is now called (for no reason I can intuit) tries to get to the troublesome cases faster by a sort of depth-first search, but exhaustive will always beat it for value. Our exhaustive tester for bitblt, first done by John Reiser if I remember right, set the stage for my own thinking about how you properly test something.

-rob

On Thu, May 23, 2024 at 11:49 PM Douglas McIlroy <douglas.mcilroy@dartmouth.edu> wrote:

> Doug McIlroy was generating random regular expressions

Actually not. I exhaustively (within limits) tested an RE recognizer without knowingly generating any RE either mechanically or by hand.

The trick: From recursive equations (easily derived from the grammar of REs), I counted how many REs exist up to various limits on token counts, Then I generated all strings that satisfied those limits, turned the recognizer loose on them and counted how many it accepted. Any disagreement of counts revealed the existence (but not any symptom) of bugs.

Unlike most diagnostic techniques, this scheme produces a certificate of (very high odds on) correctness over a representative subdomain. The scheme also agnostically checks behavior on bad inputs as well as good. It does not, however, provide a stress test of a recognizer's capacity limits. And its exponential nature limits its applicability to rather small domains. (REs have only 5 distinct kinds of token.)

Doug