Re: [PATCH v7 27/30] t/lib-unicode-nfc-nfd: helper prereqs for testing unicode nfc/nfd

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: May 24, 2022, 12:14 UTC
Message-ID: <nycvar.QRO.7.76.6.2205241400200.352@tvgsbejvaqbjf.bet>
In-Reply-To: <xmqq5ylworwy.fsf@gitster.g>

Hi Junio,

On Mon, 23 May 2022, Junio C Hamano wrote:

Show 11 quoted lines

> "Jeff Hostetler via GitGitGadget" <gitgitgadget@gmail.com> writes:
>
> A devil's advocate suggestion is to go in the completely opposite
> side of the spectrum.  Perhaps if we are willing to limit the tool's
> utility to the tests done in this script file, it might be a good
> idea to combine the latter two elements in the pipeline, i.e.
>
> 	ls | test-tool hexgrep 63 5f c3 a9
>
> that exits with 0 when the output from "ls" has the 4-byte sequence,
> exits with 1 when it does not, and exits with 139 when it segfauls ;-)

I like the idea, but from what I recall of the Knuth-Pratt algorithm [*1*], the implementation might get a bit more involved than the current `test-hexdump.c`. With non-repetitive patterns like you wrote above, you can simply re-set the needle's offset to 0 if a mismatch was seen. It's partially-repetitive patterns such as `01 02 01 02 01 ff` that make things tricker: After encountering a `01 02 01 02 01`, if the next character is a `02`, we must not reset the needle's offset completely, as the next two characters might be `01 ff`, i.e. a match.

Since the purpose of this already-long, already well-iterated patch series is not necessarily to improve the test suite in such an involved manner, it should be left as an excercise for another patch series whose purpose _is_ to improve Git's test framework.

Don't get me wrong, I am very much in favor of that `hexgrep` idea. Just in its own, dedicated patch series.

Ciao, Dscho

Footnote *1*: I actually had to look at Wikipedia page at https://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm to realize that I did unjustice to James Morris by forgetting that they had discovered the algorithm independently.