Re: [PATCH v6 6/6] blame: use a fingerprint heuristic to match ignored lines

From: Michael Platings <michael@platin.gs>
Date: Apr 14, 2019, 09:41 UTC
Message-ID: <CAJDYR9RHb89mjT65XVERJfo3cTySi++ZAwOFftBtyXkqfC=JOQ@mail.gmail.com>
In-Reply-To: <xmqqk1fxw8ad.fsf@gitster-ct.c.googlers.com>

Show 5 quoted lines

>  - I wonder if the hash used here can replace what is used in
>    diffcore-delta.c as an improvement (or obviously vice versa), as
>    using two (or more) ad-hoc fingerprinting function without having
>    a clear reason why we need two instead of a unified one feels
>    like a bad idea.

Hi Junio, If I understand correctly, the algorithm in diffcore-delta.c is intended to match files that contain identical lines (or 64-byte chunks). The fingerprinting that Barret & I are talking about is intended to match lines that contain identical byte pairs. With significant refactoring, you could make the diffcore-delta algorithm apply in both cases but I think the end result would be longer and more complicated than keeping the two separate. Unlike hashing a line, hashing a byte pair is trivial. Unlike hashing lines, all except the first and last bytes are included in two "hashes" - "hello" is hashed to "he", "el", "ll", "lo". So based on my limited understanding of diffcore-delta.c I think the two are algorithms are sufficiently different in intent and in implementation that it's appropriate to keep them separate.

Regarding the "old heuristic" I think there may still be a use case for that but I'll expand on that later.

Thanks, -Michael