Re: [PATCH 2/2] diff: teach diff to read gitattribute diff-algorithm

From: Jeff King <peff@peff.net>
Date: Feb 11, 2023, 01:59 UTC
Message-ID: <Y+b2l4Le2gTxGwO8@coredump.intra.peff.net>
In-Reply-To: <1ddac91b-7552-3e1e-9888-9e21e808104d@dunelm.org.uk>

On Thu, Feb 09, 2023 at 02:44:15PM +0000, Phillip Wood wrote:

Show 20 quoted lines

> To see the differences between the output of patience and histogram
> algorithms I diffed the output of "git log -p --no-merges
> --diff-algorithm=patience" and "git log -p --no-merges
> --diff-algorithm=histogram". The first three differences are
> 
> - 6c065f72b8 (http: support CURLOPT_PROTOCOLS_STR, 2023-01-16)
>   In get_curl_allowed_protocols() the patience algorithm shows the
>   change in the return statement more clearly
> 
> - 47cfc9bd7d (attr: add flag `--source` to work with tree-ish, 2023-01-14)
>    The histogram algorithm shows read_attr_from_index() being moved
>    whereas the patience algorithm does not making the diff easier to
>    follow.
> 
> - b0226007f0 (fsmonitor: eliminate call to deprecated FSEventStream
> function, 2022-12-14)
>   In fsm_listen__stop_async() the histogram algorithm shows
>   data->shutdown_style = SHUTDOWN_EVENT;
>   being moved, which is not as clear as the patience output which
>   shows it as a context line.

Just a small counter-point, since I happened to be looking at myers vs patience for something elsewhere in the thread, but:

  git show 35bd13fcd2caa4185bf3729655ca20b6a5fe9b6f builtin/add.c

looks slightly better to me with myers, even though it is 2 lines longer. The issue is that patience and histogram are very eager to use blank lines as anchor points, so a diff like:

  -some words
  -
  -and some more
  +unrelated content
  +
  +but it happens to also be two paragraphs

in myers becomes:

  -some words
  +unrelated content
  
  -and some more
  +but it happens to also be two paragraphs

in patience (here I'm using single lines, but in practice these may be paragraphs, or stanzas of code). I think that's also the _strength_ of patience in many cases, but it really depends on the content. Replacing a multi-stanza block with another one may be the best explanation for what happened. Or the two stanzas may be independent, and showing the change for each one may be better.

I'm not sure which one happens more often. And you'd probably want to weight it by how good/bad the change is. In the example I showed I don't find patience very much worse, since it's already a pretty ugly diff. But in cases where patience shines, it may be making things significantly more readable.

I don't have a super strong opinion, but I just wanted to chime in that it is not clear to me that patience/histogram is always a win over myers (yes, I know your examples were comparing patience vs histogram, but the larger thread is discussing the other).

-Peff