🚀 Highlighting of --word-diff output, and custom "squashed" diff output

[ryanberckmans]

Does delta support diff --word-diff?

[AvnerCohen]

In a quick check - no. Would be awesome to add though.

[dandavison]

Hi @ryanberckmans (and @AvnerCohen), thanks for this! Could you expand on exactly what you'd like to see here?

In a sense Delta does provide something like --word-diff, in that it infers and highlights within-line edits (using a Levenshtein-type DP algorithm). However, I'm not saying that there wouldn't be benefit in working with git's native output, and also extending and improving the algorithms implemented by Delta (see #140).

[dandavison]

Are you thinking of replacing git's default (and ugly) {+ and [- markers with background colors? (Presumably in this mode, Delta's native within-line edit inference would be disabled).

_SUPPORTED_PYTHONS = [-['3.7',-]{+['3.8', '3.7',+} '3.6', '3.5', '3.4',[-'3.3',-] '2.7']

[AvnerCohen]

How about something like:

_SUPPORTED_PYTHONS = [['3.7',]{['3.8', '3.7',} '3.6', '3.5', '3.4',[-'3.3',-] '2.7']

With relevant color scheme applied as well ?

What would be nice is if the color is applied not across all lin, but just on the +- parts.

[dandavison]

@AvnerCohen could you comment on how you see the role of delta's default within-line edit highlighting versus --word-diff? For example, when would you use one versus the other? Do you have examples of cases where Delta's within-line edit inference is not doing a good job? (Delta's within-line edits (like GitHub, GitLab) are displayed on two different lines, unlike --word-diff which combines the added and removed on a single line.) Note that you may want to play with --max-line-distance to adjust Delta's within-line edit inference algorithm.

Again, I'm not suggesting we shouldn't support Git's --word-diff output, I just want to understand how people view those two different, but somewhat similar, possibilities.

[AvnerCohen]

@dandavison Thanks for the time and effort.
First, I have checked back and seems like I got a bit confused, I actually meant "--color-words" and not "word-diff".
That being said, I have done couple of tests, and while I remember specific cases where color-words was pretty helpful, I couldn't really find a specific case where this would be better than the correct default you suggest, with the specific part of the word highlighted, for me, it feels like it's just a matter of getting used to it.

[ryanberckmans]

@dandavison to answer your question on what I'd like to see here-

Simply put, right now delta doesn't seem to support --word-diff. The output has no or incorrect highlighting.

Here's an example

In this screenshot, I'd expect the removals and additions to be styled according to the configured delta theme. Right now they are unstyled.

Thanks!

[dandavison]

Thanks @ryanberckmans. I agree that that would make much more sense and that, whatever features delta offers, people will still send it --word-diff output and it would be good for it to display it nicely.

Out of interest, do you have any comments on when you use --word-diff versus relying on delta's within-line edit highlighting (below)?

git diff | delta --max-line-distance=0.6

[ryanberckmans]

@dandavison delta's within-line edit highlighting displays on two lines, whereas --word-diff shows "just the words that changed" on a single diff line. For example,

^ delta, uses two lines

^ --word-diff, uses one line

[lilyball]

At the very least I'd love to see --color-words and --word-diff not confuse delta quite so much.

[pilif-pilif]

@dandavison I would like to have this feature and help coding it. Could you give me some guidelines where to start?

[dandavison]

Hi @pilif-pilif, thanks for the offer! Let's start thinking about this then.

I think that we need to start off with some design thinking before getting to code changes. Basically, what exactly are we aiming for? One question is:

• Do we give users the option for the lines with --word-diff markers in them to be syntax highlighted by delta? Or do we forget about syntax highlighting and just aim to style the output appropriately?

Syntax highlighting might be quite hard: I think we would have to parse the input to reconstruct the original minus/plus lines of valid syntax in the target language, send those to the syntax highlighter, and then recombine them.

Honestly, after writing the notes below, I am starting to wonder about a different approach here: maybe we should just emit the diff --word-diff output in raw mode, so that the colors that git applies come through unchanged?

What are your initial thoughts about how you want this to behave when you are finished with it?

---

Here are some more notes. I'm just writing down my initial thoughts, so I am probably overlooking possibilities.

Here is some raw output from git --word-diff, not using delta:

We see that

1. Git has added some special markup to the code: [-removed-] and {+added+}
2. Git is also emitting ANSI color escape sequences causing these to be displayed as red and green

Now, here is the same output, but using delta:

We see that

1. Delta has removed the red and green from git
2. Delta has syntax-highlighted the code as Rust

The [- and {+ markers mean that this is not valid Rust syntax, and so the syntax-highlighting could have gone wrong. But in fact, in this case, it doesn't look too bad. In contrast, here's an example where the syntax-highlighting isn't quite what we'd want: