Support --color-moved

[dwijnand]

Git recently added --color-moved support, to colour moved code: https://stackoverflow.com/a/48166435/463761

Would it be possible to add support for that in delta?

[dandavison]

Thanks @dwijnand, I didn't know about this feature in git. Here are some thoughts, let me know if this sounds right to you.

From a quick look at the docs and play around with the feature, it looks to me like git uses color alone to distinguish "moved" code from "changed" code: i.e. git does not, for example, use special - and + markers for moved blocks.

Now, delta cannot assume that it has access to the original color codes from git. EDIT 2021: That statement is incorrect. The colors are always available when delta is git's pager, and they are available if git is piped to delta if the user supplies --color=always So that suggests that we would have to re-implement git's algorithms for moved code detection. That in itself sounds like a blocker: it would be fragile to attempt to track the details of their algorithm.

However, if git emitted (or could be configured to emit) special +/- markers for moved code blocks then the situation would be entirely different and we could definitely have delta apply special colors to them. I do wonder whether it would make sense for git to do that. After all, without that, nothing can programmatically obtain the information from git output.

Currently delta only uses the input text and does not read anything from the git repo (i.e. delta does not use a library like libgit2, whereas e.g. bat does). So in fact delta doesn't know or care what repo the input comes from and may not even have access to that repo. I don't know if something like libgit2 might supply information about moved code blocks now or in the future.

In addition to being fragile to reimplement git's algorithms it would also be a potential perfomance hit: delta is currently implemented in a streaming fashion: as it moves through the input, it only stores a small amount of local context in memory. But for delta to do moved code detection, it seems to me that in the worst case it would have to read the diff for one file into memory and do some computation before starting to emit any output (e.g. if it encountered a - block at the beginning of the diff, it would need to read to the end of the file before knowing whether to color that as deleted or moved. I don't know how often that would result in a noticeable performance hit for delta users, but it would be a non-trivial code change. Git has the repo available to it and thus probably can do this more efficiently.

What do you think, am I understanding the situation correctly?

[dwijnand]

Thanks for the detailed write-up! I didn't spend time thinking about it too much, but I can share my thoughts based on your analysis.

Now, delta cannot assume that it has access to the original color codes from git.

What if:

1. it uses the colour codes you've separately configured in delta; or
2. it uses the .git/config/~/.gitconfig-configured colour codes, provided it can access them; or
3. it just uses git's defaults (or bombs)

?

[dandavison]

Ah-ha, I see. So currently, delta ignores any ANSI color codes in the input it receives it from git. But we could in fact inspect the color codes coming from git, and compare them to what we believe git is using for moved code, and thus figure out when we are in a special moved code block rather than a normal added/deleted block.

This wouldn't work if someone were to do git diff | delta because by default git does not write color codes into a pipe, but it would work for the most important case where delta is configured officially as the git pager via git config's core.pager variable.

Does that sound right?

[dwijnand]

Yeah, that was the setup that I had in mind. And you're right to point out the piping caveat, for the alternative usage.

[dandavison]

OK great, this seems very doable. We can always add an option to disable it / make it non-default if the color-sniffing is unreliable for some reason.

So the next question might be: what is a good way for delta to display moved code blocks? Is it simply a case of adding a third and fourth background colour? Any advice on the actual design / colour selection would be appreciated; I found it quite hard to come up with good defaults, especially for dark terminal backgrounds, and that was with just two background colours. cc @nkouevda @fdcds.

[dwijnand]

Yeah, that might be a challenge, given you're already adding both syntax highlighting and within-line highlighting... I don't have any suggestions, sorry.

[0xC0FFEE]

So the next question might be: what is a good way for delta to display moved code blocks? Is it simply a case of adding a third and fourth background colour? Any advice on the actual design / colour selection would be appreciated; I found it quite hard to come up with good defaults, especially for dark terminal backgrounds, and that was with just two background colours. cc @nkouevda @fdcds.

Choosing the right™ colors is always hard (spoiler: there will always be someone saying that color X is better). I've personally been using some variation of red/green for moved code for quite some time and I'm total used to this by now. So much, that currently this prevents me for using delta on a day to day basis :-).

Having said that, I would make the colors configurable and do not worry too much about the defaults in the beginning. IMHO defaults can be optimized after this feature has seen some real world tests and once they are good enough, the feature can be enabled by default.

Some possibilities to indicate moved lines could be:

• Make the background color configurable
• Prefix lines with some marker (e.g. +N/-N where N is the number of the moved block in this hunk)
• Maybe disable syntax highlighting for moved code and make the foreground color configurable
• Using the same background color as added/deleted lines, but draw the whole line in italics/underlined/...

[zx8]

This is the single missing feature holding me back from making the final switch to git-delta – in every other regard, git-delta seems far superior to any of the alternatives from my initial testing, but I've come to rely on quickly identifying and filtering out moved lines so much that I keep having to revert back to diff-so-fancy, solely for --color-moved support.

[dandavison]

Hey @zx8, thanks for that. I did see your comment when you made it a couple of weeks ago and I have started work on this. Hopefully there'll be something soon!