Command line option to apply line number style to unchanged lines

[clnoll]

Adds --number-zero-style option that overrides --number-minus-style and --number-plus-style on unchanged lines if --number is set.

This gives the ability to draw attention to line numbers for changed lines by deemphasizing the numbers for unchanged lines, e.g.:

[dandavison]

Thanks @clnoll! That screenshot is very compelling. I agree with this plan and I think I'd have realized soon that I wanted this also.

The option name --number-zero-style is making me pause and think we should double-check our naming scheme. It seems clear that delta is going to have lots of options, and I'm thinking that the only way to manage that complexity is going to be pedantically consistent naming...

So, let me have a go at justifying to myself what we have:

When it comes to hunk lines, we're using the terms minus, zero, and plus (e.g. --minus-style, --zero-style, etc). The implied definitions seem to be

• minus: a line that occurred in the old file only
• zero: a line that occurs in both files
• plus: a line that occurs in the new file only

Then, from #190, we have --number-minus-* and --number-plus-*. There the definitions seem to be

• minus: line numbers in the old file (they go on the left)
• plus: line numbers in the new file (they go on the right)

So, I think I'm happy with all that. Even though the identified sections are horizontal in the first case and vertical in the second, it seems to be consistent with the basic idea that there's a "minus file" (old file) and a "plus file" (new file).

When it comes to line number display, it seems we have 2 dimensions and a maximum of 6 things that could have names and distinct styles:

| X_11 | X_21 | minus line
| X_12 | X_22 | zero line
| X_13 | X_23 | plus line

So it's like we're using --number-minus-* to refer to the left column X_1* and --number-plus-* to refer to the right column, i.e. X_2*.

And finally, in this PR, we want to choose a name for X_12 and X_22...

What do you think? The only thing I've come up with so far is to add two new options

• --number-minus-zero-style
• --number-plus-zero-style

That sort of makes sense as representing indexing into a 2D array, with the first index identifying the column? And leaves open the possibility of adding options for the other things with logical names if that were ever desired (--number-minus-minus-style, etc).

Or is there another way of looking at it that yields a simpler set of names?

There are a couple of related thoughts that aren't really the concern of this PR but I'll mention anyway:

• It might be getting necessary to organize the --help text so that the commonly used options are not lost in the sea of all possible options.

• We could maybe do with better ways to express the idea of one option defaulting to the value of another option?

[clnoll]

Thanks for the suggestions, I agree re the inconsistency of row vs. column.

I have a slightly different proposal, now that I've played around with the options a bit.

The first version of the numbers options conflates the concept of minus/left and plus/right, because it colors all numbers in a column, even if the row that the number is associated with was removed or added (aka a zero-row).

I think we should alter the behavior of --number-minus-style and --number-plus-style to apply row-wise. That is, only apply the style if the line was actually removed/added. That leaves the obvious gap for the row-based --number-zero-style, which will have behavior from this PR.

We can then add separate options for styling by entire column (--number-left-style and --number-right-style). I think that if the minus/plus styles are also set they should override the column setting for their respective rows.

The last changes I'd propose would be to have the format strings & corresponding styles be left/right column-based (--number-left-format* and --number-right-format* instead of --number-minus-format* and --number-plus-format*), and instead of having the minus/plus line numbers be determined by which column you're in, we can have separate format string placeholders %lm and %lp, so if someone wanted to swap them that would be possible.

[clnoll]

BTW I'd love to make the placeholders %nm and %np but git pretty-formats use %n for new lines, and I don't want to clash with such a generalized placeholder. (I also noticed that if they use a letter in the 1-letter placeholder, they don't use that letter in any the 2-letter placeholders, and it seems worthwhile to stay consistent with their conventions in case they become useful in other areas).

That being said, let me know if you'd prefer to use something else for the placeholders.

[dandavison]

Thanks for thinking about this @clnoll! I think your proposal is better than mine: I agree with everything you wrote in your last two comments.

[dandavison]

This is looking great. I've pushed a commit that starts adding (high-level) tests for this branch. 3 are passing and one is failing.

I've added the tests in a new file features/numbers.rs. At some point we can move all the number-specific code into that file and make the implementation follow the same pattern as other "features". Incidentally, what do you think about the singular vs plural name? It seems that based on other common command line tools, a flag with the singular name --number should exist. But as a feature, the singular version seems unnatural. Maybe we need to support both as synonyms?

[dandavison]

Thanks again @clnoll! I've added some tests and changed the command line interface as discussed.

    -n, --line-numbers               Display line numbers next to the diff. See LINE NUMBERS section

    --line-numbers-minus-style <line-numbers-minus-style>
        Style (foreground, background, attributes) for line numbers in the old (minus) version of the file. See
        STYLES and LINE NUMBERS sections [default: auto]
    --line-numbers-zero-style <line-numbers-zero-style>
        Style (foreground, background, attributes) for line numbers in unchanged (zero) lines. See STYLES and LINE
        NUMBERS sections [default: auto]
    --line-numbers-plus-style <line-numbers-plus-style>
        Style (foreground, background, attributes) for line numbers in the new (plus) version of the file. See
        STYLES and LINE NUMBERS sections [default: auto]
    --line-numbers-left-format <line-numbers-left-format>
        Format string for the left column of line numbers. A typical value would be "{nm:^4}⋮" which means to
        display the line numbers of the minus file (old version), followed by a dividing character. See the LINE
        NUMBERS section [default: {nm:^4}⋮]
    --line-numbers-right-format <line-numbers-right-format>
        Format string for the right column of line numbers. A typical value would be "{np:^4}│ " which means to
        display the line numbers of the plus file (new version), followed by a dividing character, and a space. See
        the LINE NUMBERS section [default: {np:^4}│ ]
    --line-numbers-left-style <line-numbers-left-style>
        Style (foreground, background, attributes) for the left column of line numbers. See STYLES and LINE NUMBERS
        sections [default: auto]
    --line-numbers-right-style <line-numbers-right-style>
        Style (foreground, background, attributes) for the right column of line numbers. See STYLES and LINE NUMBERS
        sections [default: auto]