Support optional fields for statfmt option

[joelim-work]

For the statfmt option, I finally found a way to support display text depending on whether a field exists or not. The idea is to divide the format string into sections, and if any section contains a failed expansion (blank string), then the section is discarded and not shown.

Benefits:

• There is no need to have separate placeholders for the link target (%l), and the link target with an arrow (%L).
• There is no need to have a separate default for Windows just because some fields don't exist.
• Formatting/spacing can be 'tied' to a field, and if it doesn't exist then it won't show up in the final result.

In the example given in #1288, the \033[36m %l \033[0m part for displaying the link target would show as two cyan spaces if there wasn't one, which is not ideal. With optional fields supported, this can now work:

set statfmt "\033[7;31m %p \033[32m %c \033[33m %u %g \033[34m %s \033[35m %t |\033[36m %l |\033[0m" 

There are a couple of limitations now, though I don't think it will matter in practice:

• The | character cannot appear in the format string, as it now has a special meaning.
• Results of expansions now cannot contain a null byte (0x00) or a unit separator byte (0x1f) as they now have a special meaning (failed expansion, and field delimiter, respectively).

[gokcehan]

@joelim-work I don't have a strong opinion about this as I don't use this myself so feel free to merge this if you like. However, my two cents is that I'm not sure about the | character as the separator. Isn't this a common character to separate fields in status lines? (e.g. i3status) Have you considered other alternatives? On the top of my head, maybe : (to be more consistent with the rest of our options) or \t (similar to tsv files) could be used instead?

[joelim-work]

@gokcehan I guess your concern is fair, the irony is that I chose | as a separator because it is commonly used for separating fields. In particular I wanted to have the statfmt option remain fairly readable. I don't mind changing the separator to something else though, below are some alternatives for comparison:

set statfmt "\033[36m%p\033[0m| %c| %u| %g| %s| %t| -> %l"

set statfmt "\033[36m%p\033[0m|| %c|| %u|| %g|| %s|| %t|| -> %l"

set statfmt "\033[36m%p\033[0m: %c: %u: %g: %s: %t: -> %l"

set statfmt "\033[36m%p\033[0m\t %c\t %u\t %g\t %s\t %t\t -> %l"

Another alternative is to provide an escape character, for example %| to represent a literal |.

---

I'm not sure how useful the character | is for visually separating fields, in my opinion have spaces and different colors/styles do a much better job. So my personal preference for the implementation is:

1. Use | as a separator, and provide %| to encode a literal | for users that require it
2. Use || as a separator, as it is doesn't hurt readability by much, and is less likely to be required by users
3. Use another character like : or \t as a separator

I'm not sure of how to get feedback on this, we could either merge this and see what happens, or post this as a discussion first.

[gokcehan]

@joelim-work I guess I agree it wouldn't be an issue most of the time. So feel free to merge this whenever you feel like. You don't even need to add the literal escape (i.e. %|) at this point unless someone explicitly asks for it.

[joelim-work]

@gokcehan Thank, I wanted to get this change in before statfmt was officially released, otherwise it would be more difficult to change later on.

I agree that it would be better to just keep the current implementation for now, and only add %| if it becomes necessary. For reference, the implementation would look like this (just have to add a couple more replace calls):

	statfmt := gOpts.statfmt
	statfmt = strings.ReplaceAll(statfmt, "%|", "\x1a")
	statfmt = strings.ReplaceAll(statfmt, "|", "\x1f")
	replace := func(s string, val string) {
		if val == "" {
			val = "\x00"
		}
		statfmt = strings.ReplaceAll(statfmt, s, val)
	}
	replace("%p", curr.Mode().String())
	replace("%c", linkCount(curr))
	replace("%u", userName(curr))
	replace("%g", groupName(curr))
	replace("%s", humanize(curr.Size()))
	replace("%t", curr.ModTime().Format(gOpts.timefmt))
	replace("%l", curr.linkTarget)

	fileInfo := ""
	for _, section := range strings.Split(statfmt, "\x1f") {
		if !strings.Contains(section, "\x00") {
			fileInfo += section
		}
	}
	fileInfo = strings.ReplaceAll(fileInfo, "\x1a", "|")

	ui.echo(fileInfo)