uniq: adding support for "-b, --ignore-leading-blanks"

[Todd Lewis]

Hey, folks,

I've spent a most enjoyable few hours implementing 
`--ignore-leading-blanks` in `uniq`, including tests and texinfo 
updates. It doesn't break any other features, and it solves the problem 
that made me start playing with this in the first place.I was surprised 
at how little code was necessary to implement `--ignore-leading-blanks`, 
and how minimally invasive it was. `vc-dwim` is happy with the 
ChangeLog, and I have the output from `git format-patch --stdout -1` all 
ready to send in and stored as a draft email.

But I'm not going to submit it, because it's the wrong solution.But by 
itself, it just doesn't advance the cause. The workaround indicated in
https://www.pixelbeat.org/docs/coreutils-gotchas.html#uniq
i.e. filtering the input through `tr -s '[:blank:]'` first,
is better, if only because it deals with all blanks equitably, whereas 
my `--ignore-leading-blanks` only ignores the leading blanks of the 
first unskipped field (normally the beginning of the line). But once you 
start dealing with skipped blanks or fields, other blanks start causing 
unwelcome surprises.

The trouble with using `tr -s '[:blank:]'` as a pre-filter is that 
you've now screwed up the nice columns you probably had, and which you 
probably want to preserve, especially if you're using the `--group` option.

Fun though it's been, the right answer, as stated in 
https://www.gnu.org/software/coreutils/rejected_requests.html#uniq, is 
`uniq --key` and friends 
(https://lists.gnu.org/archive/html/bug-coreutils/2006-06/msg00211.html). 
That however is a much bigger lift.

That discussion from 2006 talks about code sharing between `uniq` and 
`sort`, which sounds great, but at first blush uniq seem to take a more 
nuanced approach to character encodings than does `sort`. To be fair, I 
haven't dug into the `sort` code yet to the degree necessary to justify 
such sweeping statements. My feeling is that if `--key` is implemented 
right, then the other features on that 2006 wish list should follow 
fairly naturally.

If there's still interest in having sort-like `--key` capabilities in 
`uniq`, I may begin studying this more seriously. Before I do, though, 
I'd like to gauge the likelihood of such work being accepted – assuming 
a clean, working implementation of course.

--
Todd Lewis<utoddl@gmail.com>