Iterating over Chunks of a Diff in Vim

Every now and then at work I’ll make broad, sweeping changes in the codebase. The one I did recently was replacing all instances of print STDERR "foo\n" with warn "foo\n". There were about 160 instances in all that I changed. After discussing more with my boss, we discussed that instead of blindly replacing all those print statements with warns (which, for those who don’t know, are easier to intercept and log) we should just log to the right log level.

Enter Quickfix

Quickfix sounds like some kind of bad guy from a slasher movie to me, but it’s actualy a super handy feature in Vim. Here’s what the manual says:

Vim has a special mode to speedup the edit-compile-edit cycle. This is inspired by the quickfix option of the Manx’s Aztec C compiler on the Amiga. The idea is to save the error messages from the compiler in a file and use Vim to jump to the errors one by one. You can examine each problem and fix it, without having to remember all the error messages.

More concretely, the quickfix commands end up giving the user a list of locations. I tend to use the quickfix list most commonly with Fugitive. You can run the command :Ggrep foo and the quickfix list will contain all of the lines that git found containing foo. Then, to iterate over those locations you can use :cnext, :cprev, :cwindow, and many others, to interact with the list.

I have wanted a way to populate the quickfix list with the locations of all of the chunks that are in the current modified files for a long time, and this week I decided to finally do it.

First off, I wrote a little tool to parse diffs and output locations:

#!/usr/bin/env perl

use strict;
use warnings;

my $filename;
my $line;
my $offset = 0;
my $printed = 0;
while (<STDIN>) {
   if (m(^\+\+\+ b/(.*)$)) {
      $printed = 0;
      $filename = $1;
   } elsif (m(^@@ -\d+(?:,\d+)? \+(\d+))) {
      $line = $1;
      $offset = 0;
      $printed = 0;
   } elsif (m(^\+(.*)$)) {
      my $data = $1 || '-';
      print "$filename:" . ($offset + $line) . ":$data\n"
         unless $printed;
      $printed = 1;
   } elsif (m(^ )) {
      $printed = 0;

The general usage is something like: git diff | diff-hunk-list, and the output will be something like:

app/lib/ZR/Plack/Middleware/  local $SIG{USR1} = sub {
bin/zr-plack-reaper:29:sub timeout { 120 }

The end result is a location for each new set of lines in a given diff. That means that deleted lines will not be included with this tool. Another tool or more options for this tool would have to be made for that functionality.

Then, I added the following to my vimrc:

command Gdiffs cexpr system('git diff \| diff-hunk-list')

So now I can simply run :Gdiffs and iterate over all of my changes, possibly tweaking them along the way!

Super Secret Bonus Content

The Quickfix is great, but there are a couple other things that I think really round out the functionality.

First: the Quickfix is global per session, so if you do :Gdiffs and then :Ggrep to refer to some other code, you’ve blown away the original quickfix list. There’s another list called the location list, which is scoped to a window. Also very useful; tends to use commands that start with l instead of c.

Second: There is another Tim Pope plugin called unimpaired which adds a ton of useful mappings; which includes [q and ]q to go back and forth in the quickfix, and [l and ]l to go back and forth in the location list. Please realize that the plugin does way more than just those two things, but I do use it for those the most.

If you’d like to learn more, I can recommend two excellent books. I first learned how to use vi from Learning the vi and Vim Editors. The new edition has a lot more information and spends more time on Vim specific features. It was helpful for me at the time, and the fundamental model of vi is still well supported in Vim and this book explores that well.

Second, if you really want to up your editing game, check out Practical Vim. It’s a very approachable book that unpacks some of the lesser used features in ways that will be clearly and immediately useful. I periodically review this book because it’s such a treasure trove of clear hints and tips.

Posted Wed, May 25, 2016