How to make vc-diff output more compact

Question

I am doing C-x v = to inspect the current differences of a file under RCS control (that is, it has an associated file with ,v suffix). However, the output is barely readable because I changed every, say, 12th line. So I get these giant chunks of diffs, where I cannot easily deduce which line was changed to what other.

Is there a way to make this diff-output more readable and still usable (so that I can jump to the source rapidly)?

(I could

1. C-x v l to see the old versions, then
2. do f on the last one to check it out.
3. M-x ediff the files

but that is not very handy)

Answer 1

When running vc-diff on a file in RCS, the diff output is a "context diff" by default. That means that the old and new chunks are displayed one above the other, with the differences highlighted. More modern version control systems produce "unified diffs", where the old and new version of each group of changed lines are adjacent.

Emacs' diff-mode can convert diffs from one format to another. Type C-c C-u to convert a context diff to a unified diff, and C-c C-d to do the opposite.

For example, this context diff:

*** bar.txt 2013/11/19 14:00:03 1.1
--- bar.txt 2013/11/19 14:00:11 1.2
***************
*** 1,5 ****
  one
  two
! tree
  four
  five
--- 1,5 ----
  one
  two
! three
  four
  five

would be transformed to this unified diff:

--- bar.txt 2013/11/19 14:00:03 1.1
+++ bar.txt 2013/11/19 14:00:11 1.2
@@ -1,5 +1,5 @@
 one
 two
-tree
+three
 four
 five

Regardless of which format the diff is currently in, you can jump to the corresponding line in the source file with C-c C-c. If it doesn't find the correct file, use M-x diff-tell-file-name.

Answer 2

You might like to try and set

(setq diff-switches '("-u"))

so that your diffs come out in unified format, which is a bit more compact. If that's not sufficient, you can try and pass additional flags to reduce the size of the "context" (by default 3 lines before and 3 lines after).