blag/post/splitting-overly-large-hunks-in-patches.mdwn

   1 [[!meta title="Splitting overly large hunks in patches"]]
   2 [[!meta author="rohieb"]]
   3 [[!meta license="CC-BY-SA 3.0"]]
   4 [[!img defaults size=x200]]
   5
   6 Today I stumbled over a lengthy patch on my harddisk. It was about half a year
   7 old, and consisted of only one hunk, which was about 1000 lines in length. Most
   8 of the contents were indentation changes from tabs to spaces, but I knew that
   9 the patch contained a small useful portion, which I wanted to extract. What was
  10 slightly more annoying was the fact the the patch did not apply cleanly to the
  11 file it was supposed to change, and `patch` only applies hunks atomically, the
  12 whole patch was rejected.
  13
  14 Since I did not want to compare each of the lines in the patch visually and
  15 decide whether they changed only whitespace, I tried to look for a way to split
  16 the patch into smaller hunks. My first try was looking at the useful tool in the
  17 [patchutils](http://cyberelk.net/tim/software/patchutils/) package, but none of
  18 them did what I wanted, they only allowed me to split patches into single hunks
  19 (but my patch already had only one hunk).
  20
  21 But after a bit of googling, I found out that Emacs has a
  22 [`diff-split-hunk`][emacs-dsh] command, so I installed Emacs (for the first time
  23 in my life), opened my patch, selected Emacs' Diff mode with `M-x diff-mode`,
  24 and split the patch into smaller hunks by pressing `C-x C-s` on appropriate
  25 context lines. After saving, the patch applied cleanly except for two smaller
  26 hunks, which I could easily identify as containing only whitespace changes. Then
  27 I could compare my patched file with the original file, this time ignoring
  28 whitespace changes with `diff -w`, and, voilà, I got the seven useful lines I
  29 wanted.
  30
  31 [emacs-dsh]: https://www.gnu.org/software/emacs/manual/html_node/emacs/Diff-Mode.html
  32
  33 For illustration, see the different [[edit stages of my patch|edit-stages]] on a
  34 separate page.
  35
  36 [[!tag patch patchutils diff GNU Emacs QDjango C++ howto]]