Add context diff support

[jan-matejka]

http://en.wikipedia.org/wiki/Diff#Context_format

[ymattw]

This is in my plan however not able to implement quickly, I will get back
to this soon. Thanks for support.

On Wednesday, March 13, 2013, yaccz wrote:

http://en.wikipedia.org/wiki/Diff#Context_format

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[ymattw]

Could you describe what is expected use case for context diff? I want to understand how robust the parse need to be to evaluate the ROI as the fix is much difficult than I expected, the Hunk and DiffOps abstraction need refactor.

[myint]

I just noticed filterdiff. You guys probably already know about this. But just in case you don't:

filterdiff --format=unified < context_diff | cdiff

[jan-matejka]

@ymattw unidiff is not standardized, while context diff is in posix. So I expect posix compliant projects to prefer using context diffs, like postgresql.

[ymattw]

@myint Thanks for the info, that tool make things easier.

[jan-matejka]

I would be perfectly happy with just detecting a context diff and doing filterdiff --format=unified < context | cdiff in the background

[ymattw]

Tried filterdiff with the example at http://en.wikipedia.org/wiki/Diff#Context_format, looks like it is broken. FYI someone put patchutils on github here.

$ cat 1.cdiff

*** /path/to/original   ''timestamp''
--- /path/to/new        ''timestamp''
***************
*** 1,3 ****
--- 1,9 ----
+ This is an important
+ notice! It should
+ therefore be located at
+ the beginning of this
+ document!
+
  This part of the
  document has stayed the
  same from version to
***************
*** 5,20 ****
  be shown if it doesn't
  change.  Otherwise, that
  would not be helping to
! compress the size of the
! changes.
!
! This paragraph contains
! text that is outdated.
! It will be deleted in the
! near future.

  It is important to spell
! check this dokument. On
  the other hand, a
  misspelled word isn't
  the end of the world.
--- 11,20 ----
  be shown if it doesn't
  change.  Otherwise, that
  would not be helping to
! compress anything.

  It is important to spell
! check this document. On
  the other hand, a
  misspelled word isn't
  the end of the world.
***************
*** 22,24 ****
--- 22,28 ----
  this paragraph needs to
  be changed. Things can
  be added after it.
+
+ This paragraph contains
+ important new additions
+ to this document.

$ cat 1.cdiff | filterdiff --format=unified

--- /path/to/original   ''timestamp''
+++ /path/to/new        ''timestamp''
@@ -1,3 +1,9 @@
+This is an important
+notice! It should
+therefore be located at
+the beginning of this
+document!
+ This part of the
 document has stayed the
 same from version to
@@ -5,16 +11,10 @@

[myint]

It looks like that is due to the trailing whitespace after the ! symbols being removed.

You can see that by generating a context diff from the unified diff on that same WIkipedia page.

$ filterdiff --format=context < unified_diff > generated_context
$ cat generated_context
*** /path/to/original   ''timestamp''
--- /path/to/new        ''timestamp''
***************
*** 1,3 ****
--- 1,9 ----
+ This is an important
+ notice! It should
+ therefore be located at
+ the beginning of this
+ document!
+ 
  This part of the
  document has stayed the
  same from version to
***************
*** 5,20 ****
  be shown if it doesn't
  change.  Otherwise, that
  would not be helping to
! compress the size of the
! changes.
! 
! This paragraph contains
! text that is outdated.
! It will be deleted in the
! near future.

  It is important to spell
! check this dokument. On
  the other hand, a
  misspelled word isn't
  the end of the world.
--- 11,20 ----
  be shown if it doesn't
  change.  Otherwise, that
  would not be helping to
! compress anything.

  It is important to spell
! check this document. On
  the other hand, a
  misspelled word isn't
  the end of the world.
***************
*** 22,24 ****
--- 22,28 ----
  this paragraph needs to
  be changed. Things can
  be added after it.
+ 
+ This paragraph contains
+ important new additions
+ to this document.

$ filterdiff --format=unified < generated_context 
--- /path/to/original   ''timestamp''
+++ /path/to/new        ''timestamp''
@@ -1,3 +1,9 @@
+This is an important
+notice! It should
+therefore be located at
+the beginning of this
+document!
+
 This part of the
 document has stayed the
 same from version to
@@ -5,16 +11,10 @@
 be shown if it doesn't
 change.  Otherwise, that
 would not be helping to
-compress the size of the
-changes.
-
-This paragraph contains
-text that is outdated.
-It will be deleted in the
-near future.
+compress anything.

 It is important to spell
-check this dokument. On
+check this document. On
 the other hand, a
 misspelled word isn't
 the end of the world.
@@ -22,3 +22,7 @@
 this paragraph needs to
 be changed. Things can
 be added after it.
+
+This paragraph contains
+important new additions
+to this document.

[ymattw]

@myint You rock! lines contain only a !, + or Space were all missing a trailing blank.

This tool seems installed on RHEL (at least on >= 5.6) and available in debian/ubuntu package repo, not sure other linux distros, it is also available on MacPorts. It most likely not installed by default, I am not sure invoking filterdiff in background to do the converting is a good idea, @yaccz how about do a lazy fix, detect context diff and print out guidance points to filterdiff? The context-diff branch already has the detection integrated.

[jan-matejka]

@myint nice catch.

@ymattw what's the issue with it? And how is just displaying info about filterdiff different from just running filterdiff in background besides annoying the user? I'd use it as an optional dependency.

[ymattw]

What do you expect when filterdiff does not exist?

On Tuesday, March 19, 2013, yaccz wrote:

@myint https://github.com/myint nice catch.

@ymattw https://github.com/ymattw what's the issue with it? And how is
just displaying info about filterdiff different from just running
filterdiff in background besides annoying the user? I'd use it as an
optional dependency.

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[myint]

Maybe print out a message like "Context diff support depends on filterdiff".

(subprocess.Popen() will raise an OSError if the program you execute does not exist so you can catch that.)

[jan-matejka]

@ymattw as @myint says.

[ymattw]

Sorry for late fix, too much occupied these days. I feel the fix isn't elegant, enhancement welcome. Document will be updated later.