[PATCH] diff-format.txt: Combined diff format documentation supplement

[Jakub Narebski <jnareb@gmail.com>]

Update example combined diff format to the current version
$ git diff-tree -p -c fec9ebf16c948bcb4a8b88d0173ee63584bcde76
and provide complete first chunk in example.

Document combined diff format headers: how "diff header" look like,
which of "extended diff headers" are used with combined diff and how
they look like, differences in two-line from-file/to-file header from
non-combined diff format, chunk header format.

It should be noted that combined diff format was designed for quick
_content_ inspection and renames would work correctly to pick which
blobs from each tree to compare but otherwise not reflected in the
output (the pathnames are not shown).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
---
Junio C Hamano napisał:
> Patches to documentation would be easier to comment on and more
> productive, I guess.

So here you have. It should perhaps get review on validity by someone
well versed in the combined diff generation code. There are some guesses
here...

It compiles, but the output was not inspected.


 Documentation/diff-format.txt |   70 ++++++++++++++++++++++++++++++++++++++---
 1 files changed, 65 insertions(+), 5 deletions(-)

diff --git a/Documentation/diff-format.txt b/Documentation/diff-format.txt
index 617d8f5..0d04b03 100644
--- a/Documentation/diff-format.txt
+++ b/Documentation/diff-format.txt
@@ -156,18 +156,78 @@ to produce 'combined diff', which looks 
 
 ------------
 diff --combined describe.c
-@@@ +98,7 @@@
-   return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
+index fabadb8,cc95eb0..4866510
+--- a/describe.c
++++ b/describe.c
+@@@ -98,20 -98,12 +98,20 @@@
+  	return (a_date > b_date) ? -1 : (a_date == b_date) ? 0 : 1;
   }
-
+  
 - static void describe(char *arg)
  -static void describe(struct commit *cmit, int last_one)
 ++static void describe(char *arg, int last_one)
   {
- +     unsigned char sha1[20];
- +     struct commit *cmit;
+ +	unsigned char sha1[20];
+ +	struct commit *cmit;
+  	struct commit_list *list;
+  	static int initialized = 0;
+  	struct commit_name *n;
+  
+ +	if (get_sha1(arg, sha1) < 0)
+ +		usage(describe_usage);
+ +	cmit = lookup_commit_reference(sha1);
+ +	if (!cmit)
+ +		usage(describe_usage);
+ +
+  	if (!initialized) {
+  		initialized = 1;
+  		for_each_ref(get_name);
 ------------
 
+1.   It is preceded with a "git diff" header, that looks like
+     this (when '-c' option is used):
+
+       diff --combined fileM
+
+     or like this (when '--cc' option is used):
+
+       diff --c fileM
+
+2.   It is followed by one or more extended header lines
+     (we assume here that we have merge with two parents):
+
+       index <hash>,<hash>..<hash>
+       mode <mode>,<mode>..<mode>
+       new file mode <mode>
+
+     The "mode <mode>,<mode>..<mode>" appears only if at least
+     one of the <mode> is diferent from the rest. Extended headers
+     with information about detected contents movement (renames
+     and copying detection) are designed to work with diff of two
+     <tree-ish> and are not used by combined diff format. Currently
+     combined diff format cannot show files which were removed
+     by merge, so "deleted file mode <mode>,<mode>" is never used.
+
+3.   It is followed by two-line from-file/to-file header
+
+       --- a/fileM
+       +++ b/fileM
+
+     Contrary to two-line header for traditional 'unified' diff
+     format, and similar to filenames in ordinary "diff header",
+     /dev/null is not used for creation combined diff.
+
+4.   Chunk header format is modified to prevent people from
+     accidentally feeding it to 'patch -p1'. Combined diff format
+     was created for review of merge commit changes, and was not
+     meant for apply. The change is similar to the change in the
+     extended 'index' header
+
+       @@@ <from-file-range> <from-file-range> <to-file-range> @@@
+
+     It might be not obvious that we have number of parents + 1
+     '@' characters in chunk header for combined diff format.
+
 Unlike the traditional 'unified' diff format, which shows two
 files A and B with a single column that has `-` (minus --
 appears in A but removed in B), `+` (plus -- missing in A but
-- 
1.4.2.1



-- 
Jakub Narebski