customizing git diff output format (own function name for my files in chunk header)

Question

I'm using git 2.14.1 on Linux/x86-64/Debian/Sid. I don't have any shell environment variable with GIT. I've read the git diff format page. Here is output of git config -l, for my current project (on github) bismon :

user.name=Basile Starynkevitch
[email protected]
push.default=simple
core.repositoryformatversion=0
core.filemode=true
core.bare=false
core.logallrefupdates=true
[email protected]:bstarynk/bismon.git
remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*
branch.master.remote=origin
branch.master.merge=refs/heads/master

When I do a git diff on some C file like main_BM.c I'm getting an output like

diff --git a/main_BM.c b/main_BM.c
index 3639a52..9c43d9e 100644
--- a/main_BM.c
+++ b/main_BM.c
@@ -371,6 +371,7 @@ rungui_BM (void)
     }
   if (gui_command_log_file_BM)
     {
+      /// the current time
       {
         time_t nowtim = time (NULL);
         struct tm nowtm = { };

Notice the chunk header line @@ -371,6 +371,7 @@ rungui_BM (void); it shows a function name (e.g. rungui_BM), and I don't understand why and how (but I like that).

I have also several .bismon textual files (machine generated, but versioned under git, and could be considered as some simplistic domain specific language). For example my store2.bismon contains stuff like

!(_0277f6mOd9j_0tVZSXnTzUK |=basiclo_temporary_module|
!~ name (~ basiclo_temporary_module
~)
!@ 1504862208.05
!$_0LK4TzFd6u1_0JFUsrQ4odG |=class|
!: _01h86SAfOfg_1q2oMegGRwW |=comment|
 "class of temporary modules containing only functions for closures"
!~ class (~ |supercl plain_temporary_module:| _1oEp0eAAyFN_4lsobepyr1T
~)

!)_0277f6mOd9j_0tVZSXnTzUK

^{(My store*.bismon have their own format -that I defined and could slightly change if convenient- and basically represents a dump of the persistent memory heap of some interpreter)}

From the point of view of git diff, I would like that any line similar to !(_0277f6mOd9j_0tVZSXnTzUK |=basiclo_temporary_module| (that is any line starting with !(_ followed by 11 alphanumerical characters followed by _ followed by 11 alphanumerical characters) be considered by git diff as a function start and displayed by git diff in the @@ chunk header.

Currently git diff store2.bismon don't give nice chunk headers like I would want them:

diff --git a/store2.bismon b/store2.bismon
index edbb2f7..3847b6b 100644
--- a/store2.bismon
+++ b/store2.bismon
@@ -6,7 +6,13 @@
 !(_0277f6mOd9j_0tVZSXnTzUK |=basiclo_temporary_module|
 !~ name (~ basiclo_temporary_module
 ~)
-!@ 1504861742.79
+!@ 1504862208.05
+!$_0LK4TzFd6u1_0JFUsrQ4odG |=class|
+!: _01h86SAfOfg_1q2oMegGRwW |=comment|
+ "class of temporary modules containing only functions for closures"
+!~ class (~ |supercl plain_temporary_module:| _1oEp0eAAyFN_4lsobepyr1T
+~)
+
 !)_0277f6mOd9j_0tVZSXnTzUK

I would like the chunk header line @@ -6,7 +6,13 @@ to be something like:

 @@ -6,7 +6,13 @@ !(_0277f6mOd9j_0tVZSXnTzUK |=basiclo_temporary_module|

Maybe I need to have some GIT_EXTERNAL_DIFF set to some script. But how is that thing invoked and by what subpart of git?

If possible, I would like the colors used by git diff to stay the same.

Answer 1

The solution is to define a custom hunk-header:

echo '*.bismon diff=bismon' > .gitattributes
git config diff.bismon.xfuncname '^!\(_.*$'

You can adapt the regexp for more accuracy