Re: Use a *real* built-in diff generator

Junio C Hamano <junkio@cox.net> writes:

The reason I like this is because I was thinking about doing in-core diffs for different purpose when I was driving to work this morning [*1*] --- to make pickaxe a more useful building block.

Currently, pickaxe tries to do an exact match to find the case where a given substring S appears in the version C of the file but not in the its parent C^n (1 <= n), and then it tells the diffcore to emit the differences. The user (probably only me on this list?) is expected to look at the change, make an intelligent decision to feed a matching substring S' found in C^n and restart from that commit.

To be a useful "content movement tracker", the process of finding matching 'old shape' in the previous version and re-feeding it to pickaxe should be automated if possible, and in-core diff machinery would be one component to help that.

For example, if I wanted to find when I stole 'ls-files -t' feature from Cogito, I would first run less ls-files.c; I see these and am reasonably sure these relate to what I am looking for:

	...
        static const char *tag_cached = "";
        static const char *tag_unmerged = "";
        static const char *tag_removed = "";
        static const char *tag_other = "";
        static const char *tag_killed = "";
        static const char *tag_modified = "";
	...

So I run:

	$ git whatchanged -S'static const char *tag_other = "";
        static const char *tag_killed = "";
	static const char *tag_modified' -p master -- ls-files.c

which finds:

        Author: Junio C Hamano <junkio@cox.net>
        Date:   Mon Sep 19 15:11:15 2005 -0700

            Show modified files in git-ls-files
	...
        @@ -28,6 +29,7 @@ static const char *tag_unmerged = "";
         static const char *tag_removed = "";
         static const char *tag_other = "";
         static const char *tag_killed = "";
        +static const char *tag_modified = "";

but that is not what I am interested in; the matching "old shape" is the version before the tag_modified was added (and it already had other tag_xxx in there). So with the current pickaxe, I manually re-run whatchanged starting from the found commit with modified string like this:

	$ git whatchanged -S'static const char *tag_removed = "";
        static const char *tag_other = "";
        static const char *tag_killed = "";' -p $that_commit -- ls-files.c

in order to further drill down.

A truly useful pickaxe should take two line numbers and a filename (to name the range of lines I am interested in) from the starting version, notice when that range changes shape, and after showing the found commit, replace the range with the one matching from the older commit and continue.

[Footnote]

*1* When you are bogged down in a boring day-job, your brain tends to try to compensate by spending as much your waking time as possible on thinking about more interesting and more useful stuff -- like git ;-).