[RFC] [GSoC] Port git-add--interactive.perl:status_cmd to C

Hey there,

So, in the GSoC proposal I sent about porting git-add--interactive to C[1], I expected I would be able to do a couple of small patches to git-add to familiarize myself with the Git API and have a better clue of how the porting process would go by. Due to the unexpected size my microproject ended up taking (though in a good way, I think) I chose to focus on that and ended up not sending those anticipated patches. However, I *did* have time to study how I'd go for the port more closely, and I'd like some comments on what you think of it. Be I accepted or not into GSoC some days from now, I think the topic might interest others that might want to tackle this task.

As I went through the code, I grew to believe that a good way to make a step-by-step conversion might be to get add--interactive "commands" (as in status_cmd, update_cmd, add_untracked_cmd etc.) ported one at a time over a set of patch series. For each series, one X_cmd Perl subroutine would become a one-line call to "git-add-interactive--helper X <args>" (a builtin). This would make understanding a function as complex as list_and_choose() to being ported to C way easier: for each command, a new functionality would be added on-demand instead of having the whole function sent over at once. Afterwards, a final series would likely set up the "interactive main loop" in C and retire the Perl script.

I tried to apply this philosophy to porting `status_cmd` to C, and easily got most of the configuration/color logic into C by mimicking what builtins like clean.c do.

The next issue I came up with (and do not yet have a solution) is how reproduce git-add--interactive's list_modified(). The Perl script runs git-diff-index and git-diff-files, does some clever string parsing of their outputs and "merges" the output of both in a single table to display the number of added/removed lines in a given file BOTH between HEAD+index and index+tree.

Reproducing either of these comparisons "natively" would simply require running run_diff_index() or run_diff_files() with DIFF_FORMAT_NUMSTAT and tweaking diff_flush() to format appropriately for the "interactive--add case". Furthermore, the current output prefix options would manage to cover the eventual case of `update_cmd` selecting files. To output both comparisons, however, I can't come up with any simple solution. Would you have any ideas?

-- Daniel.

[1]: https://public-inbox.org/git/CAEA2_RJEf4vjgcaux8a1KWh1-vxLLmv1--Vjf9wiEQoF+gVDtA@mail.gmail.com/