Re: [PATCH 2/2] diff-filter: be more careful when looking for negative bits

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Taylor,

On Tue, 25 Jan 2022, Taylor Blau wrote:

> On Tue, Jan 25, 2022 at 10:29:19PM +0000, Johannes Schindelin via GitGitGadget wrote:
> > diff --git a/diff.c b/diff.c
> > index c862771a589..fc1151b9c73 100644
> > --- a/diff.c
> > +++ b/diff.c
> > @@ -4821,17 +4821,15 @@ static int diff_opt_diff_filter(const struct option *option,
> >  	prepare_filter_bits();
> >
> >  	/*
> > -	 * If there is a negation e.g. 'd' in the input, and we haven't
> > +	 * If the input starts with a negation e.g. 'd', and we haven't
> >  	 * initialized the filter field with another --diff-filter, start
> >  	 * from full set of bits, except for AON.
> >  	 */
> >  	if (!opt->filter) {
> > -		for (i = 0; (optch = optarg[i]) != '\0'; i++) {
> > -			if (optch < 'a' || 'z' < optch)
> > -				continue;
> > +		optch = optarg[0];
> > +		if (optch >= 'a' && 'z' >= optch) {
> >  			opt->filter = (1 << (ARRAY_SIZE(diff_status_letters) - 1)) - 1;
> >  			opt->filter &= ~filter_bit[DIFF_STATUS_FILTER_AON];
> > -			break;
> >  		}
> >  	}
>
> Thinking through how this would have worked before with
> `--diff-filter=Dr`, I think it goes something like:
>
>   1. We set all bits (except the all-or-none bit) on via the first loop.
>   2. Then we OR in the bit for deletions, which does not change the
>      overall filter (since it was already set by the previous step).
>   3. Then we unset the bit corresponding to renames.
>
> That leaves us with all bits on except two: DIFF_STATUS_RENAMED and
> DIFF_STATUS_FILTER_AON.

Correct. And since we asked only for "Deleted", we get way more than we
bargained for.

> As far as I can understand, the AON "filter" shows all files as long as
> at least one of them matches the filter, otherwise it shows nothing at
> all.

Right, so on its own, it is quite useless. It needs to be combined with
another diff filter to make sense.

> But that doesn't save us, since we have many more bits on than we should
> have, meaning that `--diff-filter=Dr` doesn't work at all (assuming you
> expected it to show just deletions, like `--diff-filter=D` does).

Correct.

Ciao,
Dscho