Re: [PATCH 3/8] merge-ort: record the reason that we want a rename for a directory

From: Junio C Hamano <gitster@pobox.com>
To: Elijah Newren <newren@gmail.com>
Cc: "Derrick Stolee" <stolee@gmail.com>,
	"Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com>,
	"Git Mailing List" <git@vger.kernel.org>,
	"Derrick Stolee" <dstolee@microsoft.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Jonathan Tan" <jonathantanmy@google.com>,
	"Taylor Blau" <me@ttaylorr.com>
Subject: Re: [PATCH 3/8] merge-ort: record the reason that we want a rename for a directory
Date: Sat, 27 Mar 2021 19:01:45 -0700	[thread overview]
Message-ID: <xmqq4kgwox2u.fsf@gitster.g> (raw)
In-Reply-To: <CABPp-BHLL_u_E-Ss=AYWUZM+_9hbC6D6WzRjxEUwmexTEgQ2zw@mail.gmail.com> (Elijah Newren's message of "Mon, 15 Mar 2021 08:27:48 -0700")

Elijah Newren <newren@gmail.com> writes:

> On Mon, Mar 15, 2021 at 7:31 AM Derrick Stolee <stolee@gmail.com> wrote:
>>
>> On 3/13/2021 5:22 PM, Elijah Newren via GitGitGadget wrote:
>> > From: Elijah Newren <newren@gmail.com>
>> >
>> > When one side of history renames a directory, and the other side of
>> > history added files to the old directory, directory rename detection is
>> > used to warn about the location of the added files so the user can
>> > move them to the old directory or keep them with the new one.
>> >
>> > This sets up three different types of directories:
>> >   * directories that had new files added to them
>> >   * directories underneath a directory that had new files added to them
>> >   * directories where no new files were added to it or any leading path
>> >
>> > Save this information in dirs_removed; the next several commits will
>> > make use of this information.
>> ...
>> > +/* dir_rename_relevance: the reason we want rename information for a dir */
>> > +enum dir_rename_relevance {
>> > +     NOT_RELEVANT = 0,
>> > +     RELEVANT_FOR_ANCESTOR = 1,
>> > +     RELEVANT_FOR_SELF = 2
>> > +};
>>
>> Is this potentially a flag list? It's hard to tell because we don't
>> have another item (3 or 4?).
>>
>> >               unsigned sides = (0x07 - dirmask)/2;
>> > +             unsigned relevance = (renames->dir_rename_mask == 0x07) ?
>> > +                                     RELEVANT_FOR_ANCESTOR : NOT_RELEVANT;
>> > +             /*
>> > +              * Record relevance of this directory.  However, note that
>> > +              * when collect_merge_info_callback() recurses into this
>> > +              * directory and calls collect_rename_info() on paths
>> > +              * within that directory, if we find a path that was added
>> > +              * to this directory on the other side of history, we will
>> > +              * upgrade this value to RELEVANT_FOR_SELF; see below.
>> > +              */
>>
>> This comment seems to imply that RELEVANT_FOR_SELF is "more important"
>> than RELEVANT_FOR_ANCESTOR, so the value will just be changed (not a
>> flag).
>
> Yes.
>
>> > +     /*
>> > +      * Here's the block that potentially upgrades to RELEVANT_FOR_SELF.
>> > +      * When we run across a file added to a directory.  In such a case,
>> > +      * find the directory of the file and upgrade its relevance.
>> > +      */
>> > +     if (renames->dir_rename_mask == 0x07 &&
>> > +         (filemask == 2 || filemask == 4)) {
>> > +             /*
>> > +              * Need directory rename for parent directory on other side
>> > +              * of history from added file.  Thus
>> > +              *    side = (~filemask & 0x06) >> 1
>> > +              * or
>> > +              *    side = 3 - (filemask/2).
>> > +              */
>> > +             unsigned side = 3 - (filemask >> 1);
>> > +             strintmap_set(&renames->dirs_removed[side], dirname,
>> > +                           RELEVANT_FOR_SELF);
>>
>> Yes, using "RELEVANT_FOR_SELF" here, not "relevance | RELEVANT_FOR_SELF".
>> OK. This should make the later consumers simpler.
>
> Yep, indeed.  Would it make it clearer earlier if I were to stop
> assigning the explicit values in the enum?  Would adding a comment
> when I introduce the enum be easier?  Or was it just "thinking out
> loud"?

You are not asking me, but if you were, I'd say not using enum for
bitmask would be a good discipline to follow, and an enum like this
one that is used only for uniqueness of the values would benefit from
not having explicit value assignments.