Re: [PATCH 3/5] roaring: teach Git to write roaring bitmaps

[Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>]

On Fri, Sep 30, 2022 at 11:51 AM Junio C Hamano <gitster@pobox.com> wrote:
>
> "Abhradeep Chakraborty via GitGitGadget" <gitgitgadget@gmail.com>
> writes:
>
> > From: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
> >
> > Roaring bitmaps are said to be more efficient (most of the time) than
> > ewah bitmaps. So Git might gain some optimization if it support roaring
> > bitmaps. As Roaring library has all the changes it needed to implement
> > roaring bitmaps in Git, Git can learn to write roaring bitmaps. However,
> > all the changes are backward-compatible.
> >
> > Teach Git to write roaring bitmaps.
>
> That is way underexplained.   At least cover what the plans are, so
> that readers do not have to ask these questions:
>
>  * When is the choice of bitmap type is made?  Is it fixed at
>    repository initialization time and once chosen other kinds cannot
>    be used?
>
>  * Is the bitmap file self describing?  How does a reader know
>    between ewah and roaring codepaths to use to read a given bitmap
>    file?  Is there enough room for extending the set of bitmap
>    formats, or we cannot add other formats easily?

Hey Junio,

First of all, sorry that the next version is taking so much time to
land. We have a festival ("Durga Puja"; it is the biggest festival for
Bengalis) going on here now. So I am not that active.

I will explain briefly in the next version.

>
> Do you really need the global variable that holds the bitmap type?
>
> Wouldn't it be easier to write code that needs to deal with both
> types (e.g. in a repository with existing ewah bitmap, you want to
> do a repack and index the result using the roaring bitmap) if you
> passed the type through the callchain as a parameter?

I didn't want to go for "passing the type through the callchain as a
parameter" because that would cause changes to every affected function
definition. I found the "global variable" approach simpler for this
reason. Here we have to initialize the type once and the affected
functions will work accordingly.

If you like the "callchain" approach, I have no problem to implement it.

> It may be that the codepath that reads from an existing bitmap file
> says "ah, the file given to us seems to be in format X (either EWAH
> or ROARING or perhaps something else), so let's call bitmap_init(X)
> to obtain the in-core data structure to deal with that file".  When
> that happens, you may probably need to have two cases in the default:
> arm of this switch statement, i.e. one to diagnose a BUG() to pass
> an uninitialized bitmap type to the codepath, and the other to
> diagnose a runtime error() to have read a bitmap file whose format
> this version of Git does not understand.

Ok, understood. Thanks.

> These repetitive patterns makes me wonder if void *bitmap
> is a good type to be passing around.  Shouldn't it be a struct with
> its first member being a bitmap_type, and another member being what
> these functions are passing to the underlying bitmap format specific
> functions as "bitmap"?  E.g.
>
>     void bitmap_unset(struct bitmap *bm, uint32_t i)
>     {
>         switch (bm->type) {
>         case EWAH:
>                 ewah_bitmap_remove(bm->u.ewah, i);
>                 break;
>         ...

Good idea! Thanks.

> > +
> > +enum bitmap_type {
> > +     INIT_BITMAP_TYPE = 0,
>
> "UNINITIALIZED_BITMAP_TYPE", probably.

Ok.

> > +void *roaring_or_ewah_bitmap_init(void);
>
> I would strongly suggest reconsider these names.  What if you later
> want to add the third variant?  roaring_or_ewah_or_xyzzy_bitmap_init()?
>
> Instead just use the most generic name, like "bitmap_init", perhaps
> something along the lines of ...
>
>     struct bitmap {
>         enum bitmap_type type;
>         union {
>             struct ewah_bitmap *ewah;
>             struct roaring_bitmap *roaring;
>         } u;
>     };
>
>     struct bitmap *bitmap_new(enum bitmap_type type)
>     {
>         struct bitmap *bm = xmalloc(sizeof(*bm));
>
>         bm->type = type;
>         switch (bm->type) {
>         case EWAH:
>             bm->u.ewah = ewah_new();
>             break;
>         case ROARING:
>             bm->u.roaring = roaring_bitmap_create();
>             break;
>         default:
>             die(_("unknown bitmap type %d"), (int)type);
>         }
>         return bm;
>     }

Got it. It seems a better option than the current one.

Thanks )