"Abhradeep Chakraborty via GitGitGadget" <gitgitgadget@xxxxxxxxx>
writes:
> From: Abhradeep Chakraborty <chakrabortyabhradeep79@xxxxxxxxx>
>
> 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?
> Mentored-by: Taylor Blau <me@xxxxxxxxxxxx>
> Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@xxxxxxxxx>
> Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@xxxxxxxxx>
> ---
> Makefile | 1 +
> bitmap.c | 225 +++++++++++++++++++++++++++
> bitmap.h | 33 ++++
> builtin/diff.c | 10 +-
> ewah/bitmap.c | 61 +++++---
> ewah/ewok.h | 37 ++---
> pack-bitmap-write.c | 326 ++++++++++++++++++++++++++++++----------
> pack-bitmap.c | 114 +++++++-------
> pack-bitmap.h | 22 ++-
> t/t5310-pack-bitmaps.sh | 17 +++
> 10 files changed, 664 insertions(+), 182 deletions(-)
> create mode 100644 bitmap.c
> create mode 100644 bitmap.h
>
> diff --git a/Makefile b/Makefile
> index e9537951105..9ca19b3ca8d 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -900,6 +900,7 @@ LIB_OBJS += archive.o
> LIB_OBJS += attr.o
> LIB_OBJS += base85.o
> LIB_OBJS += bisect.o
> +LIB_OBJS += bitmap.o
> LIB_OBJS += blame.o
> LIB_OBJS += blob.o
> LIB_OBJS += bloom.o
> diff --git a/bitmap.c b/bitmap.c
> new file mode 100644
> index 00000000000..7d547eb9f53
> --- /dev/null
> +++ b/bitmap.c
> @@ -0,0 +1,225 @@
> +#include "bitmap.h"
> +#include "cache.h"
> +
> +static enum bitmap_type bitmap_type = INIT_BITMAP_TYPE;
"INIT" is a strange name for "UNINITIALIZED". Especially ...
> +void *roaring_or_ewah_bitmap_init(void)
> +{
> + switch (bitmap_type)
> + {
(Style)
> + case EWAH:
> + return ewah_new();
> + case ROARING:
> + return roaring_bitmap_create();
> + default:
... here, you use it to mean exactly that.
> + error(_("bitmap type not initialized\n"));
> + return NULL;
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?
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.
> +void *roaring_or_raw_bitmap_copy(void *bitmap)
> +{
> + switch (bitmap_type)
> + {
> + case EWAH:
> ...
> +int roaring_or_ewah_bitmap_set(void *bitmap, uint32_t i)
> +{
> + switch (bitmap_type) {
> + case EWAH:
> +...
> +void roaring_or_raw_bitmap_set(void *bitmap, uint32_t i)
> +{
> + switch (bitmap_type)
> + {
> + case EWAH:
> +...
> +void roaring_or_raw_bitmap_unset(void *bitmap, uint32_t i)
> +{
> + switch (bitmap_type)
> + {
> + case EWAH:
> +...
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;
...
> \ No newline at end of file
Careful.
> diff --git a/bitmap.h b/bitmap.h
> new file mode 100644
> index 00000000000..d75400922cc
> --- /dev/null
> +++ b/bitmap.h
> @@ -0,0 +1,33 @@
> +#ifndef __BITMAP_H__
> +#define __BITMAP_H__
> +
> +
> +#include "git-compat-util.h"
> +#include "ewah/ewok.h"
> +#include "roaring/roaring.h"
> +
> +enum bitmap_type {
> + INIT_BITMAP_TYPE = 0,
"UNINITIALIZED_BITMAP_TYPE", probably.
> +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;
}
I dunno.
