Jeff King <peff@xxxxxxxx> writes:
>> OK, as Linus's "count at the point of use" is already in 'next',
>> could you make it incremental with a log message?
>
> Sure. I wasn't sure if you actually liked my direction or not, so I was
> mostly just showing off what the completed one would look like.
To be quite honest, I am not just unsure if I liked your direction;
rather I am not sure if I actually understood what you perceived as
a difference that matters between the two approaches. I wanted to
hear you explain the difference in terms of "Linus's does this, but
it is bad in X and Y way, so let's avoid it and do it like Z
instead". One effective way to extract that out of you was to force
you to justify the "incremental" update.
And it seems that I succeeded ;-).
I am still not sure if I 100% agree with your first paragraph, but
at least now I think I see where you are coming from.
You probably will hear from Ramsay about extern-ness of msb().
> -- >8 --
> Subject: [PATCH] find_unique_abbrev: move logic out of get_short_sha1()
>
> The get_short_sha1() is only about reading short sha1s; we
> do call it in a loop to check "is this long enough" for each
> object, but otherwise it should not need to know about
> things like our default_abbrev setting.
>
> So instead of asking it to set default_automatic_abbrev as a
> side-effect, let's just have find_unique_abbrev() pick the
> right place to start its loop. This requires a separate
> approximate_object_count() function, but that naturally
> belongs with the rest of sha1_file.c.
>
> Signed-off-by: Jeff King <peff@xxxxxxxx>
> ---
> cache.h | 7 ++++++-
> sha1_file.c | 27 +++++++++++++++++++++++++++
> sha1_name.c | 60 +++++++++++++++++++++++++++++++++++-------------------------
> 3 files changed, 68 insertions(+), 26 deletions(-)
>
> diff --git a/cache.h b/cache.h
> index 0e2a059..f22ace5 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1204,7 +1204,6 @@ struct object_context {
> #define GET_SHA1_TREEISH 020
> #define GET_SHA1_BLOB 040
> #define GET_SHA1_FOLLOW_SYMLINKS 0100
> -#define GET_SHA1_AUTOMATIC 0200
> #define GET_SHA1_ONLY_TO_DIE 04000
>
> #define GET_SHA1_DISAMBIGUATORS \
> @@ -1456,6 +1455,12 @@ extern void prepare_packed_git(void);
> extern void reprepare_packed_git(void);
> extern void install_packed_git(struct packed_git *pack);
>
> +/*
> + * Give a rough count of objects in the repository. This sacrifices accuracy
> + * for speed.
> + */
> +unsigned long approximate_object_count(void);
> +
> extern struct packed_git *find_sha1_pack(const unsigned char *sha1,
> struct packed_git *packs);
>
> diff --git a/sha1_file.c b/sha1_file.c
> index b9c1fa3..4882440 100644
> --- a/sha1_file.c
> +++ b/sha1_file.c
> @@ -1381,6 +1381,32 @@ static void prepare_packed_git_one(char *objdir, int local)
> strbuf_release(&path);
> }
>
> +static int approximate_object_count_valid;
> +
> +/*
> + * Give a fast, rough count of the number of objects in the repository. This
> + * ignores loose objects completely. If you have a lot of them, then either
> + * you should repack because your performance will be awful, or they are
> + * all unreachable objects about to be pruned, in which case they're not really
> + * interesting as a measure of repo size in the first place.
> + */
> +unsigned long approximate_object_count(void)
> +{
> + static unsigned long count;
> + if (!approximate_object_count_valid) {
> + struct packed_git *p;
> +
> + prepare_packed_git();
> + count = 0;
> + for (p = packed_git; p; p = p->next) {
> + if (open_pack_index(p))
> + continue;
> + count += p->num_objects;
> + }
> + }
> + return count;
> +}
> +
> static void *get_next_packed_git(const void *p)
> {
> return ((const struct packed_git *)p)->next;
> @@ -1455,6 +1481,7 @@ void prepare_packed_git(void)
>
> void reprepare_packed_git(void)
> {
> + approximate_object_count_valid = 0;
> prepare_packed_git_run_once = 0;
> prepare_packed_git();
> }
> diff --git a/sha1_name.c b/sha1_name.c
> index beb7ab5..76e6885 100644
> --- a/sha1_name.c
> +++ b/sha1_name.c
> @@ -15,7 +15,6 @@ typedef int (*disambiguate_hint_fn)(const unsigned char *, void *);
>
> struct disambiguate_state {
> int len; /* length of prefix in hex chars */
> - unsigned int nrobjects;
> char hex_pfx[GIT_SHA1_HEXSZ + 1];
> unsigned char bin_pfx[GIT_SHA1_RAWSZ];
>
> @@ -119,14 +118,6 @@ static void find_short_object_filename(struct disambiguate_state *ds)
>
> if (strlen(de->d_name) != 38)
> continue;
> -
> - /*
> - * We only look at the one subdirectory, and we assume
> - * each subdirectory is roughly similar, so each
> - * object we find probably has 255 other objects in
> - * the other fan-out directories.
> - */
> - ds->nrobjects += 256;
> if (memcmp(de->d_name, ds->hex_pfx + 2, ds->len - 2))
> continue;
> memcpy(hex + 2, de->d_name, 38);
> @@ -160,7 +151,6 @@ static void unique_in_pack(struct packed_git *p,
>
> open_pack_index(p);
> num = p->num_objects;
> - ds->nrobjects += num;
> last = num;
> while (first < last) {
> uint32_t mid = (first + last) / 2;
> @@ -390,9 +380,6 @@ static int show_ambiguous_object(const unsigned char *sha1, void *data)
> return 0;
> }
>
> -/* start from our historical default before the automatic abbreviation */
> -static int default_automatic_abbrev = FALLBACK_DEFAULT_ABBREV;
> -
> static int get_short_sha1(const char *name, int len, unsigned char *sha1,
> unsigned flags)
> {
> @@ -439,14 +426,6 @@ static int get_short_sha1(const char *name, int len, unsigned char *sha1,
> for_each_abbrev(ds.hex_pfx, show_ambiguous_object, &ds);
> }
>
> - if (len < 16 && !status && (flags & GET_SHA1_AUTOMATIC)) {
> - unsigned int expect_collision = 1 << (len * 2);
> - if (ds.nrobjects > expect_collision) {
> - default_automatic_abbrev = len+1;
> - return SHORT_NAME_AMBIGUOUS;
> - }
> - }
> -
> return status;
> }
>
> @@ -476,22 +455,53 @@ int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
> return ret;
> }
>
> +/*
> + * Return the slot of the most-significant bit set in "val". There are various
> + * ways to do this quickly with fls() or __builtin_clzl(), but speed is
> + * probably not a big deal here.
> + */
> +unsigned msb(unsigned long val)
> +{
> + unsigned r = 0;
> + while (val >>= 1)
> + r++;
> + return r;
> +}
> +
> int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
> {
> int status, exists;
> - int flags = GET_SHA1_QUIETLY;
>
> if (len < 0) {
> - flags |= GET_SHA1_AUTOMATIC;
> - len = default_automatic_abbrev;
> + unsigned long count = approximate_object_count();
> + /*
> + * Add one because the MSB only tells us the highest bit set,
> + * not including the value of all the _other_ bits (so "15"
> + * is only one off of 2^4, but the MSB is the 3rd bit.
> + */
> + len = msb(count) + 1;
> + /*
> + * We now know we have on the order of 2^len objects, which
> + * expects a collision at 2^(len/2). But we also care about hex
> + * chars, not bits, and there are 4 bits per hex. So all
> + * together we need to divide by 2; but we also want to round
> + * odd numbers up, hence adding one before dividing.
> + */
> + len = (len + 1) / 2;
> + /*
> + * For very small repos, we stick with our regular fallback.
> + */
> + if (len < FALLBACK_DEFAULT_ABBREV)
> + len = FALLBACK_DEFAULT_ABBREV;
> }
> +
> sha1_to_hex_r(hex, sha1);
> if (len == 40 || !len)
> return 40;
> exists = has_sha1_file(sha1);
> while (len < 40) {
> unsigned char sha1_ret[20];
> - status = get_short_sha1(hex, len, sha1_ret, flags);
> + status = get_short_sha1(hex, len, sha1_ret, GET_SHA1_QUIETLY);
> if (exists
> ? !status
> : status == SHORT_NAME_NOT_FOUND) {