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@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/cache.h b/cache.h
index 0e2a0595e516445e833b24d8a2acd47dc07f5b49..f22ace507932353c744b32bdc376670e17786656 100644 (file)
--- 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 b9c1fa3f1a0c24fa411d590987b60362cae6f943..48824407aa2c4713ec4a4ac72bd0cd044421a1fe 100644 (file)
--- 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 beb7ab588b4da529080301576447762a17e4da11..4a6f8426921231a16089f6ac5498e1bfd5956451 100644 (file)
--- 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.
+ */
+static 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) {