} while (lo < hi);
return -lo-1;
}
-
-/*
- * Conventional binary search loop looks like this:
- *
- * unsigned lo, hi;
- * do {
- * unsigned mi = (lo + hi) / 2;
- * int cmp = "entry pointed at by mi" minus "target";
- * if (!cmp)
- * return (mi is the wanted one)
- * if (cmp > 0)
- * hi = mi; "mi is larger than target"
- * else
- * lo = mi+1; "mi is smaller than target"
- * } while (lo < hi);
- *
- * The invariants are:
- *
- * - When entering the loop, lo points at a slot that is never
- * above the target (it could be at the target), hi points at a
- * slot that is guaranteed to be above the target (it can never
- * be at the target).
- *
- * - We find a point 'mi' between lo and hi (mi could be the same
- * as lo, but never can be as same as hi), and check if it hits
- * the target. There are three cases:
- *
- * - if it is a hit, we are happy.
- *
- * - if it is strictly higher than the target, we set it to hi,
- * and repeat the search.
- *
- * - if it is strictly lower than the target, we update lo to
- * one slot after it, because we allow lo to be at the target.
- *
- * If the loop exits, there is no matching entry.
- *
- * When choosing 'mi', we do not have to take the "middle" but
- * anywhere in between lo and hi, as long as lo <= mi < hi is
- * satisfied. When we somehow know that the distance between the
- * target and lo is much shorter than the target and hi, we could
- * pick mi that is much closer to lo than the midway.
- *
- * Now, we can take advantage of the fact that SHA-1 is a good hash
- * function, and as long as there are enough entries in the table, we
- * can expect uniform distribution. An entry that begins with for
- * example "deadbeef..." is much likely to appear much later than in
- * the midway of the table. It can reasonably be expected to be near
- * 87% (222/256) from the top of the table.
- *
- * However, we do not want to pick "mi" too precisely. If the entry at
- * the 87% in the above example turns out to be higher than the target
- * we are looking for, we would end up narrowing the search space down
- * only by 13%, instead of 50% we would get if we did a simple binary
- * search. So we would want to hedge our bets by being less aggressive.
- *
- * The table at "table" holds at least "nr" entries of "elem_size"
- * bytes each. Each entry has the SHA-1 key at "key_offset". The
- * table is sorted by the SHA-1 key of the entries. The caller wants
- * to find the entry with "key", and knows that the entry at "lo" is
- * not higher than the entry it is looking for, and that the entry at
- * "hi" is higher than the entry it is looking for.
- */
-int sha1_entry_pos(const void *table,
- size_t elem_size,
- size_t key_offset,
- unsigned lo, unsigned hi, unsigned nr,
- const unsigned char *key)
-{
- const unsigned char *base = table;
- const unsigned char *hi_key, *lo_key;
- unsigned ofs_0;
- static int debug_lookup = -1;
-
- if (debug_lookup < 0)
- debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
-
- if (!nr || lo >= hi)
- return -1;
-
- if (nr == hi)
- hi_key = NULL;
- else
- hi_key = base + elem_size * hi + key_offset;
- lo_key = base + elem_size * lo + key_offset;
-
- ofs_0 = 0;
- do {
- int cmp;
- unsigned ofs, mi, range;
- unsigned lov, hiv, kyv;
- const unsigned char *mi_key;
-
- range = hi - lo;
- if (hi_key) {
- for (ofs = ofs_0; ofs < 20; ofs++)
- if (lo_key[ofs] != hi_key[ofs])
- break;
- ofs_0 = ofs;
- /*
- * byte 0 thru (ofs-1) are the same between
- * lo and hi; ofs is the first byte that is
- * different.
- *
- * If ofs==20, then no bytes are different,
- * meaning we have entries with duplicate
- * keys. We know that we are in a solid run
- * of this entry (because the entries are
- * sorted, and our lo and hi are the same,
- * there can be nothing but this single key
- * in between). So we can stop the search.
- * Either one of these entries is it (and
- * we do not care which), or we do not have
- * it.
- *
- * Furthermore, we know that one of our
- * endpoints must be the edge of the run of
- * duplicates. For example, given this
- * sequence:
- *
- * idx 0 1 2 3 4 5
- * key A C C C C D
- *
- * If we are searching for "B", we might
- * hit the duplicate run at lo=1, hi=3
- * (e.g., by first mi=3, then mi=0). But we
- * can never have lo > 1, because B < C.
- * That is, if our key is less than the
- * run, we know that "lo" is the edge, but
- * we can say nothing of "hi". Similarly,
- * if our key is greater than the run, we
- * know that "hi" is the edge, but we can
- * say nothing of "lo".
- *
- * Therefore if we do not find it, we also
- * know where it would go if it did exist:
- * just on the far side of the edge that we
- * know about.
- */
- if (ofs == 20) {
- mi = lo;
- mi_key = base + elem_size * mi + key_offset;
- cmp = memcmp(mi_key, key, 20);
- if (!cmp)
- return mi;
- if (cmp < 0)
- return -1 - hi;
- else
- return -1 - lo;
- }
-
- hiv = hi_key[ofs_0];
- if (ofs_0 < 19)
- hiv = (hiv << 8) | hi_key[ofs_0+1];
- } else {
- hiv = 256;
- if (ofs_0 < 19)
- hiv <<= 8;
- }
- lov = lo_key[ofs_0];
- kyv = key[ofs_0];
- if (ofs_0 < 19) {
- lov = (lov << 8) | lo_key[ofs_0+1];
- kyv = (kyv << 8) | key[ofs_0+1];
- }
- assert(lov < hiv);
-
- if (kyv < lov)
- return -1 - lo;
- if (hiv < kyv)
- return -1 - hi;
-
- /*
- * Even if we know the target is much closer to 'hi'
- * than 'lo', if we pick too precisely and overshoot
- * (e.g. when we know 'mi' is closer to 'hi' than to
- * 'lo', pick 'mi' that is higher than the target), we
- * end up narrowing the search space by a smaller
- * amount (i.e. the distance between 'mi' and 'hi')
- * than what we would have (i.e. about half of 'lo'
- * and 'hi'). Hedge our bets to pick 'mi' less
- * aggressively, i.e. make 'mi' a bit closer to the
- * middle than we would otherwise pick.
- */
- kyv = (kyv * 6 + lov + hiv) / 8;
- if (lov < hiv - 1) {
- if (kyv == lov)
- kyv++;
- else if (kyv == hiv)
- kyv--;
- }
- mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
-
- if (debug_lookup) {
- printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
- printf("ofs %u lov %x, hiv %x, kyv %x\n",
- ofs_0, lov, hiv, kyv);
- }
- if (!(lo <= mi && mi < hi))
- die("assertion failure lo %u mi %u hi %u %s",
- lo, mi, hi, sha1_to_hex(key));
-
- mi_key = base + elem_size * mi + key_offset;
- cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
- if (!cmp)
- return mi;
- if (cmp > 0) {
- hi = mi;
- hi_key = mi_key;
- } else {
- lo = mi + 1;
- lo_key = mi_key + elem_size;
- }
- } while (lo < hi);
- return -lo-1;
-}