diff options
Diffstat (limited to 'sha1-lookup.c')
| -rw-r--r-- | sha1-lookup.c | 216 |
1 files changed, 0 insertions, 216 deletions
diff --git a/sha1-lookup.c b/sha1-lookup.c index 5f06921..2552b79 100644 --- a/sha1-lookup.c +++ b/sha1-lookup.c @@ -99,219 +99,3 @@ int sha1_pos(const unsigned char *sha1, void *table, size_t nr, } 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; -} |