blob: da357479cf19aad4bebc64f874c76fdf8566712b [file] [log] [blame]
 #include "cache.h" #include "sha1-lookup.h" /* * 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. */ 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; }