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Diffstat (limited to 'sha1-lookup.c')
| -rw-r--r-- | sha1-lookup.c | 319 |
1 files changed, 319 insertions, 0 deletions
diff --git a/sha1-lookup.c b/sha1-lookup.c new file mode 100644 index 0000000000..2dd851598a --- /dev/null +++ b/sha1-lookup.c @@ -0,0 +1,319 @@ +#include "cache.h" +#include "sha1-lookup.h" + +static uint32_t take2(const unsigned char *sha1) +{ + return ((sha1[0] << 8) | sha1[1]); +} + +/* + * Conventional binary search loop looks like this: + * + * do { + * int 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 the 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 update hi with + * it. + * + * - if it is strictly lower than the target, we update lo to be + * one slot after it, because we allow lo to be at the target. + * + * 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. + */ +/* + * The table should contain "nr" elements. + * The sha1 of element i (between 0 and nr - 1) should be returned + * by "fn(i, table)". + */ +int sha1_pos(const unsigned char *sha1, void *table, size_t nr, + sha1_access_fn fn) +{ + size_t hi = nr; + size_t lo = 0; + size_t mi = 0; + + if (!nr) + return -1; + + if (nr != 1) { + size_t lov, hiv, miv, ofs; + + for (ofs = 0; ofs < 18; ofs += 2) { + lov = take2(fn(0, table) + ofs); + hiv = take2(fn(nr - 1, table) + ofs); + miv = take2(sha1 + ofs); + if (miv < lov) + return -1; + if (hiv < miv) + return -1 - nr; + if (lov != hiv) { + /* + * At this point miv could be equal + * to hiv (but sha1 could still be higher); + * the invariant of (mi < hi) should be + * kept. + */ + mi = (nr - 1) * (miv - lov) / (hiv - lov); + if (lo <= mi && mi < hi) + break; + die("BUG: assertion failed in binary search"); + } + } + if (18 <= ofs) + die("cannot happen -- lo and hi are identical"); + } + + do { + int cmp; + cmp = hashcmp(fn(mi, table), sha1); + if (!cmp) + return mi; + if (cmp > 0) + hi = mi; + else + lo = mi + 1; + mi = (hi + lo) / 2; + } 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; +} |