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Diffstat (limited to 'sha1-lookup.c')
| -rw-r--r-- | sha1-lookup.c | 152 |
1 files changed, 152 insertions, 0 deletions
diff --git a/sha1-lookup.c b/sha1-lookup.c new file mode 100644 index 0000000..4faa638 --- /dev/null +++ b/sha1-lookup.c @@ -0,0 +1,152 @@ +#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. + * + * 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; + + if (kyv == lov && lov < hiv - 1) + kyv++; + else if (kyv == hiv - 1 && lov < kyv) + 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; +} |