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| -rw-r--r-- | sha1-lookup.c | 101 | ||||
| -rw-r--r-- | sha1-lookup.h | 7 |
2 files changed, 108 insertions, 0 deletions
diff --git a/sha1-lookup.c b/sha1-lookup.c index da357479cf..055dd87dc1 100644 --- a/sha1-lookup.c +++ b/sha1-lookup.c @@ -1,6 +1,107 @@ #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("oops"); + } + } + 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: * diff --git a/sha1-lookup.h b/sha1-lookup.h index 3249a81b3d..20af285681 100644 --- a/sha1-lookup.h +++ b/sha1-lookup.h @@ -1,6 +1,13 @@ #ifndef SHA1_LOOKUP_H #define SHA1_LOOKUP_H +typedef const unsigned char *sha1_access_fn(size_t index, void *table); + +extern int sha1_pos(const unsigned char *sha1, + void *table, + size_t nr, + sha1_access_fn fn); + extern int sha1_entry_pos(const void *table, size_t elem_size, size_t key_offset, |