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hash-lookup.c
129 lines (118 loc) · 3.22 KB
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hash-lookup.c
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#include "cache.h"
#include "hash-lookup.h"
static uint32_t take2(const struct object_id *oid, size_t ofs)
{
return ((oid->hash[ofs] << 8) | oid->hash[ofs + 1]);
}
/*
* Conventional binary search loop looks like this:
*
* do {
* int mi = lo + (hi - lo) / 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 oid of element i (between 0 and nr - 1) should be returned
* by "fn(i, table)".
*/
int oid_pos(const struct object_id *oid, const void *table, size_t nr,
oid_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 < the_hash_algo->rawsz - 2; ofs += 2) {
lov = take2(fn(0, table), ofs);
hiv = take2(fn(nr - 1, table), ofs);
miv = take2(oid, ofs);
if (miv < lov)
return -1;
if (hiv < miv)
return index_pos_to_insert_pos(nr);
if (lov != hiv) {
/*
* At this point miv could be equal
* to hiv (but hash 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;
BUG("assertion failed in binary search");
}
}
}
do {
int cmp;
cmp = oidcmp(fn(mi, table), oid);
if (!cmp)
return mi;
if (cmp > 0)
hi = mi;
else
lo = mi + 1;
mi = lo + (hi - lo) / 2;
} while (lo < hi);
return index_pos_to_insert_pos(lo);
}
int bsearch_hash(const unsigned char *hash, const uint32_t *fanout_nbo,
const unsigned char *table, size_t stride, uint32_t *result)
{
uint32_t hi, lo;
hi = ntohl(fanout_nbo[*hash]);
lo = ((*hash == 0x0) ? 0 : ntohl(fanout_nbo[*hash - 1]));
while (lo < hi) {
unsigned mi = lo + (hi - lo) / 2;
int cmp = hashcmp(table + mi * stride, hash);
if (!cmp) {
if (result)
*result = mi;
return 1;
}
if (cmp > 0)
hi = mi;
else
lo = mi + 1;
}
if (result)
*result = lo;
return 0;
}