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/*
* Copyright (C) 2009-2011 the libgit2 contributors
*
* This file is part of libgit2, distributed under the GNU GPL v2 with
* a Linking Exception. For full terms see the included COPYING file.
*/
#include <stdio.h>
#include "sha1_lookup.h"
#include "common.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 = (const unsigned char*)table;
const unsigned char *hi_key, *lo_key;
unsigned ofs_0;
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;
#ifdef INDEX_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);
#endif
if (!(lo <= mi && mi < hi)) {
return git__throw(GIT_ERROR, "Assertion failure. Binary search invariant is false");
}
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 -((int)lo)-1;
}
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