320 lines
9.3 KiB
C
320 lines
9.3 KiB
C
#include "cache.h"
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#include "sha1-lookup.h"
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static uint32_t take2(const unsigned char *sha1)
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{
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return ((sha1[0] << 8) | sha1[1]);
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}
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/*
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* Conventional binary search loop looks like this:
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*
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* do {
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* int mi = (lo + hi) / 2;
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* int cmp = "entry pointed at by mi" minus "target";
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* if (!cmp)
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* return (mi is the wanted one)
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* if (cmp > 0)
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* hi = mi; "mi is larger than target"
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* else
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* lo = mi+1; "mi is smaller than target"
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* } while (lo < hi);
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*
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* The invariants are:
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*
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* - When entering the loop, lo points at a slot that is never
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* above the target (it could be at the target), hi points at a
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* slot that is guaranteed to be above the target (it can never
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* be at the target).
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*
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* - We find a point 'mi' between lo and hi (mi could be the same
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* as lo, but never can be the same as hi), and check if it hits
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* the target. There are three cases:
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*
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* - if it is a hit, we are happy.
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*
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* - if it is strictly higher than the target, we update hi with
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* it.
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*
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* - if it is strictly lower than the target, we update lo to be
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* one slot after it, because we allow lo to be at the target.
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*
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* When choosing 'mi', we do not have to take the "middle" but
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* anywhere in between lo and hi, as long as lo <= mi < hi is
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* satisfied. When we somehow know that the distance between the
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* target and lo is much shorter than the target and hi, we could
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* pick mi that is much closer to lo than the midway.
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*/
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/*
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* The table should contain "nr" elements.
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* The sha1 of element i (between 0 and nr - 1) should be returned
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* by "fn(i, table)".
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*/
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int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
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sha1_access_fn fn)
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{
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size_t hi = nr;
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size_t lo = 0;
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size_t mi = 0;
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if (!nr)
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return -1;
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if (nr != 1) {
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size_t lov, hiv, miv, ofs;
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for (ofs = 0; ofs < 18; ofs += 2) {
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lov = take2(fn(0, table) + ofs);
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hiv = take2(fn(nr - 1, table) + ofs);
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miv = take2(sha1 + ofs);
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if (miv < lov)
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return -1;
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if (hiv < miv)
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return -1 - nr;
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if (lov != hiv) {
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/*
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* At this point miv could be equal
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* to hiv (but sha1 could still be higher);
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* the invariant of (mi < hi) should be
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* kept.
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*/
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mi = (nr - 1) * (miv - lov) / (hiv - lov);
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if (lo <= mi && mi < hi)
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break;
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die("BUG: assertion failed in binary search");
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}
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}
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if (18 <= ofs)
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die("cannot happen -- lo and hi are identical");
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}
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do {
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int cmp;
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cmp = hashcmp(fn(mi, table), sha1);
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if (!cmp)
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return mi;
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if (cmp > 0)
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hi = mi;
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else
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lo = mi + 1;
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mi = (hi + lo) / 2;
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} while (lo < hi);
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return -lo-1;
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}
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/*
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* Conventional binary search loop looks like this:
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*
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* unsigned lo, hi;
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* do {
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* unsigned mi = (lo + hi) / 2;
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* int cmp = "entry pointed at by mi" minus "target";
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* if (!cmp)
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* return (mi is the wanted one)
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* if (cmp > 0)
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* hi = mi; "mi is larger than target"
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* else
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* lo = mi+1; "mi is smaller than target"
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* } while (lo < hi);
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*
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* The invariants are:
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*
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* - When entering the loop, lo points at a slot that is never
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* above the target (it could be at the target), hi points at a
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* slot that is guaranteed to be above the target (it can never
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* be at the target).
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*
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* - We find a point 'mi' between lo and hi (mi could be the same
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* as lo, but never can be as same as hi), and check if it hits
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* the target. There are three cases:
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*
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* - if it is a hit, we are happy.
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*
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* - if it is strictly higher than the target, we set it to hi,
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* and repeat the search.
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*
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* - if it is strictly lower than the target, we update lo to
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* one slot after it, because we allow lo to be at the target.
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*
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* If the loop exits, there is no matching entry.
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*
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* When choosing 'mi', we do not have to take the "middle" but
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* anywhere in between lo and hi, as long as lo <= mi < hi is
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* satisfied. When we somehow know that the distance between the
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* target and lo is much shorter than the target and hi, we could
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* pick mi that is much closer to lo than the midway.
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*
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* Now, we can take advantage of the fact that SHA-1 is a good hash
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* function, and as long as there are enough entries in the table, we
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* can expect uniform distribution. An entry that begins with for
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* example "deadbeef..." is much likely to appear much later than in
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* the midway of the table. It can reasonably be expected to be near
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* 87% (222/256) from the top of the table.
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*
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* However, we do not want to pick "mi" too precisely. If the entry at
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* the 87% in the above example turns out to be higher than the target
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* we are looking for, we would end up narrowing the search space down
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* only by 13%, instead of 50% we would get if we did a simple binary
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* search. So we would want to hedge our bets by being less aggressive.
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*
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* The table at "table" holds at least "nr" entries of "elem_size"
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* bytes each. Each entry has the SHA-1 key at "key_offset". The
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* table is sorted by the SHA-1 key of the entries. The caller wants
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* to find the entry with "key", and knows that the entry at "lo" is
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* not higher than the entry it is looking for, and that the entry at
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* "hi" is higher than the entry it is looking for.
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*/
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int sha1_entry_pos(const void *table,
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size_t elem_size,
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size_t key_offset,
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unsigned lo, unsigned hi, unsigned nr,
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const unsigned char *key)
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{
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const unsigned char *base = table;
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const unsigned char *hi_key, *lo_key;
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unsigned ofs_0;
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static int debug_lookup = -1;
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if (debug_lookup < 0)
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debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
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if (!nr || lo >= hi)
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return -1;
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if (nr == hi)
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hi_key = NULL;
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else
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hi_key = base + elem_size * hi + key_offset;
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lo_key = base + elem_size * lo + key_offset;
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ofs_0 = 0;
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do {
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int cmp;
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unsigned ofs, mi, range;
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unsigned lov, hiv, kyv;
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const unsigned char *mi_key;
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range = hi - lo;
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if (hi_key) {
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for (ofs = ofs_0; ofs < 20; ofs++)
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if (lo_key[ofs] != hi_key[ofs])
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break;
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ofs_0 = ofs;
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/*
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* byte 0 thru (ofs-1) are the same between
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* lo and hi; ofs is the first byte that is
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* different.
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*
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* If ofs==20, then no bytes are different,
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* meaning we have entries with duplicate
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* keys. We know that we are in a solid run
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* of this entry (because the entries are
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* sorted, and our lo and hi are the same,
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* there can be nothing but this single key
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* in between). So we can stop the search.
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* Either one of these entries is it (and
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* we do not care which), or we do not have
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* it.
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*
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* Furthermore, we know that one of our
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* endpoints must be the edge of the run of
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* duplicates. For example, given this
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* sequence:
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*
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* idx 0 1 2 3 4 5
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* key A C C C C D
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*
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* If we are searching for "B", we might
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* hit the duplicate run at lo=1, hi=3
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* (e.g., by first mi=3, then mi=0). But we
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* can never have lo > 1, because B < C.
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* That is, if our key is less than the
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* run, we know that "lo" is the edge, but
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* we can say nothing of "hi". Similarly,
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* if our key is greater than the run, we
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* know that "hi" is the edge, but we can
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* say nothing of "lo".
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*
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* Therefore if we do not find it, we also
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* know where it would go if it did exist:
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* just on the far side of the edge that we
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* know about.
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*/
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if (ofs == 20) {
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mi = lo;
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mi_key = base + elem_size * mi + key_offset;
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cmp = memcmp(mi_key, key, 20);
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if (!cmp)
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return mi;
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if (cmp < 0)
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return -1 - hi;
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else
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return -1 - lo;
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}
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hiv = hi_key[ofs_0];
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if (ofs_0 < 19)
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hiv = (hiv << 8) | hi_key[ofs_0+1];
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} else {
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hiv = 256;
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if (ofs_0 < 19)
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hiv <<= 8;
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}
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lov = lo_key[ofs_0];
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kyv = key[ofs_0];
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if (ofs_0 < 19) {
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lov = (lov << 8) | lo_key[ofs_0+1];
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kyv = (kyv << 8) | key[ofs_0+1];
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}
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assert(lov < hiv);
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if (kyv < lov)
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return -1 - lo;
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if (hiv < kyv)
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return -1 - hi;
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/*
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* Even if we know the target is much closer to 'hi'
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* than 'lo', if we pick too precisely and overshoot
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* (e.g. when we know 'mi' is closer to 'hi' than to
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* 'lo', pick 'mi' that is higher than the target), we
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* end up narrowing the search space by a smaller
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* amount (i.e. the distance between 'mi' and 'hi')
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* than what we would have (i.e. about half of 'lo'
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* and 'hi'). Hedge our bets to pick 'mi' less
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* aggressively, i.e. make 'mi' a bit closer to the
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* middle than we would otherwise pick.
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*/
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kyv = (kyv * 6 + lov + hiv) / 8;
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if (lov < hiv - 1) {
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if (kyv == lov)
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kyv++;
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else if (kyv == hiv)
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kyv--;
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}
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mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
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if (debug_lookup) {
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printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
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printf("ofs %u lov %x, hiv %x, kyv %x\n",
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ofs_0, lov, hiv, kyv);
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}
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if (!(lo <= mi && mi < hi))
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die("assertion failure lo %u mi %u hi %u %s",
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lo, mi, hi, sha1_to_hex(key));
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mi_key = base + elem_size * mi + key_offset;
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cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
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if (!cmp)
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return mi;
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if (cmp > 0) {
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hi = mi;
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hi_key = mi_key;
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} else {
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lo = mi + 1;
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lo_key = mi_key + elem_size;
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}
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} while (lo < hi);
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return -lo-1;
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}
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