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[git.git] / sha1-lookup.c
blob4cf3ebd9212f6d5c9b9829373e58c34b83f0a548
1 #include "cache.h"
2 #include "sha1-lookup.h"
4 static uint32_t take2(const unsigned char *sha1)
6 return ((sha1[0] << 8) | sha1[1]);
9 /*
10 * Conventional binary search loop looks like this:
12 * do {
13 * int mi = lo + (hi - lo) / 2;
14 * int cmp = "entry pointed at by mi" minus "target";
15 * if (!cmp)
16 * return (mi is the wanted one)
17 * if (cmp > 0)
18 * hi = mi; "mi is larger than target"
19 * else
20 * lo = mi+1; "mi is smaller than target"
21 * } while (lo < hi);
23 * The invariants are:
25 * - When entering the loop, lo points at a slot that is never
26 * above the target (it could be at the target), hi points at a
27 * slot that is guaranteed to be above the target (it can never
28 * be at the target).
30 * - We find a point 'mi' between lo and hi (mi could be the same
31 * as lo, but never can be the same as hi), and check if it hits
32 * the target. There are three cases:
34 * - if it is a hit, we are happy.
36 * - if it is strictly higher than the target, we update hi with
37 * it.
39 * - if it is strictly lower than the target, we update lo to be
40 * one slot after it, because we allow lo to be at the target.
42 * When choosing 'mi', we do not have to take the "middle" but
43 * anywhere in between lo and hi, as long as lo <= mi < hi is
44 * satisfied. When we somehow know that the distance between the
45 * target and lo is much shorter than the target and hi, we could
46 * pick mi that is much closer to lo than the midway.
49 * The table should contain "nr" elements.
50 * The sha1 of element i (between 0 and nr - 1) should be returned
51 * by "fn(i, table)".
53 int sha1_pos(const unsigned char *sha1, void *table, size_t nr,
54 sha1_access_fn fn)
56 size_t hi = nr;
57 size_t lo = 0;
58 size_t mi = 0;
60 if (!nr)
61 return -1;
63 if (nr != 1) {
64 size_t lov, hiv, miv, ofs;
66 for (ofs = 0; ofs < 18; ofs += 2) {
67 lov = take2(fn(0, table) + ofs);
68 hiv = take2(fn(nr - 1, table) + ofs);
69 miv = take2(sha1 + ofs);
70 if (miv < lov)
71 return -1;
72 if (hiv < miv)
73 return -1 - nr;
74 if (lov != hiv) {
76 * At this point miv could be equal
77 * to hiv (but sha1 could still be higher);
78 * the invariant of (mi < hi) should be
79 * kept.
81 mi = (nr - 1) * (miv - lov) / (hiv - lov);
82 if (lo <= mi && mi < hi)
83 break;
84 die("BUG: assertion failed in binary search");
89 do {
90 int cmp;
91 cmp = hashcmp(fn(mi, table), sha1);
92 if (!cmp)
93 return mi;
94 if (cmp > 0)
95 hi = mi;
96 else
97 lo = mi + 1;
98 mi = lo + (hi - lo) / 2;
99 } while (lo < hi);
100 return -lo-1;