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Commit | Line | Data |
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628522ec JH |
1 | #include "cache.h" |
2 | #include "sha1-lookup.h" | |
3 | ||
96beef8c CC |
4 | static uint32_t take2(const unsigned char *sha1) |
5 | { | |
6 | return ((sha1[0] << 8) | sha1[1]); | |
7 | } | |
8 | ||
9 | /* | |
10 | * Conventional binary search loop looks like this: | |
11 | * | |
12 | * do { | |
19716b21 | 13 | * int mi = lo + (hi - lo) / 2; |
96beef8c CC |
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); | |
22 | * | |
23 | * The invariants are: | |
24 | * | |
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). | |
29 | * | |
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: | |
33 | * | |
34 | * - if it is a hit, we are happy. | |
35 | * | |
36 | * - if it is strictly higher than the target, we update hi with | |
37 | * it. | |
38 | * | |
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. | |
41 | * | |
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. | |
47 | */ | |
48 | /* | |
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)". | |
52 | */ | |
53 | int sha1_pos(const unsigned char *sha1, void *table, size_t nr, | |
54 | sha1_access_fn fn) | |
55 | { | |
56 | size_t hi = nr; | |
57 | size_t lo = 0; | |
58 | size_t mi = 0; | |
59 | ||
60 | if (!nr) | |
61 | return -1; | |
62 | ||
63 | if (nr != 1) { | |
64 | size_t lov, hiv, miv, ofs; | |
65 | ||
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) { | |
75 | /* | |
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. | |
80 | */ | |
81 | mi = (nr - 1) * (miv - lov) / (hiv - lov); | |
82 | if (lo <= mi && mi < hi) | |
83 | break; | |
1a7b1f6b | 84 | die("BUG: assertion failed in binary search"); |
96beef8c CC |
85 | } |
86 | } | |
96beef8c CC |
87 | } |
88 | ||
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; | |
19716b21 | 98 | mi = lo + (hi - lo) / 2; |
96beef8c CC |
99 | } while (lo < hi); |
100 | return -lo-1; | |
101 | } | |
b4e00f73 JT |
102 | |
103 | int bsearch_hash(const unsigned char *sha1, const uint32_t *fanout_nbo, | |
104 | const unsigned char *table, size_t stride, uint32_t *result) | |
105 | { | |
106 | uint32_t hi, lo; | |
107 | ||
108 | hi = ntohl(fanout_nbo[*sha1]); | |
109 | lo = ((*sha1 == 0x0) ? 0 : ntohl(fanout_nbo[*sha1 - 1])); | |
110 | ||
111 | while (lo < hi) { | |
112 | unsigned mi = lo + (hi - lo) / 2; | |
113 | int cmp = hashcmp(table + mi * stride, sha1); | |
114 | ||
115 | if (!cmp) { | |
116 | if (result) | |
117 | *result = mi; | |
118 | return 1; | |
119 | } | |
120 | if (cmp > 0) | |
121 | hi = mi; | |
122 | else | |
123 | lo = mi + 1; | |
124 | } | |
125 | ||
126 | if (result) | |
127 | *result = lo; | |
128 | return 0; | |
129 | } |