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1 | #include "git-compat-util.h" | |
2 | #include "hash.h" | |
3 | #include "hash-lookup.h" | |
4 | #include "read-cache-ll.h" | |
5 | ||
6 | static uint32_t take2(const struct object_id *oid, size_t ofs) | |
7 | { | |
8 | return ((oid->hash[ofs] << 8) | oid->hash[ofs + 1]); | |
9 | } | |
10 | ||
11 | /* | |
12 | * Conventional binary search loop looks like this: | |
13 | * | |
14 | * do { | |
15 | * int mi = lo + (hi - lo) / 2; | |
16 | * int cmp = "entry pointed at by mi" minus "target"; | |
17 | * if (!cmp) | |
18 | * return (mi is the wanted one) | |
19 | * if (cmp > 0) | |
20 | * hi = mi; "mi is larger than target" | |
21 | * else | |
22 | * lo = mi+1; "mi is smaller than target" | |
23 | * } while (lo < hi); | |
24 | * | |
25 | * The invariants are: | |
26 | * | |
27 | * - When entering the loop, lo points at a slot that is never | |
28 | * above the target (it could be at the target), hi points at a | |
29 | * slot that is guaranteed to be above the target (it can never | |
30 | * be at the target). | |
31 | * | |
32 | * - We find a point 'mi' between lo and hi (mi could be the same | |
33 | * as lo, but never can be the same as hi), and check if it hits | |
34 | * the target. There are three cases: | |
35 | * | |
36 | * - if it is a hit, we are happy. | |
37 | * | |
38 | * - if it is strictly higher than the target, we update hi with | |
39 | * it. | |
40 | * | |
41 | * - if it is strictly lower than the target, we update lo to be | |
42 | * one slot after it, because we allow lo to be at the target. | |
43 | * | |
44 | * When choosing 'mi', we do not have to take the "middle" but | |
45 | * anywhere in between lo and hi, as long as lo <= mi < hi is | |
46 | * satisfied. When we somehow know that the distance between the | |
47 | * target and lo is much shorter than the target and hi, we could | |
48 | * pick mi that is much closer to lo than the midway. | |
49 | */ | |
50 | /* | |
51 | * The table should contain "nr" elements. | |
52 | * The oid of element i (between 0 and nr - 1) should be returned | |
53 | * by "fn(i, table)". | |
54 | */ | |
55 | int oid_pos(const struct object_id *oid, const void *table, size_t nr, | |
56 | oid_access_fn fn) | |
57 | { | |
58 | size_t hi = nr; | |
59 | size_t lo = 0; | |
60 | size_t mi = 0; | |
61 | ||
62 | if (!nr) | |
63 | return -1; | |
64 | ||
65 | if (nr != 1) { | |
66 | size_t lov, hiv, miv, ofs; | |
67 | ||
68 | for (ofs = 0; ofs < the_hash_algo->rawsz - 2; ofs += 2) { | |
69 | lov = take2(fn(0, table), ofs); | |
70 | hiv = take2(fn(nr - 1, table), ofs); | |
71 | miv = take2(oid, ofs); | |
72 | if (miv < lov) | |
73 | return -1; | |
74 | if (hiv < miv) | |
75 | return index_pos_to_insert_pos(nr); | |
76 | if (lov != hiv) { | |
77 | /* | |
78 | * At this point miv could be equal | |
79 | * to hiv (but hash could still be higher); | |
80 | * the invariant of (mi < hi) should be | |
81 | * kept. | |
82 | */ | |
83 | mi = (nr - 1) * (miv - lov) / (hiv - lov); | |
84 | if (lo <= mi && mi < hi) | |
85 | break; | |
86 | BUG("assertion failed in binary search"); | |
87 | } | |
88 | } | |
89 | } | |
90 | ||
91 | do { | |
92 | int cmp; | |
93 | cmp = oidcmp(fn(mi, table), oid); | |
94 | if (!cmp) | |
95 | return mi; | |
96 | if (cmp > 0) | |
97 | hi = mi; | |
98 | else | |
99 | lo = mi + 1; | |
100 | mi = lo + (hi - lo) / 2; | |
101 | } while (lo < hi); | |
102 | return index_pos_to_insert_pos(lo); | |
103 | } | |
104 | ||
105 | int bsearch_hash(const unsigned char *hash, const uint32_t *fanout_nbo, | |
106 | const unsigned char *table, size_t stride, uint32_t *result) | |
107 | { | |
108 | uint32_t hi, lo; | |
109 | ||
110 | hi = ntohl(fanout_nbo[*hash]); | |
111 | lo = ((*hash == 0x0) ? 0 : ntohl(fanout_nbo[*hash - 1])); | |
112 | ||
113 | while (lo < hi) { | |
114 | unsigned mi = lo + (hi - lo) / 2; | |
115 | int cmp = hashcmp(table + mi * stride, hash); | |
116 | ||
117 | if (!cmp) { | |
118 | if (result) | |
119 | *result = mi; | |
120 | return 1; | |
121 | } | |
122 | if (cmp > 0) | |
123 | hi = mi; | |
124 | else | |
125 | lo = mi + 1; | |
126 | } | |
127 | ||
128 | if (result) | |
129 | *result = lo; | |
130 | return 0; | |
131 | } |