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628522ec JH |
1 | #include "cache.h" |
2 | #include "sha1-lookup.h" | |
3 | ||
4 | /* | |
5 | * Conventional binary search loop looks like this: | |
6 | * | |
7 | * unsigned lo, hi; | |
8 | * do { | |
9 | * unsigned mi = (lo + hi) / 2; | |
10 | * int cmp = "entry pointed at by mi" minus "target"; | |
11 | * if (!cmp) | |
12 | * return (mi is the wanted one) | |
13 | * if (cmp > 0) | |
14 | * hi = mi; "mi is larger than target" | |
15 | * else | |
16 | * lo = mi+1; "mi is smaller than target" | |
17 | * } while (lo < hi); | |
18 | * | |
19 | * The invariants are: | |
20 | * | |
21 | * - When entering the loop, lo points at a slot that is never | |
22 | * above the target (it could be at the target), hi points at a | |
23 | * slot that is guaranteed to be above the target (it can never | |
24 | * be at the target). | |
25 | * | |
26 | * - We find a point 'mi' between lo and hi (mi could be the same | |
27 | * as lo, but never can be as same as hi), and check if it hits | |
28 | * the target. There are three cases: | |
29 | * | |
30 | * - if it is a hit, we are happy. | |
31 | * | |
32 | * - if it is strictly higher than the target, we set it to hi, | |
33 | * and repeat the search. | |
34 | * | |
35 | * - if it is strictly lower than the target, we update lo to | |
36 | * one slot after it, because we allow lo to be at the target. | |
37 | * | |
38 | * If the loop exits, there is no matching entry. | |
39 | * | |
40 | * When choosing 'mi', we do not have to take the "middle" but | |
41 | * anywhere in between lo and hi, as long as lo <= mi < hi is | |
42 | * satisfied. When we somehow know that the distance between the | |
43 | * target and lo is much shorter than the target and hi, we could | |
44 | * pick mi that is much closer to lo than the midway. | |
45 | * | |
46 | * Now, we can take advantage of the fact that SHA-1 is a good hash | |
47 | * function, and as long as there are enough entries in the table, we | |
48 | * can expect uniform distribution. An entry that begins with for | |
49 | * example "deadbeef..." is much likely to appear much later than in | |
50 | * the midway of the table. It can reasonably be expected to be near | |
51 | * 87% (222/256) from the top of the table. | |
52 | * | |
12ecb011 JH |
53 | * However, we do not want to pick "mi" too precisely. If the entry at |
54 | * the 87% in the above example turns out to be higher than the target | |
55 | * we are looking for, we would end up narrowing the search space down | |
56 | * only by 13%, instead of 50% we would get if we did a simple binary | |
57 | * search. So we would want to hedge our bets by being less aggressive. | |
58 | * | |
628522ec JH |
59 | * The table at "table" holds at least "nr" entries of "elem_size" |
60 | * bytes each. Each entry has the SHA-1 key at "key_offset". The | |
61 | * table is sorted by the SHA-1 key of the entries. The caller wants | |
62 | * to find the entry with "key", and knows that the entry at "lo" is | |
63 | * not higher than the entry it is looking for, and that the entry at | |
64 | * "hi" is higher than the entry it is looking for. | |
65 | */ | |
66 | int sha1_entry_pos(const void *table, | |
67 | size_t elem_size, | |
68 | size_t key_offset, | |
69 | unsigned lo, unsigned hi, unsigned nr, | |
70 | const unsigned char *key) | |
71 | { | |
72 | const unsigned char *base = table; | |
73 | const unsigned char *hi_key, *lo_key; | |
74 | unsigned ofs_0; | |
75 | static int debug_lookup = -1; | |
76 | ||
77 | if (debug_lookup < 0) | |
78 | debug_lookup = !!getenv("GIT_DEBUG_LOOKUP"); | |
79 | ||
80 | if (!nr || lo >= hi) | |
81 | return -1; | |
82 | ||
83 | if (nr == hi) | |
84 | hi_key = NULL; | |
85 | else | |
86 | hi_key = base + elem_size * hi + key_offset; | |
87 | lo_key = base + elem_size * lo + key_offset; | |
88 | ||
89 | ofs_0 = 0; | |
90 | do { | |
91 | int cmp; | |
92 | unsigned ofs, mi, range; | |
93 | unsigned lov, hiv, kyv; | |
94 | const unsigned char *mi_key; | |
95 | ||
96 | range = hi - lo; | |
97 | if (hi_key) { | |
98 | for (ofs = ofs_0; ofs < 20; ofs++) | |
99 | if (lo_key[ofs] != hi_key[ofs]) | |
100 | break; | |
101 | ofs_0 = ofs; | |
102 | /* | |
103 | * byte 0 thru (ofs-1) are the same between | |
104 | * lo and hi; ofs is the first byte that is | |
105 | * different. | |
106 | */ | |
107 | hiv = hi_key[ofs_0]; | |
108 | if (ofs_0 < 19) | |
109 | hiv = (hiv << 8) | hi_key[ofs_0+1]; | |
110 | } else { | |
111 | hiv = 256; | |
112 | if (ofs_0 < 19) | |
113 | hiv <<= 8; | |
114 | } | |
115 | lov = lo_key[ofs_0]; | |
116 | kyv = key[ofs_0]; | |
117 | if (ofs_0 < 19) { | |
118 | lov = (lov << 8) | lo_key[ofs_0+1]; | |
119 | kyv = (kyv << 8) | key[ofs_0+1]; | |
120 | } | |
121 | assert(lov < hiv); | |
122 | ||
123 | if (kyv < lov) | |
124 | return -1 - lo; | |
125 | if (hiv < kyv) | |
126 | return -1 - hi; | |
127 | ||
12ecb011 JH |
128 | /* |
129 | * Even if we know the target is much closer to 'hi' | |
130 | * than 'lo', if we pick too precisely and overshoot | |
131 | * (e.g. when we know 'mi' is closer to 'hi' than to | |
132 | * 'lo', pick 'mi' that is higher than the target), we | |
133 | * end up narrowing the search space by a smaller | |
134 | * amount (i.e. the distance between 'mi' and 'hi') | |
135 | * than what we would have (i.e. about half of 'lo' | |
136 | * and 'hi'). Hedge our bets to pick 'mi' less | |
137 | * aggressively, i.e. make 'mi' a bit closer to the | |
138 | * middle than we would otherwise pick. | |
139 | */ | |
140 | kyv = (kyv * 6 + lov + hiv) / 8; | |
141 | if (lov < hiv - 1) { | |
142 | if (kyv == lov) | |
143 | kyv++; | |
144 | else if (kyv == hiv) | |
145 | kyv--; | |
146 | } | |
628522ec JH |
147 | mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo; |
148 | ||
149 | if (debug_lookup) { | |
150 | printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi); | |
151 | printf("ofs %u lov %x, hiv %x, kyv %x\n", | |
152 | ofs_0, lov, hiv, kyv); | |
153 | } | |
154 | if (!(lo <= mi && mi < hi)) | |
155 | die("assertion failure lo %u mi %u hi %u %s", | |
156 | lo, mi, hi, sha1_to_hex(key)); | |
157 | ||
158 | mi_key = base + elem_size * mi + key_offset; | |
159 | cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0); | |
160 | if (!cmp) | |
161 | return mi; | |
162 | if (cmp > 0) { | |
163 | hi = mi; | |
164 | hi_key = mi_key; | |
12ecb011 | 165 | } else { |
628522ec JH |
166 | lo = mi + 1; |
167 | lo_key = mi_key + elem_size; | |
168 | } | |
169 | } while (lo < hi); | |
170 | return -lo-1; | |
171 | } |