sha1-lookup: more memory efficient search in sorted list of SHA-1
[git/git.git] / sha1-lookup.c
CommitLineData
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 *
53 * The table at "table" holds at least "nr" entries of "elem_size"
54 * bytes each. Each entry has the SHA-1 key at "key_offset". The
55 * table is sorted by the SHA-1 key of the entries. The caller wants
56 * to find the entry with "key", and knows that the entry at "lo" is
57 * not higher than the entry it is looking for, and that the entry at
58 * "hi" is higher than the entry it is looking for.
59 */
60int sha1_entry_pos(const void *table,
61 size_t elem_size,
62 size_t key_offset,
63 unsigned lo, unsigned hi, unsigned nr,
64 const unsigned char *key)
65{
66 const unsigned char *base = table;
67 const unsigned char *hi_key, *lo_key;
68 unsigned ofs_0;
69 static int debug_lookup = -1;
70
71 if (debug_lookup < 0)
72 debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
73
74 if (!nr || lo >= hi)
75 return -1;
76
77 if (nr == hi)
78 hi_key = NULL;
79 else
80 hi_key = base + elem_size * hi + key_offset;
81 lo_key = base + elem_size * lo + key_offset;
82
83 ofs_0 = 0;
84 do {
85 int cmp;
86 unsigned ofs, mi, range;
87 unsigned lov, hiv, kyv;
88 const unsigned char *mi_key;
89
90 range = hi - lo;
91 if (hi_key) {
92 for (ofs = ofs_0; ofs < 20; ofs++)
93 if (lo_key[ofs] != hi_key[ofs])
94 break;
95 ofs_0 = ofs;
96 /*
97 * byte 0 thru (ofs-1) are the same between
98 * lo and hi; ofs is the first byte that is
99 * different.
100 */
101 hiv = hi_key[ofs_0];
102 if (ofs_0 < 19)
103 hiv = (hiv << 8) | hi_key[ofs_0+1];
104 } else {
105 hiv = 256;
106 if (ofs_0 < 19)
107 hiv <<= 8;
108 }
109 lov = lo_key[ofs_0];
110 kyv = key[ofs_0];
111 if (ofs_0 < 19) {
112 lov = (lov << 8) | lo_key[ofs_0+1];
113 kyv = (kyv << 8) | key[ofs_0+1];
114 }
115 assert(lov < hiv);
116
117 if (kyv < lov)
118 return -1 - lo;
119 if (hiv < kyv)
120 return -1 - hi;
121
122 if (kyv == lov && lov < hiv - 1)
123 kyv++;
124 else if (kyv == hiv - 1 && lov < kyv)
125 kyv--;
126
127 mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
128
129 if (debug_lookup) {
130 printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
131 printf("ofs %u lov %x, hiv %x, kyv %x\n",
132 ofs_0, lov, hiv, kyv);
133 }
134 if (!(lo <= mi && mi < hi))
135 die("assertion failure lo %u mi %u hi %u %s",
136 lo, mi, hi, sha1_to_hex(key));
137
138 mi_key = base + elem_size * mi + key_offset;
139 cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
140 if (!cmp)
141 return mi;
142 if (cmp > 0) {
143 hi = mi;
144 hi_key = mi_key;
145 }
146 else {
147 lo = mi + 1;
148 lo_key = mi_key + elem_size;
149 }
150 } while (lo < hi);
151 return -lo-1;
152}