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[alt-git.git] / sha1-lookup.c
blobc4dc55d1f5cd07adcf46865354f841cc587c51f6
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) / 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");
87 if (18 <= ofs)
88 die("cannot happen -- lo and hi are identical");
91 do {
92 int cmp;
93 cmp = hashcmp(fn(mi, table), sha1);
94 if (!cmp)
95 return mi;
96 if (cmp > 0)
97 hi = mi;
98 else
99 lo = mi + 1;
100 mi = (hi + lo) / 2;
101 } while (lo < hi);
102 return -lo-1;
106 * Conventional binary search loop looks like this:
108 * unsigned lo, hi;
109 * do {
110 * unsigned mi = (lo + hi) / 2;
111 * int cmp = "entry pointed at by mi" minus "target";
112 * if (!cmp)
113 * return (mi is the wanted one)
114 * if (cmp > 0)
115 * hi = mi; "mi is larger than target"
116 * else
117 * lo = mi+1; "mi is smaller than target"
118 * } while (lo < hi);
120 * The invariants are:
122 * - When entering the loop, lo points at a slot that is never
123 * above the target (it could be at the target), hi points at a
124 * slot that is guaranteed to be above the target (it can never
125 * be at the target).
127 * - We find a point 'mi' between lo and hi (mi could be the same
128 * as lo, but never can be as same as hi), and check if it hits
129 * the target. There are three cases:
131 * - if it is a hit, we are happy.
133 * - if it is strictly higher than the target, we set it to hi,
134 * and repeat the search.
136 * - if it is strictly lower than the target, we update lo to
137 * one slot after it, because we allow lo to be at the target.
139 * If the loop exits, there is no matching entry.
141 * When choosing 'mi', we do not have to take the "middle" but
142 * anywhere in between lo and hi, as long as lo <= mi < hi is
143 * satisfied. When we somehow know that the distance between the
144 * target and lo is much shorter than the target and hi, we could
145 * pick mi that is much closer to lo than the midway.
147 * Now, we can take advantage of the fact that SHA-1 is a good hash
148 * function, and as long as there are enough entries in the table, we
149 * can expect uniform distribution. An entry that begins with for
150 * example "deadbeef..." is much likely to appear much later than in
151 * the midway of the table. It can reasonably be expected to be near
152 * 87% (222/256) from the top of the table.
154 * However, we do not want to pick "mi" too precisely. If the entry at
155 * the 87% in the above example turns out to be higher than the target
156 * we are looking for, we would end up narrowing the search space down
157 * only by 13%, instead of 50% we would get if we did a simple binary
158 * search. So we would want to hedge our bets by being less aggressive.
160 * The table at "table" holds at least "nr" entries of "elem_size"
161 * bytes each. Each entry has the SHA-1 key at "key_offset". The
162 * table is sorted by the SHA-1 key of the entries. The caller wants
163 * to find the entry with "key", and knows that the entry at "lo" is
164 * not higher than the entry it is looking for, and that the entry at
165 * "hi" is higher than the entry it is looking for.
167 int sha1_entry_pos(const void *table,
168 size_t elem_size,
169 size_t key_offset,
170 unsigned lo, unsigned hi, unsigned nr,
171 const unsigned char *key)
173 const unsigned char *base = table;
174 const unsigned char *hi_key, *lo_key;
175 unsigned ofs_0;
176 static int debug_lookup = -1;
178 if (debug_lookup < 0)
179 debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
181 if (!nr || lo >= hi)
182 return -1;
184 if (nr == hi)
185 hi_key = NULL;
186 else
187 hi_key = base + elem_size * hi + key_offset;
188 lo_key = base + elem_size * lo + key_offset;
190 ofs_0 = 0;
191 do {
192 int cmp;
193 unsigned ofs, mi, range;
194 unsigned lov, hiv, kyv;
195 const unsigned char *mi_key;
197 range = hi - lo;
198 if (hi_key) {
199 for (ofs = ofs_0; ofs < 20; ofs++)
200 if (lo_key[ofs] != hi_key[ofs])
201 break;
202 ofs_0 = ofs;
204 * byte 0 thru (ofs-1) are the same between
205 * lo and hi; ofs is the first byte that is
206 * different.
208 hiv = hi_key[ofs_0];
209 if (ofs_0 < 19)
210 hiv = (hiv << 8) | hi_key[ofs_0+1];
211 } else {
212 hiv = 256;
213 if (ofs_0 < 19)
214 hiv <<= 8;
216 lov = lo_key[ofs_0];
217 kyv = key[ofs_0];
218 if (ofs_0 < 19) {
219 lov = (lov << 8) | lo_key[ofs_0+1];
220 kyv = (kyv << 8) | key[ofs_0+1];
222 assert(lov < hiv);
224 if (kyv < lov)
225 return -1 - lo;
226 if (hiv < kyv)
227 return -1 - hi;
230 * Even if we know the target is much closer to 'hi'
231 * than 'lo', if we pick too precisely and overshoot
232 * (e.g. when we know 'mi' is closer to 'hi' than to
233 * 'lo', pick 'mi' that is higher than the target), we
234 * end up narrowing the search space by a smaller
235 * amount (i.e. the distance between 'mi' and 'hi')
236 * than what we would have (i.e. about half of 'lo'
237 * and 'hi'). Hedge our bets to pick 'mi' less
238 * aggressively, i.e. make 'mi' a bit closer to the
239 * middle than we would otherwise pick.
241 kyv = (kyv * 6 + lov + hiv) / 8;
242 if (lov < hiv - 1) {
243 if (kyv == lov)
244 kyv++;
245 else if (kyv == hiv)
246 kyv--;
248 mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
250 if (debug_lookup) {
251 printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
252 printf("ofs %u lov %x, hiv %x, kyv %x\n",
253 ofs_0, lov, hiv, kyv);
255 if (!(lo <= mi && mi < hi))
256 die("assertion failure lo %u mi %u hi %u %s",
257 lo, mi, hi, sha1_to_hex(key));
259 mi_key = base + elem_size * mi + key_offset;
260 cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
261 if (!cmp)
262 return mi;
263 if (cmp > 0) {
264 hi = mi;
265 hi_key = mi_key;
266 } else {
267 lo = mi + 1;
268 lo_key = mi_key + elem_size;
270 } while (lo < hi);
271 return -lo-1;