1#include "cache.h" 2#include "sha1-lookup.h" 3 4static 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 { 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); 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 */ 53int 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; 84 die("BUG: assertion failed in binary search"); 85 } 86 } 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; 98 mi = (hi + lo) / 2; 99 } while (lo < hi); 100 return -lo-1; 101} 102 103/* 104 * Conventional binary search loop looks like this: 105 * 106 * unsigned lo, hi; 107 * do { 108 * unsigned mi = (lo + hi) / 2; 109 * int cmp = "entry pointed at by mi" minus "target"; 110 * if (!cmp) 111 * return (mi is the wanted one) 112 * if (cmp > 0) 113 * hi = mi; "mi is larger than target" 114 * else 115 * lo = mi+1; "mi is smaller than target" 116 * } while (lo < hi); 117 * 118 * The invariants are: 119 * 120 * - When entering the loop, lo points at a slot that is never 121 * above the target (it could be at the target), hi points at a 122 * slot that is guaranteed to be above the target (it can never 123 * be at the target). 124 * 125 * - We find a point 'mi' between lo and hi (mi could be the same 126 * as lo, but never can be as same as hi), and check if it hits 127 * the target. There are three cases: 128 * 129 * - if it is a hit, we are happy. 130 * 131 * - if it is strictly higher than the target, we set it to hi, 132 * and repeat the search. 133 * 134 * - if it is strictly lower than the target, we update lo to 135 * one slot after it, because we allow lo to be at the target. 136 * 137 * If the loop exits, there is no matching entry. 138 * 139 * When choosing 'mi', we do not have to take the "middle" but 140 * anywhere in between lo and hi, as long as lo <= mi < hi is 141 * satisfied. When we somehow know that the distance between the 142 * target and lo is much shorter than the target and hi, we could 143 * pick mi that is much closer to lo than the midway. 144 * 145 * Now, we can take advantage of the fact that SHA-1 is a good hash 146 * function, and as long as there are enough entries in the table, we 147 * can expect uniform distribution. An entry that begins with for 148 * example "deadbeef..." is much likely to appear much later than in 149 * the midway of the table. It can reasonably be expected to be near 150 * 87% (222/256) from the top of the table. 151 * 152 * However, we do not want to pick "mi" too precisely. If the entry at 153 * the 87% in the above example turns out to be higher than the target 154 * we are looking for, we would end up narrowing the search space down 155 * only by 13%, instead of 50% we would get if we did a simple binary 156 * search. So we would want to hedge our bets by being less aggressive. 157 * 158 * The table at "table" holds at least "nr" entries of "elem_size" 159 * bytes each. Each entry has the SHA-1 key at "key_offset". The 160 * table is sorted by the SHA-1 key of the entries. The caller wants 161 * to find the entry with "key", and knows that the entry at "lo" is 162 * not higher than the entry it is looking for, and that the entry at 163 * "hi" is higher than the entry it is looking for. 164 */ 165int sha1_entry_pos(const void *table, 166 size_t elem_size, 167 size_t key_offset, 168 unsigned lo, unsigned hi, unsigned nr, 169 const unsigned char *key) 170{ 171 const unsigned char *base = table; 172 const unsigned char *hi_key, *lo_key; 173 unsigned ofs_0; 174 static int debug_lookup = -1; 175 176 if (debug_lookup < 0) 177 debug_lookup = !!getenv("GIT_DEBUG_LOOKUP"); 178 179 if (!nr || lo >= hi) 180 return -1; 181 182 if (nr == hi) 183 hi_key = NULL; 184 else 185 hi_key = base + elem_size * hi + key_offset; 186 lo_key = base + elem_size * lo + key_offset; 187 188 ofs_0 = 0; 189 do { 190 int cmp; 191 unsigned ofs, mi, range; 192 unsigned lov, hiv, kyv; 193 const unsigned char *mi_key; 194 195 range = hi - lo; 196 if (hi_key) { 197 for (ofs = ofs_0; ofs < 20; ofs++) 198 if (lo_key[ofs] != hi_key[ofs]) 199 break; 200 ofs_0 = ofs; 201 /* 202 * byte 0 thru (ofs-1) are the same between 203 * lo and hi; ofs is the first byte that is 204 * different. 205 * 206 * If ofs==20, then no bytes are different, 207 * meaning we have entries with duplicate 208 * keys. We know that we are in a solid run 209 * of this entry (because the entries are 210 * sorted, and our lo and hi are the same, 211 * there can be nothing but this single key 212 * in between). So we can stop the search. 213 * Either one of these entries is it (and 214 * we do not care which), or we do not have 215 * it. 216 * 217 * Furthermore, we know that one of our 218 * endpoints must be the edge of the run of 219 * duplicates. For example, given this 220 * sequence: 221 * 222 * idx 0 1 2 3 4 5 223 * key A C C C C D 224 * 225 * If we are searching for "B", we might 226 * hit the duplicate run at lo=1, hi=3 227 * (e.g., by first mi=3, then mi=0). But we 228 * can never have lo > 1, because B < C. 229 * That is, if our key is less than the 230 * run, we know that "lo" is the edge, but 231 * we can say nothing of "hi". Similarly, 232 * if our key is greater than the run, we 233 * know that "hi" is the edge, but we can 234 * say nothing of "lo". 235 * 236 * Therefore if we do not find it, we also 237 * know where it would go if it did exist: 238 * just on the far side of the edge that we 239 * know about. 240 */ 241 if (ofs == 20) { 242 mi = lo; 243 mi_key = base + elem_size * mi + key_offset; 244 cmp = memcmp(mi_key, key, 20); 245 if (!cmp) 246 return mi; 247 if (cmp < 0) 248 return -1 - hi; 249 else 250 return -1 - lo; 251 } 252 253 hiv = hi_key[ofs_0]; 254 if (ofs_0 < 19) 255 hiv = (hiv << 8) | hi_key[ofs_0+1]; 256 } else { 257 hiv = 256; 258 if (ofs_0 < 19) 259 hiv <<= 8; 260 } 261 lov = lo_key[ofs_0]; 262 kyv = key[ofs_0]; 263 if (ofs_0 < 19) { 264 lov = (lov << 8) | lo_key[ofs_0+1]; 265 kyv = (kyv << 8) | key[ofs_0+1]; 266 } 267 assert(lov < hiv); 268 269 if (kyv < lov) 270 return -1 - lo; 271 if (hiv < kyv) 272 return -1 - hi; 273 274 /* 275 * Even if we know the target is much closer to 'hi' 276 * than 'lo', if we pick too precisely and overshoot 277 * (e.g. when we know 'mi' is closer to 'hi' than to 278 * 'lo', pick 'mi' that is higher than the target), we 279 * end up narrowing the search space by a smaller 280 * amount (i.e. the distance between 'mi' and 'hi') 281 * than what we would have (i.e. about half of 'lo' 282 * and 'hi'). Hedge our bets to pick 'mi' less 283 * aggressively, i.e. make 'mi' a bit closer to the 284 * middle than we would otherwise pick. 285 */ 286 kyv = (kyv * 6 + lov + hiv) / 8; 287 if (lov < hiv - 1) { 288 if (kyv == lov) 289 kyv++; 290 else if (kyv == hiv) 291 kyv--; 292 } 293 mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo; 294 295 if (debug_lookup) { 296 printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi); 297 printf("ofs %u lov %x, hiv %x, kyv %x\n", 298 ofs_0, lov, hiv, kyv); 299 } 300 if (!(lo <= mi && mi < hi)) 301 die("assertion failure lo %u mi %u hi %u %s", 302 lo, mi, hi, sha1_to_hex(key)); 303 304 mi_key = base + elem_size * mi + key_offset; 305 cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0); 306 if (!cmp) 307 return mi; 308 if (cmp > 0) { 309 hi = mi; 310 hi_key = mi_key; 311 } else { 312 lo = mi + 1; 313 lo_key = mi_key + elem_size; 314 } 315 } while (lo < hi); 316 return -lo-1; 317}