# git / sha1-lookup.c

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