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}