hash-lookup.c

   1 #include "cache.h"
   2 #include "hash.h"
   3 #include "hash-lookup.h"
   4
   5 static uint32_t take2(const struct object_id *oid, size_t ofs)
   6 {
   7         return ((oid->hash[ofs] << 8) | oid->hash[ofs + 1]);
   8 }
   9
  10 /*
  11  * Conventional binary search loop looks like this:
  12  *
  13  *      do {
  14  *              int mi = lo + (hi - lo) / 2;
  15  *              int cmp = "entry pointed at by mi" minus "target";
  16  *              if (!cmp)
  17  *                      return (mi is the wanted one)
  18  *              if (cmp > 0)
  19  *                      hi = mi; "mi is larger than target"
  20  *              else
  21  *                      lo = mi+1; "mi is smaller than target"
  22  *      } while (lo < hi);
  23  *
  24  * The invariants are:
  25  *
  26  * - When entering the loop, lo points at a slot that is never
  27  *   above the target (it could be at the target), hi points at a
  28  *   slot that is guaranteed to be above the target (it can never
  29  *   be at the target).
  30  *
  31  * - We find a point 'mi' between lo and hi (mi could be the same
  32  *   as lo, but never can be the same as hi), and check if it hits
  33  *   the target.  There are three cases:
  34  *
  35  *    - if it is a hit, we are happy.
  36  *
  37  *    - if it is strictly higher than the target, we update hi with
  38  *      it.
  39  *
  40  *    - if it is strictly lower than the target, we update lo to be
  41  *      one slot after it, because we allow lo to be at the target.
  42  *
  43  * When choosing 'mi', we do not have to take the "middle" but
  44  * anywhere in between lo and hi, as long as lo <= mi < hi is
  45  * satisfied.  When we somehow know that the distance between the
  46  * target and lo is much shorter than the target and hi, we could
  47  * pick mi that is much closer to lo than the midway.
  48  */
  49 /*
  50  * The table should contain "nr" elements.
  51  * The oid of element i (between 0 and nr - 1) should be returned
  52  * by "fn(i, table)".
  53  */
  54 int oid_pos(const struct object_id *oid, const void *table, size_t nr,
  55             oid_access_fn fn)
  56 {
  57         size_t hi = nr;
  58         size_t lo = 0;
  59         size_t mi = 0;
  60
  61         if (!nr)
  62                 return -1;
  63
  64         if (nr != 1) {
  65                 size_t lov, hiv, miv, ofs;
  66
  67                 for (ofs = 0; ofs < the_hash_algo->rawsz - 2; ofs += 2) {
  68                         lov = take2(fn(0, table), ofs);
  69                         hiv = take2(fn(nr - 1, table), ofs);
  70                         miv = take2(oid, ofs);
  71                         if (miv < lov)
  72                                 return -1;
  73                         if (hiv < miv)
  74                                 return index_pos_to_insert_pos(nr);
  75                         if (lov != hiv) {
  76                                 /*
  77                                  * At this point miv could be equal
  78                                  * to hiv (but hash could still be higher);
  79                                  * the invariant of (mi < hi) should be
  80                                  * kept.
  81                                  */
  82                                 mi = (nr - 1) * (miv - lov) / (hiv - lov);
  83                                 if (lo <= mi && mi < hi)
  84                                         break;
  85                                 BUG("assertion failed in binary search");
  86                         }
  87                 }
  88         }
  89
  90         do {
  91                 int cmp;
  92                 cmp = oidcmp(fn(mi, table), oid);
  93                 if (!cmp)
  94                         return mi;
  95                 if (cmp > 0)
  96                         hi = mi;
  97                 else
  98                         lo = mi + 1;
  99                 mi = lo + (hi - lo) / 2;
 100         } while (lo < hi);
 101         return index_pos_to_insert_pos(lo);
 102 }
 103
 104 int bsearch_hash(const unsigned char *hash, const uint32_t *fanout_nbo,
 105                  const unsigned char *table, size_t stride, uint32_t *result)
 106 {
 107         uint32_t hi, lo;
 108
 109         hi = ntohl(fanout_nbo[*hash]);
 110         lo = ((*hash == 0x0) ? 0 : ntohl(fanout_nbo[*hash - 1]));
 111
 112         while (lo < hi) {
 113                 unsigned mi = lo + (hi - lo) / 2;
 114                 int cmp = hashcmp(table + mi * stride, hash);
 115
 116                 if (!cmp) {
 117                         if (result)
 118                                 *result = mi;
 119                         return 1;
 120                 }
 121                 if (cmp > 0)
 122                         hi = mi;
 123                 else
 124                         lo = mi + 1;
 125         }
 126
 127         if (result)
 128                 *result = lo;
 129         return 0;
 130 }