sha1-lookup.c

   1 #include "cache.h"
   2 #include "sha1-lookup.h"
   3
   4 static 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 - lo) / 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  */
  53 int 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 = lo + (hi - lo) / 2;
  99         } while (lo < hi);
 100         return -lo-1;
 101 }