Andrew's git - gitweb.git/blob

   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}