Andrew's git - gitweb.git/blob

   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}