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 * However, we do not want to pick "mi" too precisely.  If the entry at
  54 * the 87% in the above example turns out to be higher than the target
  55 * we are looking for, we would end up narrowing the search space down
  56 * only by 13%, instead of 50% we would get if we did a simple binary
  57 * search.  So we would want to hedge our bets by being less aggressive.
  58 *
  59 * The table at "table" holds at least "nr" entries of "elem_size"
  60 * bytes each.  Each entry has the SHA-1 key at "key_offset".  The
  61 * table is sorted by the SHA-1 key of the entries.  The caller wants
  62 * to find the entry with "key", and knows that the entry at "lo" is
  63 * not higher than the entry it is looking for, and that the entry at
  64 * "hi" is higher than the entry it is looking for.
  65 */
  66int sha1_entry_pos(const void *table,
  67                   size_t elem_size,
  68                   size_t key_offset,
  69                   unsigned lo, unsigned hi, unsigned nr,
  70                   const unsigned char *key)
  71{
  72        const unsigned char *base = table;
  73        const unsigned char *hi_key, *lo_key;
  74        unsigned ofs_0;
  75        static int debug_lookup = -1;
  76
  77        if (debug_lookup < 0)
  78                debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
  79
  80        if (!nr || lo >= hi)
  81                return -1;
  82
  83        if (nr == hi)
  84                hi_key = NULL;
  85        else
  86                hi_key = base + elem_size * hi + key_offset;
  87        lo_key = base + elem_size * lo + key_offset;
  88
  89        ofs_0 = 0;
  90        do {
  91                int cmp;
  92                unsigned ofs, mi, range;
  93                unsigned lov, hiv, kyv;
  94                const unsigned char *mi_key;
  95
  96                range = hi - lo;
  97                if (hi_key) {
  98                        for (ofs = ofs_0; ofs < 20; ofs++)
  99                                if (lo_key[ofs] != hi_key[ofs])
 100                                        break;
 101                        ofs_0 = ofs;
 102                        /*
 103                         * byte 0 thru (ofs-1) are the same between
 104                         * lo and hi; ofs is the first byte that is
 105                         * different.
 106                         */
 107                        hiv = hi_key[ofs_0];
 108                        if (ofs_0 < 19)
 109                                hiv = (hiv << 8) | hi_key[ofs_0+1];
 110                } else {
 111                        hiv = 256;
 112                        if (ofs_0 < 19)
 113                                hiv <<= 8;
 114                }
 115                lov = lo_key[ofs_0];
 116                kyv = key[ofs_0];
 117                if (ofs_0 < 19) {
 118                        lov = (lov << 8) | lo_key[ofs_0+1];
 119                        kyv = (kyv << 8) | key[ofs_0+1];
 120                }
 121                assert(lov < hiv);
 122
 123                if (kyv < lov)
 124                        return -1 - lo;
 125                if (hiv < kyv)
 126                        return -1 - hi;
 127
 128                /*
 129                 * Even if we know the target is much closer to 'hi'
 130                 * than 'lo', if we pick too precisely and overshoot
 131                 * (e.g. when we know 'mi' is closer to 'hi' than to
 132                 * 'lo', pick 'mi' that is higher than the target), we
 133                 * end up narrowing the search space by a smaller
 134                 * amount (i.e. the distance between 'mi' and 'hi')
 135                 * than what we would have (i.e. about half of 'lo'
 136                 * and 'hi').  Hedge our bets to pick 'mi' less
 137                 * aggressively, i.e. make 'mi' a bit closer to the
 138                 * middle than we would otherwise pick.
 139                 */
 140                kyv = (kyv * 6 + lov + hiv) / 8;
 141                if (lov < hiv - 1) {
 142                        if (kyv == lov)
 143                                kyv++;
 144                        else if (kyv == hiv)
 145                                kyv--;
 146                }
 147                mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
 148
 149                if (debug_lookup) {
 150                        printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
 151                        printf("ofs %u lov %x, hiv %x, kyv %x\n",
 152                               ofs_0, lov, hiv, kyv);
 153                }
 154                if (!(lo <= mi && mi < hi))
 155                        die("assertion failure lo %u mi %u hi %u %s",
 156                            lo, mi, hi, sha1_to_hex(key));
 157
 158                mi_key = base + elem_size * mi + key_offset;
 159                cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
 160                if (!cmp)
 161                        return mi;
 162                if (cmp > 0) {
 163                        hi = mi;
 164                        hi_key = mi_key;
 165                } else {
 166                        lo = mi + 1;
 167                        lo_key = mi_key + elem_size;
 168                }
 169        } while (lo < hi);
 170        return -lo-1;
 171}