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) / 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 = (hi + lo) / 2;
  99         } while (lo < hi);
 100         return -lo-1;
 101 }
 102
 103 /*
 104  * Conventional binary search loop looks like this:
 105  *
 106  *      unsigned lo, hi;
 107  *      do {
 108  *              unsigned mi = (lo + hi) / 2;
 109  *              int cmp = "entry pointed at by mi" minus "target";
 110  *              if (!cmp)
 111  *                      return (mi is the wanted one)
 112  *              if (cmp > 0)
 113  *                      hi = mi; "mi is larger than target"
 114  *              else
 115  *                      lo = mi+1; "mi is smaller than target"
 116  *      } while (lo < hi);
 117  *
 118  * The invariants are:
 119  *
 120  * - When entering the loop, lo points at a slot that is never
 121  *   above the target (it could be at the target), hi points at a
 122  *   slot that is guaranteed to be above the target (it can never
 123  *   be at the target).
 124  *
 125  * - We find a point 'mi' between lo and hi (mi could be the same
 126  *   as lo, but never can be as same as hi), and check if it hits
 127  *   the target.  There are three cases:
 128  *
 129  *    - if it is a hit, we are happy.
 130  *
 131  *    - if it is strictly higher than the target, we set it to hi,
 132  *      and repeat the search.
 133  *
 134  *    - if it is strictly lower than the target, we update lo to
 135  *      one slot after it, because we allow lo to be at the target.
 136  *
 137  *   If the loop exits, there is no matching entry.
 138  *
 139  * When choosing 'mi', we do not have to take the "middle" but
 140  * anywhere in between lo and hi, as long as lo <= mi < hi is
 141  * satisfied.  When we somehow know that the distance between the
 142  * target and lo is much shorter than the target and hi, we could
 143  * pick mi that is much closer to lo than the midway.
 144  *
 145  * Now, we can take advantage of the fact that SHA-1 is a good hash
 146  * function, and as long as there are enough entries in the table, we
 147  * can expect uniform distribution.  An entry that begins with for
 148  * example "deadbeef..." is much likely to appear much later than in
 149  * the midway of the table.  It can reasonably be expected to be near
 150  * 87% (222/256) from the top of the table.
 151  *
 152  * However, we do not want to pick "mi" too precisely.  If the entry at
 153  * the 87% in the above example turns out to be higher than the target
 154  * we are looking for, we would end up narrowing the search space down
 155  * only by 13%, instead of 50% we would get if we did a simple binary
 156  * search.  So we would want to hedge our bets by being less aggressive.
 157  *
 158  * The table at "table" holds at least "nr" entries of "elem_size"
 159  * bytes each.  Each entry has the SHA-1 key at "key_offset".  The
 160  * table is sorted by the SHA-1 key of the entries.  The caller wants
 161  * to find the entry with "key", and knows that the entry at "lo" is
 162  * not higher than the entry it is looking for, and that the entry at
 163  * "hi" is higher than the entry it is looking for.
 164  */
 165 int sha1_entry_pos(const void *table,
 166                    size_t elem_size,
 167                    size_t key_offset,
 168                    unsigned lo, unsigned hi, unsigned nr,
 169                    const unsigned char *key)
 170 {
 171         const unsigned char *base = table;
 172         const unsigned char *hi_key, *lo_key;
 173         unsigned ofs_0;
 174         static int debug_lookup = -1;
 175
 176         if (debug_lookup < 0)
 177                 debug_lookup = !!getenv("GIT_DEBUG_LOOKUP");
 178
 179         if (!nr || lo >= hi)
 180                 return -1;
 181
 182         if (nr == hi)
 183                 hi_key = NULL;
 184         else
 185                 hi_key = base + elem_size * hi + key_offset;
 186         lo_key = base + elem_size * lo + key_offset;
 187
 188         ofs_0 = 0;
 189         do {
 190                 int cmp;
 191                 unsigned ofs, mi, range;
 192                 unsigned lov, hiv, kyv;
 193                 const unsigned char *mi_key;
 194
 195                 range = hi - lo;
 196                 if (hi_key) {
 197                         for (ofs = ofs_0; ofs < 20; ofs++)
 198                                 if (lo_key[ofs] != hi_key[ofs])
 199                                         break;
 200                         ofs_0 = ofs;
 201                         /*
 202                          * byte 0 thru (ofs-1) are the same between
 203                          * lo and hi; ofs is the first byte that is
 204                          * different.
 205                          *
 206                          * If ofs==20, then no bytes are different,
 207                          * meaning we have entries with duplicate
 208                          * keys. We know that we are in a solid run
 209                          * of this entry (because the entries are
 210                          * sorted, and our lo and hi are the same,
 211                          * there can be nothing but this single key
 212                          * in between). So we can stop the search.
 213                          * Either one of these entries is it (and
 214                          * we do not care which), or we do not have
 215                          * it.
 216                          *
 217                          * Furthermore, we know that one of our
 218                          * endpoints must be the edge of the run of
 219                          * duplicates. For example, given this
 220                          * sequence:
 221                          *
 222                          *     idx 0 1 2 3 4 5
 223                          *     key A C C C C D
 224                          *
 225                          * If we are searching for "B", we might
 226                          * hit the duplicate run at lo=1, hi=3
 227                          * (e.g., by first mi=3, then mi=0). But we
 228                          * can never have lo > 1, because B < C.
 229                          * That is, if our key is less than the
 230                          * run, we know that "lo" is the edge, but
 231                          * we can say nothing of "hi". Similarly,
 232                          * if our key is greater than the run, we
 233                          * know that "hi" is the edge, but we can
 234                          * say nothing of "lo".
 235                          *
 236                          * Therefore if we do not find it, we also
 237                          * know where it would go if it did exist:
 238                          * just on the far side of the edge that we
 239                          * know about.
 240                          */
 241                         if (ofs == 20) {
 242                                 mi = lo;
 243                                 mi_key = base + elem_size * mi + key_offset;
 244                                 cmp = memcmp(mi_key, key, 20);
 245                                 if (!cmp)
 246                                         return mi;
 247                                 if (cmp < 0)
 248                                         return -1 - hi;
 249                                 else
 250                                         return -1 - lo;
 251                         }
 252
 253                         hiv = hi_key[ofs_0];
 254                         if (ofs_0 < 19)
 255                                 hiv = (hiv << 8) | hi_key[ofs_0+1];
 256                 } else {
 257                         hiv = 256;
 258                         if (ofs_0 < 19)
 259                                 hiv <<= 8;
 260                 }
 261                 lov = lo_key[ofs_0];
 262                 kyv = key[ofs_0];
 263                 if (ofs_0 < 19) {
 264                         lov = (lov << 8) | lo_key[ofs_0+1];
 265                         kyv = (kyv << 8) | key[ofs_0+1];
 266                 }
 267                 assert(lov < hiv);
 268
 269                 if (kyv < lov)
 270                         return -1 - lo;
 271                 if (hiv < kyv)
 272                         return -1 - hi;
 273
 274                 /*
 275                  * Even if we know the target is much closer to 'hi'
 276                  * than 'lo', if we pick too precisely and overshoot
 277                  * (e.g. when we know 'mi' is closer to 'hi' than to
 278                  * 'lo', pick 'mi' that is higher than the target), we
 279                  * end up narrowing the search space by a smaller
 280                  * amount (i.e. the distance between 'mi' and 'hi')
 281                  * than what we would have (i.e. about half of 'lo'
 282                  * and 'hi').  Hedge our bets to pick 'mi' less
 283                  * aggressively, i.e. make 'mi' a bit closer to the
 284                  * middle than we would otherwise pick.
 285                  */
 286                 kyv = (kyv * 6 + lov + hiv) / 8;
 287                 if (lov < hiv - 1) {
 288                         if (kyv == lov)
 289                                 kyv++;
 290                         else if (kyv == hiv)
 291                                 kyv--;
 292                 }
 293                 mi = (range - 1) * (kyv - lov) / (hiv - lov) + lo;
 294
 295                 if (debug_lookup) {
 296                         printf("lo %u hi %u rg %u mi %u ", lo, hi, range, mi);
 297                         printf("ofs %u lov %x, hiv %x, kyv %x\n",
 298                                ofs_0, lov, hiv, kyv);
 299                 }
 300                 if (!(lo <= mi && mi < hi))
 301                         die("assertion failure lo %u mi %u hi %u %s",
 302                             lo, mi, hi, sha1_to_hex(key));
 303
 304                 mi_key = base + elem_size * mi + key_offset;
 305                 cmp = memcmp(mi_key + ofs_0, key + ofs_0, 20 - ofs_0);
 306                 if (!cmp)
 307                         return mi;
 308                 if (cmp > 0) {
 309                         hi = mi;
 310                         hi_key = mi_key;
 311                 } else {
 312                         lo = mi + 1;
 313                         lo_key = mi_key + elem_size;
 314                 }
 315         } while (lo < hi);
 316         return -lo-1;
 317 }