BaseTools/Source/C/BrotliCompress/enc/backward_references_inc.h

   1 /* NOLINT(build/header_guard) */
   2 /* Copyright 2013 Google Inc. All Rights Reserved.
   3
   4    Distributed under MIT license.
   5    See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
   6 */
   7
   8 /* template parameters: EXPORT_FN, FN */
   9
  10 static BROTLI_NOINLINE void EXPORT_FN(CreateBackwardReferences)(
  11     size_t num_bytes, size_t position,
  12     const uint8_t* ringbuffer, size_t ringbuffer_mask,
  13     const BrotliEncoderParams* params, HasherHandle hasher, int* dist_cache,
  14     size_t* last_insert_len, Command* commands, size_t* num_commands,
  15     size_t* num_literals) {
  16   /* Set maximum distance, see section 9.1. of the spec. */
  17   const size_t max_backward_limit = BROTLI_MAX_BACKWARD_LIMIT(params->lgwin);
  18
  19   const Command* const orig_commands = commands;
  20   size_t insert_length = *last_insert_len;
  21   const size_t pos_end = position + num_bytes;
  22   const size_t store_end = num_bytes >= FN(StoreLookahead)() ?
  23       position + num_bytes - FN(StoreLookahead)() + 1 : position;
  24
  25   /* For speed up heuristics for random data. */
  26   const size_t random_heuristics_window_size =
  27       LiteralSpreeLengthForSparseSearch(params);
  28   size_t apply_random_heuristics = position + random_heuristics_window_size;
  29   const size_t gap = 0;
  30
  31   /* Minimum score to accept a backward reference. */
  32   const score_t kMinScore = BROTLI_SCORE_BASE + 100;
  33
  34   FN(PrepareDistanceCache)(hasher, dist_cache);
  35
  36   while (position + FN(HashTypeLength)() < pos_end) {
  37     size_t max_length = pos_end - position;
  38     size_t max_distance = BROTLI_MIN(size_t, position, max_backward_limit);
  39     HasherSearchResult sr;
  40     sr.len = 0;
  41     sr.len_code_delta = 0;
  42     sr.distance = 0;
  43     sr.score = kMinScore;
  44     FN(FindLongestMatch)(hasher, &params->dictionary,
  45                          ringbuffer, ringbuffer_mask, dist_cache, position,
  46                          max_length, max_distance, gap,
  47                          params->dist.max_distance, &sr);
  48     if (sr.score > kMinScore) {
  49       /* Found a match. Let's look for something even better ahead. */
  50       int delayed_backward_references_in_row = 0;
  51       --max_length;
  52       for (;; --max_length) {
  53         const score_t cost_diff_lazy = 175;
  54         HasherSearchResult sr2;
  55         sr2.len = params->quality < MIN_QUALITY_FOR_EXTENSIVE_REFERENCE_SEARCH ?
  56             BROTLI_MIN(size_t, sr.len - 1, max_length) : 0;
  57         sr2.len_code_delta = 0;
  58         sr2.distance = 0;
  59         sr2.score = kMinScore;
  60         max_distance = BROTLI_MIN(size_t, position + 1, max_backward_limit);
  61         FN(FindLongestMatch)(hasher, &params->dictionary,
  62             ringbuffer, ringbuffer_mask, dist_cache, position + 1, max_length,
  63             max_distance, gap, params->dist.max_distance, &sr2);
  64         if (sr2.score >= sr.score + cost_diff_lazy) {
  65           /* Ok, let's just write one byte for now and start a match from the
  66              next byte. */
  67           ++position;
  68           ++insert_length;
  69           sr = sr2;
  70           if (++delayed_backward_references_in_row < 4 &&
  71               position + FN(HashTypeLength)() < pos_end) {
  72             continue;
  73           }
  74         }
  75         break;
  76       }
  77       apply_random_heuristics =
  78           position + 2 * sr.len + random_heuristics_window_size;
  79       max_distance = BROTLI_MIN(size_t, position, max_backward_limit);
  80       {
  81         /* The first 16 codes are special short-codes,
  82            and the minimum offset is 1. */
  83         size_t distance_code =
  84             ComputeDistanceCode(sr.distance, max_distance + gap, dist_cache);
  85         if ((sr.distance <= (max_distance + gap)) && distance_code > 0) {
  86           dist_cache[3] = dist_cache[2];
  87           dist_cache[2] = dist_cache[1];
  88           dist_cache[1] = dist_cache[0];
  89           dist_cache[0] = (int)sr.distance;
  90           FN(PrepareDistanceCache)(hasher, dist_cache);
  91         }
  92         InitCommand(commands++, &params->dist, insert_length,
  93             sr.len, sr.len_code_delta, distance_code);
  94       }
  95       *num_literals += insert_length;
  96       insert_length = 0;
  97       /* Put the hash keys into the table, if there are enough bytes left.
  98          Depending on the hasher implementation, it can push all positions
  99          in the given range or only a subset of them.
 100          Avoid hash poisoning with RLE data. */
 101       {
 102         size_t range_start = position + 2;
 103         size_t range_end = BROTLI_MIN(size_t, position + sr.len, store_end);
 104         if (sr.distance < (sr.len >> 2)) {
 105           range_start = BROTLI_MIN(size_t, range_end, BROTLI_MAX(size_t,
 106               range_start, position + sr.len - (sr.distance << 2)));
 107         }
 108         FN(StoreRange)(hasher, ringbuffer, ringbuffer_mask, range_start,
 109                        range_end);
 110       }
 111       position += sr.len;
 112     } else {
 113       ++insert_length;
 114       ++position;
 115       /* If we have not seen matches for a long time, we can skip some
 116          match lookups. Unsuccessful match lookups are very very expensive
 117          and this kind of a heuristic speeds up compression quite
 118          a lot. */
 119       if (position > apply_random_heuristics) {
 120         /* Going through uncompressible data, jump. */
 121         if (position >
 122             apply_random_heuristics + 4 * random_heuristics_window_size) {
 123           /* It is quite a long time since we saw a copy, so we assume
 124              that this data is not compressible, and store hashes less
 125              often. Hashes of non compressible data are less likely to
 126              turn out to be useful in the future, too, so we store less of
 127              them to not to flood out the hash table of good compressible
 128              data. */
 129           const size_t kMargin =
 130               BROTLI_MAX(size_t, FN(StoreLookahead)() - 1, 4);
 131           size_t pos_jump =
 132               BROTLI_MIN(size_t, position + 16, pos_end - kMargin);
 133           for (; position < pos_jump; position += 4) {
 134             FN(Store)(hasher, ringbuffer, ringbuffer_mask, position);
 135             insert_length += 4;
 136           }
 137         } else {
 138           const size_t kMargin =
 139               BROTLI_MAX(size_t, FN(StoreLookahead)() - 1, 2);
 140           size_t pos_jump =
 141               BROTLI_MIN(size_t, position + 8, pos_end - kMargin);
 142           for (; position < pos_jump; position += 2) {
 143             FN(Store)(hasher, ringbuffer, ringbuffer_mask, position);
 144             insert_length += 2;
 145           }
 146         }
 147       }
 148     }
 149   }
 150   insert_length += pos_end - position;
 151   *last_insert_len = insert_length;
 152   *num_commands += (size_t)(commands - orig_commands);
 153 }