src/liblzma/lzma/lzma_encoder_optimum_fast.c

   1 ///////////////////////////////////////////////////////////////////////////////
   2 //
   3 /// \file       lzma_encoder_optimum_fast.c
   4 //
   5 //  Author:     Igor Pavlov
   6 //
   7 //  This file has been put into the public domain.
   8 //  You can do whatever you want with this file.
   9 //
  10 ///////////////////////////////////////////////////////////////////////////////
  11
  12 #include "lzma_encoder_private.h"
  13 #include "memcmplen.h"
  14
  15
  16 #define change_pair(small_dist, big_dist) \
  17         (((big_dist) >> 7) > (small_dist))
  18
  19
  20 extern void
  21 lzma_lzma_optimum_fast(lzma_lzma1_encoder *restrict coder,
  22                 lzma_mf *restrict mf,
  23                 uint32_t *restrict back_res, uint32_t *restrict len_res)
  24 {
  25         const uint32_t nice_len = mf->nice_len;
  26
  27         uint32_t len_main;
  28         uint32_t matches_count;
  29         if (mf->read_ahead == 0) {
  30                 len_main = mf_find(mf, &matches_count, coder->matches);
  31         } else {
  32                 assert(mf->read_ahead == 1);
  33                 len_main = coder->longest_match_length;
  34                 matches_count = coder->matches_count;
  35         }
  36
  37         const uint8_t *buf = mf_ptr(mf) - 1;
  38         const uint32_t buf_avail = my_min(mf_avail(mf) + 1, MATCH_LEN_MAX);
  39
  40         if (buf_avail < 2) {
  41                 // There's not enough input left to encode a match.
  42                 *back_res = UINT32_MAX;
  43                 *len_res = 1;
  44                 return;
  45         }
  46
  47         // Look for repeated matches; scan the previous four match distances
  48         uint32_t rep_len = 0;
  49         uint32_t rep_index = 0;
  50
  51         for (uint32_t i = 0; i < REPS; ++i) {
  52                 // Pointer to the beginning of the match candidate
  53                 const uint8_t *const buf_back = buf - coder->reps[i] - 1;
  54
  55                 // If the first two bytes (2 == MATCH_LEN_MIN) do not match,
  56                 // this rep is not useful.
  57                 if (not_equal_16(buf, buf_back))
  58                         continue;
  59
  60                 // The first two bytes matched.
  61                 // Calculate the length of the match.
  62                 const uint32_t len = lzma_memcmplen(
  63                                 buf, buf_back, 2, buf_avail);
  64
  65                 // If we have found a repeated match that is at least
  66                 // nice_len long, return it immediately.
  67                 if (len >= nice_len) {
  68                         *back_res = i;
  69                         *len_res = len;
  70                         mf_skip(mf, len - 1);
  71                         return;
  72                 }
  73
  74                 if (len > rep_len) {
  75                         rep_index = i;
  76                         rep_len = len;
  77                 }
  78         }
  79
  80         // We didn't find a long enough repeated match. Encode it as a normal
  81         // match if the match length is at least nice_len.
  82         if (len_main >= nice_len) {
  83                 *back_res = coder->matches[matches_count - 1].dist + REPS;
  84                 *len_res = len_main;
  85                 mf_skip(mf, len_main - 1);
  86                 return;
  87         }
  88
  89         uint32_t back_main = 0;
  90         if (len_main >= 2) {
  91                 back_main = coder->matches[matches_count - 1].dist;
  92
  93                 while (matches_count > 1 && len_main ==
  94                                 coder->matches[matches_count - 2].len + 1) {
  95                         if (!change_pair(coder->matches[
  96                                                 matches_count - 2].dist,
  97                                         back_main))
  98                                 break;
  99
 100                         --matches_count;
 101                         len_main = coder->matches[matches_count - 1].len;
 102                         back_main = coder->matches[matches_count - 1].dist;
 103                 }
 104
 105                 if (len_main == 2 && back_main >= 0x80)
 106                         len_main = 1;
 107         }
 108
 109         if (rep_len >= 2) {
 110                 if (rep_len + 1 >= len_main
 111                                 || (rep_len + 2 >= len_main
 112                                         && back_main > (UINT32_C(1) << 9))
 113                                 || (rep_len + 3 >= len_main
 114                                         && back_main > (UINT32_C(1) << 15))) {
 115                         *back_res = rep_index;
 116                         *len_res = rep_len;
 117                         mf_skip(mf, rep_len - 1);
 118                         return;
 119                 }
 120         }
 121
 122         if (len_main < 2 || buf_avail <= 2) {
 123                 *back_res = UINT32_MAX;
 124                 *len_res = 1;
 125                 return;
 126         }
 127
 128         // Get the matches for the next byte. If we find a better match,
 129         // the current byte is encoded as a literal.
 130         coder->longest_match_length = mf_find(mf,
 131                         &coder->matches_count, coder->matches);
 132
 133         if (coder->longest_match_length >= 2) {
 134                 const uint32_t new_dist = coder->matches[
 135                                 coder->matches_count - 1].dist;
 136
 137                 if ((coder->longest_match_length >= len_main
 138                                         && new_dist < back_main)
 139                                 || (coder->longest_match_length == len_main + 1
 140                                         && !change_pair(back_main, new_dist))
 141                                 || (coder->longest_match_length > len_main + 1)
 142                                 || (coder->longest_match_length + 1 >= len_main
 143                                         && len_main >= 3
 144                                         && change_pair(new_dist, back_main))) {
 145                         *back_res = UINT32_MAX;
 146                         *len_res = 1;
 147                         return;
 148                 }
 149         }
 150
 151         // In contrast to LZMA SDK, dictionary could not have been moved
 152         // between mf_find() calls, thus it is safe to just increment
 153         // the old buf pointer instead of recalculating it with mf_ptr().
 154         ++buf;
 155
 156         const uint32_t limit = my_max(2, len_main - 1);
 157
 158         for (uint32_t i = 0; i < REPS; ++i) {
 159                 if (memcmp(buf, buf - coder->reps[i] - 1, limit) == 0) {
 160                         *back_res = UINT32_MAX;
 161                         *len_res = 1;
 162                         return;
 163                 }
 164         }
 165
 166         *back_res = back_main + REPS;
 167         *len_res = len_main;
 168         mf_skip(mf, len_main - 2);
 169         return;
 170 }