};
/* Adaptive state that exists after an approximate minimum-cost path to
- * reach this position is taken. */
+ * reach this position is taken.
+ *
+ * Note: we update this whenever we update the pending minimum-cost
+ * path. This is in contrast to LZMA, which also has an optimal parser
+ * that maintains a repeat offset queue per position, but will only
+ * compute the queue once that position is actually reached in the
+ * parse, meaning that matches are being considered *starting* at that
+ * position. However, the two methods seem to have approximately the
+ * same performance if appropriate optimizations are used. Intuitively
+ * the LZMA method seems faster, but it actually suffers from 1-2 extra
+ * hard-to-predict branches at each position. Probably it works better
+ * for LZMA than LZX because LZMA has a larger adaptive state than LZX,
+ * and the LZMA encoder considers more possibilities. */
struct lzx_lru_queue queue;
};
c->optimum_cur_idx = 0;
c->optimum_end_idx = 0;
- /* Search for matches at recent offsets. Only keep the one with the
- * longest match length. */
+ /* Search for matches at repeat offsets. As a heuristic, we only keep
+ * the one with the longest match length. */
longest_rep_len = LZX_MIN_MATCH_LEN - 1;
if (c->match_window_pos >= 1) {
unsigned limit = min(LZX_MAX_MATCH_LEN,
}
}
- /* If there's a long match with a recent offset, take it. */
+ /* If there's a long match with a repeat offset, choose it immediately. */
if (longest_rep_len >= c->params.nice_match_length) {
lzx_skip_bytes(c, longest_rep_len);
return (struct lz_match) {
};
}
- /* Search other matches. */
+ /* Find other matches. */
num_matches = lzx_get_matches(c, &matches);
- /* If there's a long match, take it. */
+ /* If there's a long match, choose it immediately. */
if (num_matches) {
longest_len = matches[num_matches - 1].len;
if (longest_len >= c->params.nice_match_length) {
longest_len = 1;
}
- /* Calculate the cost to reach the next position by coding a literal.
- */
+ /* Calculate the cost to reach the next position by coding a literal. */
optimum[1].queue = c->queue;
optimum[1].cost = lzx_literal_cost(c->cur_window[c->match_window_pos - 1],
&c->costs);
* position. The algorithm may find multiple paths to reach each
* position; only the lowest-cost path is saved.
*
- * The progress of the parse is tracked in the @optimum array, which
- * for each position contains the minimum cost to reach that position,
- * the index of the start of the match/literal taken to reach that
- * position through the minimum-cost path, the offset of the match taken
- * (not relevant for literals), and the adaptive state that will exist
- * at that position after the minimum-cost path is taken. The @cur_pos
+ * The progress of the parse is tracked in the @optimum array, which for
+ * each position contains the minimum cost to reach that position, the
+ * index of the start of the match/literal taken to reach that position
+ * through the minimum-cost path, the offset of the match taken (not
+ * relevant for literals), and the adaptive state that will exist at
+ * that position after the minimum-cost path is taken. The @cur_pos
* variable stores the position at which the algorithm is currently
* considering coding choices, and the @end_pos variable stores the
* greatest position at which the costs of coding choices have been
- * saved. (Actually, the algorithm guarantees that all positions up to
- * and including @end_pos are reachable by at least one path.)
+ * saved.
*
* The loop terminates when any one of the following conditions occurs:
*
if (cur_pos == end_pos || cur_pos == LZX_OPTIM_ARRAY_LENGTH)
return lzx_match_chooser_reverse_list(c, cur_pos);
- /* Search for matches at recent offsets. */
+ /* Search for matches at repeat offsets. Again, as a heuristic
+ * we only keep the longest one. */
longest_rep_len = LZX_MIN_MATCH_LEN - 1;
unsigned limit = min(LZX_MAX_MATCH_LEN,
c->match_window_end - c->match_window_pos);
}
}
- /* If we found a long match at a recent offset, choose it
+ /* If we found a long match at a repeat offset, choose it
* immediately. */
if (longest_rep_len >= c->params.nice_match_length) {
/* Build the list of matches to return and get
return match;
}
- /* Search other matches. */
+ /* Find other matches. */
num_matches = lzx_get_matches(c, &matches);
- /* If there's a long match, take it. */
+ /* If there's a long match, choose it immediately. */
if (num_matches) {
longest_len = matches[num_matches - 1].len;
if (longest_len >= c->params.nice_match_length) {
longest_len = 1;
}
+ /* If we are reaching any positions for the first time, we need
+ * to initialize their costs to infinity. */
while (end_pos < cur_pos + longest_len)
optimum[++end_pos].cost = MC_INFINITE_COST;
* length. */
for (unsigned i = 0, len = 2; i < num_matches; i++) {
u32 offset;
- struct lzx_lru_queue queue;
u32 position_cost;
unsigned position_slot;
unsigned num_extra_bits;
offset = matches[i].offset;
- queue = optimum[cur_pos].queue;
position_cost = optimum[cur_pos].cost;
- position_slot = lzx_get_position_slot(offset, &queue);
+ /* Yet another optimization: instead of calling
+ * lzx_get_position_slot(), hand-inline the search of
+ * the repeat offset queue. Then we can omit the
+ * extra_bits calculation for repeat offset matches, and
+ * also only compute the updated queue if we actually do
+ * find a new lowest cost path. */
+ for (position_slot = 0; position_slot < LZX_NUM_RECENT_OFFSETS; position_slot++)
+ if (offset == optimum[cur_pos].queue.R[position_slot])
+ goto have_position_cost;
+
+ position_slot = lzx_get_position_slot_raw(offset + LZX_OFFSET_OFFSET);
+
num_extra_bits = lzx_get_num_extra_bits(position_slot);
if (num_extra_bits >= 3) {
position_cost += num_extra_bits - 3;
position_cost += num_extra_bits;
}
+ have_position_cost:
+
do {
unsigned len_header;
unsigned main_symbol;
LZX_NUM_PRIMARY_LENS];
}
if (cost < optimum[cur_pos + len].cost) {
- optimum[cur_pos + len].queue = queue;
+ if (position_slot < LZX_NUM_RECENT_OFFSETS) {
+ optimum[cur_pos + len].queue = optimum[cur_pos].queue;
+ swap(optimum[cur_pos + len].queue.R[0],
+ optimum[cur_pos + len].queue.R[position_slot]);
+ } else {
+ optimum[cur_pos + len].queue.R[0] = offset;
+ optimum[cur_pos + len].queue.R[1] = optimum[cur_pos].queue.R[0];
+ optimum[cur_pos + len].queue.R[2] = optimum[cur_pos].queue.R[1];
+ }
optimum[cur_pos + len].prev.link = cur_pos;
optimum[cur_pos + len].prev.match_offset = offset;
optimum[cur_pos + len].cost = cost;
} while (++len <= matches[i].len);
}
+ /* Consider coding a repeat offset match.
+ *
+ * As a heuristic, we only consider the longest length of the
+ * longest repeat offset match. This does not, however,
+ * necessarily mean that we will never consider any other repeat
+ * offsets, because above we detect repeat offset matches that
+ * were found by the regular match-finder. Therefore, this
+ * special handling of the longest repeat-offset match is only
+ * helpful for coding a repeat offset match that was *not* found
+ * by the match-finder, e.g. due to being obscured by a less
+ * distant match that is at least as long.
+ *
+ * Note: an alternative, used in LZMA, is to consider every
+ * length of every repeat offset match. This is a more thorough
+ * search, and it makes it unnecessary to detect repeat offset
+ * matches that were found by the regular match-finder. But by
+ * my tests, for LZX the LZMA method slows down the compressor
+ * by ~10% and doesn't actually help the compression ratio too
+ * much.
+ *
+ * Also tested a compromise approach: consider every 3rd length
+ * of the longest repeat offset match. Still didn't seem quite
+ * worth it, though.
+ */
if (longest_rep_len >= LZX_MIN_MATCH_LEN) {
while (end_pos < cur_pos + longest_rep_len)
git://wimlib.net / wimlib / blobdiff