* that position at previous positions in the window. With LZX, the minimum
* match length is 2 and the maximum match length is 257. The only restriction
* on offsets is that LZX does not allow the last 2 bytes of the window to match
- * the the beginning of the window.
+ * the beginning of the window.
*
* There are a number of algorithms that can be used for this, including hash
* chains, binary trees, and suffix arrays. Binary trees generally work well
};
/* 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;
};
return costs->main[c];
}
-/* Given a (length, offset) pair that could be turned into a valid LZX match as
- * well as costs for the codewords in the main, length, and aligned Huffman
- * codes, return the approximate number of bits it will take to represent this
- * match in the compressed output. Take into account the match offset LRU
- * queue and also updates it. */
+/* Returns the cost, in bits, to output a repeat offset match of the specified
+ * length and position slot (repeat index) using the specified cost model. */
static u32
-lzx_match_cost(unsigned length, u32 offset, const struct lzx_costs *costs,
- struct lzx_lru_queue *queue)
+lzx_repmatch_cost(u32 len, unsigned position_slot, const struct lzx_costs *costs)
{
- unsigned position_slot;
unsigned len_header, main_symbol;
- unsigned num_extra_bits;
u32 cost = 0;
- position_slot = lzx_get_position_slot(offset, queue);
-
- len_header = min(length - LZX_MIN_MATCH_LEN, LZX_NUM_PRIMARY_LENS);
+ len_header = min(len - LZX_MIN_MATCH_LEN, LZX_NUM_PRIMARY_LENS);
main_symbol = ((position_slot << 3) | len_header) + LZX_NUM_CHARS;
/* Account for main symbol. */
cost += costs->main[main_symbol];
- /* Account for extra position information. */
- num_extra_bits = lzx_get_num_extra_bits(position_slot);
- if (num_extra_bits >= 3) {
- cost += num_extra_bits - 3;
- cost += costs->aligned[(offset + LZX_OFFSET_OFFSET) & 7];
- } else {
- cost += num_extra_bits;
- }
-
/* Account for extra length information. */
if (len_header == LZX_NUM_PRIMARY_LENS)
- cost += costs->len[length - LZX_MIN_MATCH_LEN - LZX_NUM_PRIMARY_LENS];
+ cost += costs->len[len - LZX_MIN_MATCH_LEN - LZX_NUM_PRIMARY_LENS];
return cost;
-
}
-
/* Set the cost model @c->costs from the Huffman codeword lengths specified in
* @lens.
*
unsigned num_matches;
const struct lz_match *matches;
struct lz_match match;
- unsigned longest_len;
- unsigned longest_rep_len;
- u32 longest_rep_offset;
+ u32 longest_len;
+ u32 longest_rep_len;
+ unsigned longest_rep_slot;
unsigned cur_pos;
unsigned end_pos;
+ struct lzx_mc_pos_data *optimum = c->optimum;
if (c->optimum_cur_idx != c->optimum_end_idx) {
/* Case 2: Return the next match/literal already found. */
- match.len = c->optimum[c->optimum_cur_idx].next.link -
+ match.len = optimum[c->optimum_cur_idx].next.link -
c->optimum_cur_idx;
- match.offset = c->optimum[c->optimum_cur_idx].next.match_offset;
+ match.offset = optimum[c->optimum_cur_idx].next.match_offset;
- c->optimum_cur_idx = c->optimum[c->optimum_cur_idx].next.link;
+ c->optimum_cur_idx = optimum[c->optimum_cur_idx].next.link;
return match;
}
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,
len++;
if (len > longest_rep_len) {
longest_rep_len = len;
- longest_rep_offset = offset;
+ longest_rep_slot = i;
}
}
}
- /* 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) {
.len = longest_rep_len,
- .offset = longest_rep_offset,
+ .offset = c->queue.R[longest_rep_slot],
};
}
- /* 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.
- */
- c->optimum[1].queue = c->queue;
- c->optimum[1].cost = lzx_literal_cost(c->cur_window[c->match_window_pos - 1],
+ /* 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);
- c->optimum[1].prev.link = 0;
+ optimum[1].prev.link = 0;
/* Calculate the cost to reach any position up to and including that
* reached by the longest match.
if (len_header == LZX_NUM_PRIMARY_LENS)
cost += c->costs.len[len - LZX_MIN_MATCH_LEN - LZX_NUM_PRIMARY_LENS];
- c->optimum[len].queue = queue;
- c->optimum[len].prev.link = 0;
- c->optimum[len].prev.match_offset = offset;
- c->optimum[len].cost = cost;
+ optimum[len].queue = queue;
+ optimum[len].prev.link = 0;
+ optimum[len].prev.match_offset = offset;
+ optimum[len].cost = cost;
} while (++len <= matches[i].len);
}
end_pos = longest_len;
if (longest_rep_len >= LZX_MIN_MATCH_LEN) {
- struct lzx_lru_queue queue;
u32 cost;
while (end_pos < longest_rep_len)
- c->optimum[++end_pos].cost = MC_INFINITE_COST;
+ optimum[++end_pos].cost = MC_INFINITE_COST;
- queue = c->queue;
- cost = lzx_match_cost(longest_rep_len, longest_rep_offset,
- &c->costs, &queue);
- if (cost <= c->optimum[longest_rep_len].cost) {
- c->optimum[longest_rep_len].queue = queue;
- c->optimum[longest_rep_len].prev.link = 0;
- c->optimum[longest_rep_len].prev.match_offset = longest_rep_offset;
- c->optimum[longest_rep_len].cost = cost;
+ cost = lzx_repmatch_cost(longest_rep_len, longest_rep_slot,
+ &c->costs);
+ if (cost <= optimum[longest_rep_len].cost) {
+ optimum[longest_rep_len].queue = c->queue;
+ swap(optimum[longest_rep_len].queue.R[0],
+ optimum[longest_rep_len].queue.R[longest_rep_slot]);
+ optimum[longest_rep_len].prev.link = 0;
+ optimum[longest_rep_len].prev.match_offset =
+ optimum[longest_rep_len].queue.R[0];
+ optimum[longest_rep_len].cost = cost;
}
}
* 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 @c->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:
*
* match/literal list.
*
* 3. Failing either of the above in a degenerate case, the loop
- * terminates when space in the @c->optimum array is exhausted.
+ * terminates when space in the @optimum array is exhausted.
* This terminates the algorithm and forces it to start returning
* matches/literals even though they may not be globally optimal.
*
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);
for (int i = 0; i < LZX_NUM_RECENT_OFFSETS; i++) {
- u32 offset = c->optimum[cur_pos].queue.R[i];
+ u32 offset = optimum[cur_pos].queue.R[i];
const u8 *strptr = &c->cur_window[c->match_window_pos];
const u8 *matchptr = strptr - offset;
unsigned len = 0;
len++;
if (len > longest_rep_len) {
longest_rep_len = len;
- longest_rep_offset = offset;
+ longest_rep_slot = i;
}
}
- /* 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
match = lzx_match_chooser_reverse_list(c, cur_pos);
/* Append the long match to the end of the list. */
- c->optimum[cur_pos].next.match_offset = longest_rep_offset;
- c->optimum[cur_pos].next.link = cur_pos + longest_rep_len;
+ optimum[cur_pos].next.match_offset =
+ optimum[cur_pos].queue.R[longest_rep_slot];
+ optimum[cur_pos].next.link = cur_pos + longest_rep_len;
c->optimum_end_idx = cur_pos + longest_rep_len;
/* Skip over the remaining bytes of the long match. */
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) {
match = lzx_match_chooser_reverse_list(c, cur_pos);
/* Append the long match to the end of the list. */
- c->optimum[cur_pos].next.match_offset =
+ optimum[cur_pos].next.match_offset =
matches[num_matches - 1].offset;
- c->optimum[cur_pos].next.link = cur_pos + longest_len;
+ optimum[cur_pos].next.link = cur_pos + longest_len;
c->optimum_end_idx = cur_pos + longest_len;
/* Skip over the remaining bytes of the long match. */
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)
- c->optimum[++end_pos].cost = MC_INFINITE_COST;
+ optimum[++end_pos].cost = MC_INFINITE_COST;
/* Consider coding a literal. */
- cost = c->optimum[cur_pos].cost +
+ cost = optimum[cur_pos].cost +
lzx_literal_cost(c->cur_window[c->match_window_pos - 1],
&c->costs);
- if (cost < c->optimum[cur_pos + 1].cost) {
- c->optimum[cur_pos + 1].queue = c->optimum[cur_pos].queue;
- c->optimum[cur_pos + 1].cost = cost;
- c->optimum[cur_pos + 1].prev.link = cur_pos;
+ if (cost < optimum[cur_pos + 1].cost) {
+ optimum[cur_pos + 1].queue = optimum[cur_pos].queue;
+ optimum[cur_pos + 1].cost = cost;
+ optimum[cur_pos + 1].prev.link = cur_pos;
}
/* Consider coding a match.
* 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 = c->optimum[cur_pos].queue;
- position_cost = c->optimum[cur_pos].cost;
+ position_cost = optimum[cur_pos].cost;
+
+ /* 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);
- position_slot = lzx_get_position_slot(offset, &queue);
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_MIN_MATCH_LEN -
LZX_NUM_PRIMARY_LENS];
}
- if (cost < c->optimum[cur_pos + len].cost) {
- c->optimum[cur_pos + len].queue = queue;
- c->optimum[cur_pos + len].prev.link = cur_pos;
- c->optimum[cur_pos + len].prev.match_offset = offset;
- c->optimum[cur_pos + len].cost = cost;
+ if (cost < optimum[cur_pos + len].cost) {
+ 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) {
- struct lzx_lru_queue queue;
while (end_pos < cur_pos + longest_rep_len)
- c->optimum[++end_pos].cost = MC_INFINITE_COST;
-
- queue = c->optimum[cur_pos].queue;
-
- cost = c->optimum[cur_pos].cost +
- lzx_match_cost(longest_rep_len, longest_rep_offset,
- &c->costs, &queue);
- if (cost <= c->optimum[cur_pos + longest_rep_len].cost) {
- c->optimum[cur_pos + longest_rep_len].queue =
- queue;
- c->optimum[cur_pos + longest_rep_len].prev.link =
+ optimum[++end_pos].cost = MC_INFINITE_COST;
+
+ cost = optimum[cur_pos].cost +
+ lzx_repmatch_cost(longest_rep_len, longest_rep_slot,
+ &c->costs);
+ if (cost <= optimum[cur_pos + longest_rep_len].cost) {
+ optimum[cur_pos + longest_rep_len].queue =
+ optimum[cur_pos].queue;
+ swap(optimum[cur_pos + longest_rep_len].queue.R[0],
+ optimum[cur_pos + longest_rep_len].queue.R[longest_rep_slot]);
+ optimum[cur_pos + longest_rep_len].prev.link =
cur_pos;
- c->optimum[cur_pos + longest_rep_len].prev.match_offset =
- longest_rep_offset;
- c->optimum[cur_pos + longest_rep_len].cost =
+ optimum[cur_pos + longest_rep_len].prev.match_offset =
+ optimum[cur_pos + longest_rep_len].queue.R[0];
+ optimum[cur_pos + longest_rep_len].cost =
cost;
}
}
git://wimlib.net / wimlib / blobdiff