+++ /dev/null
-/*
- * lz_optimal.h
- *
- * Near-optimal LZ (Lempel-Ziv) parsing, or "match choosing".
- * See lz_get_near_optimal_match() for details of the algorithm.
- *
- * This code is not concerned with actually *finding* LZ matches, as it relies
- * on an underlying match-finder implementation that can do so.
- */
-
-/*
- * Copyright (c) 2013 Eric Biggers. All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- *
- * 1. Redistributions of source code must retain the above copyright
- * notice, this list of conditions and the following disclaimer.
- *
- * 2. Redistributions in binary form must reproduce the above copyright
- * notice, this list of conditions and the following disclaimer in the
- * documentation and/or other materials provided with the distribution.
- *
- * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
- * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE
- * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
- * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
- * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
- * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
- * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
- * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
- * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
- */
-
-/* Define the following structures before including this header:
- *
- * LZ_COMPRESSOR
- * LZ_ADAPTIVE_STATE
- *
- * Also, the type lz_mc_cost_t can be optionally overridden by providing an
- * appropriate typedef and defining LZ_MC_COST_T_DEFINED. */
-
-#ifndef _LZ_OPTIMAL_H
-#define _LZ_OPTIMAL_H
-
-#include "wimlib/lz.h"
-
-#ifndef LZ_MC_COST_T_DEFINED
- typedef input_idx_t lz_mc_cost_t;
-#endif
-
-#define LZ_MC_INFINITE_COST (~(lz_mc_cost_t)0)
-
-struct lz_mc_pos_data;
-
-/* State of the Lempel-Ziv match-chooser.
- *
- * This is defined here for benefit of the inlined code. It's not intended for
- * code outside the match-chooser itself to read or write members from this
- * structure. */
-struct lz_match_chooser {
- /* Temporary space used for the match-choosing algorithm. The size of
- * this array must be at least one more than @nice_len but otherwise is
- * arbitrary. More space decreases the frequency at which the algorithm
- * is forced to terminate early. 4096 spaces seems sufficient for most
- * real data. */
- struct lz_mc_pos_data *optimum;
- input_idx_t array_space;
-
- /* When a match with length greater than or equal to this length is
- * found, choose it immediately without further consideration. */
- input_idx_t nice_len;
-
- /* When matches have been chosen, optimum_cur_idx is set to the position
- * in the window of the next match/literal to return and optimum_end_idx
- * is set to the position in the window at the end of the last
- * match/literal to return. */
- input_idx_t optimum_cur_idx;
- input_idx_t optimum_end_idx;
-};
-
-/*
- * Match chooser position data:
- *
- * An array of these structures is used during the match-choosing algorithm.
- * They correspond to consecutive positions in the window and are used to keep
- * track of the cost to reach each position, and the match/literal choices that
- * need to be chosen to reach that position.
- */
-struct lz_mc_pos_data {
- /* The approximate minimum cost, in bits, to reach this position in the
- * window which has been found so far. */
- lz_mc_cost_t cost;
-
- /* The union here is just for clarity, since the fields are used in two
- * slightly different ways. Initially, the @prev structure is filled in
- * first, and links go from later in the window to earlier in the
- * window. Later, @next structure is filled in and links go from
- * earlier in the window to later in the window. */
- union {
- struct {
- /* Position of the start of the match or literal that
- * was taken to get to this position in the approximate
- * minimum-cost parse. */
- input_idx_t link;
-
- /* Offset (as in an LZ (length, offset) pair) of the
- * match or literal that was taken to get to this
- * position in the approximate minimum-cost parse. */
- input_idx_t match_offset;
- } prev;
- struct {
- /* Position at which the match or literal starting at
- * this position ends in the minimum-cost parse. */
- input_idx_t link;
-
- /* Offset (as in an LZ (length, offset) pair) of the
- * match or literal starting at this position in the
- * approximate minimum-cost parse. */
- input_idx_t match_offset;
- } next;
- };
-
- /* Format-dependent adaptive state that exists after an approximate
- * minimum-cost path to reach this position is taken. For example, for
- * LZX this is the list of recently used match offsets. If the format
- * does not have any adaptive state that affects match costs,
- * LZ_ADAPTIVE_STATE could be set to a dummy structure of size 0. */
- LZ_ADAPTIVE_STATE state;
-};
-
-/* Initialize the match-chooser.
- *
- * After calling this, multiple data buffers can be scanned with it if each is
- * preceded with a call to lz_match_chooser_begin(). */
-static bool
-lz_match_chooser_init(struct lz_match_chooser *mc,
- input_idx_t array_space,
- input_idx_t nice_len, input_idx_t max_match_len)
-{
- input_idx_t extra_len = min(nice_len, max_match_len);
-
- LZ_ASSERT(array_space > 0);
- mc->optimum = MALLOC((array_space + extra_len) * sizeof(mc->optimum[0]));
- if (mc->optimum == NULL)
- return false;
- mc->array_space = array_space;
- mc->nice_len = nice_len;
- return true;
-}
-
-static inline u64
-lz_match_chooser_get_needed_memory(input_idx_t array_space,
- input_idx_t nice_len,
- input_idx_t max_match_len)
-{
- input_idx_t extra_len = min(nice_len, max_match_len);
- return ((u64)(array_space + extra_len) *
- sizeof(((struct lz_match_chooser*)0)->optimum[0]));
-}
-
-/* Free memory allocated in lz_match_chooser_init(). */
-static void
-lz_match_chooser_destroy(struct lz_match_chooser *mc)
-{
- FREE(mc->optimum);
-}
-
-/* Call this before starting to parse each new input string. */
-static void
-lz_match_chooser_begin(struct lz_match_chooser *mc)
-{
- mc->optimum_cur_idx = 0;
- mc->optimum_end_idx = 0;
-}
-
-/*
- * Reverse the linked list of near-optimal matches so that they can be returned
- * in forwards order.
- *
- * Returns the first match in the list.
- */
-static _always_inline_attribute struct raw_match
-lz_match_chooser_reverse_list(struct lz_match_chooser *mc, input_idx_t cur_pos)
-{
- unsigned prev_link, saved_prev_link;
- unsigned prev_match_offset, saved_prev_match_offset;
-
- mc->optimum_end_idx = cur_pos;
-
- saved_prev_link = mc->optimum[cur_pos].prev.link;
- saved_prev_match_offset = mc->optimum[cur_pos].prev.match_offset;
-
- do {
- prev_link = saved_prev_link;
- prev_match_offset = saved_prev_match_offset;
-
- saved_prev_link = mc->optimum[prev_link].prev.link;
- saved_prev_match_offset = mc->optimum[prev_link].prev.match_offset;
-
- mc->optimum[prev_link].next.link = cur_pos;
- mc->optimum[prev_link].next.match_offset = prev_match_offset;
-
- cur_pos = prev_link;
- } while (cur_pos != 0);
-
- mc->optimum_cur_idx = mc->optimum[0].next.link;
-
- return (struct raw_match)
- { .len = mc->optimum_cur_idx,
- .offset = mc->optimum[0].next.match_offset,
- };
-}
-
-/* Format-specific functions inlined into lz_get_near_optimal_match(). */
-
-/* Get the list of possible matches at the next position. The return value must
- * be the number of matches found (which may be 0) and a pointer to the returned
- * matches must be written into @matches_ret. Matches must be of distinct
- * lengths and sorted in decreasing order by length. Furthermore, match lengths
- * may not exceed the @max_match_len passed to lz_match_chooser_init(), and all
- * match lengths must be at least 2. */
-typedef u32 (*lz_get_matches_t)(LZ_COMPRESSOR *ctx,
- const LZ_ADAPTIVE_STATE *state,
- struct raw_match **matches_ret);
-
-/* Skip the specified number of bytes (don't search for matches at them). This
- * is expected to be faster than simply getting the matches at each position,
- * but the exact performance difference will be dependent on the match-finder
- * implementation. */
-typedef void (*lz_skip_bytes_t)(LZ_COMPRESSOR *ctx, input_idx_t n);
-
-/* Get the cost of the literal located at the position at which matches have
- * most recently been searched. This can optionally update the @state to take
- * into account format-dependent state that affects match costs, such as repeat
- * offsets. */
-typedef lz_mc_cost_t (*lz_get_prev_literal_cost_t)(LZ_COMPRESSOR *ctx,
- LZ_ADAPTIVE_STATE *state);
-
-/* Get the cost of a match. This can optionally update the @state to take into
- * account format-dependent state that affects match costs, such as repeat
- * offsets. */
-typedef lz_mc_cost_t (*lz_get_match_cost_t)(LZ_COMPRESSOR *ctx,
- LZ_ADAPTIVE_STATE *state,
- input_idx_t length,
- input_idx_t offset);
-
-/*
- * lz_get_near_optimal_match() -
- *
- * Choose an approximately optimal match or literal to use at the next position
- * in the string, or "window", being LZ-encoded.
- *
- * This is based on the algorithm used in 7-Zip's DEFLATE encoder, written by
- * Igor Pavlov. However it also attempts to account for adaptive state, such as
- * an LRU queue of recent match offsets.
- *
- * Unlike a greedy parser that always takes the longest match, or even a "lazy"
- * parser with one match/literal look-ahead like zlib, the algorithm used here
- * may look ahead many matches/literals to determine the approximately optimal
- * match/literal to code next. The motivation is that the compression ratio is
- * improved if the compressor can do things like use a shorter-than-possible
- * match in order to allow a longer match later, and also take into account the
- * estimated real cost of coding each match/literal based on the underlying
- * entropy encoding.
- *
- * Still, this is not a true optimal parser for several reasons:
- *
- * - Very long matches (at least @nice_len) are taken immediately. This is
- * because locations with long matches are likely to have many possible
- * alternatives that would cause slow optimal parsing, but also such locations
- * are already highly compressible so it is not too harmful to just grab the
- * longest match.
- *
- * - Not all possible matches at each location are considered. Users of this
- * code are expected to provide a @get_matches() function that returns a list
- * of potentially good matches at the current position, but no more than one
- * per length. It therefore must use some sort of heuristic (e.g. smallest or
- * repeat offset) to choose a good match to consider for a given length, if
- * multiple exist. Furthermore, the @get_matches() implementation may limit
- * the total number of matches returned and/or the number of computational
- * steps spent searching for matches at each position.
- *
- * - This function relies on the user-provided @get_match_cost() and
- * @get_prev_literal_cost() functions to evaluate match and literal costs,
- * respectively, but real compression formats use entropy encoding of the
- * literal/match sequence, so the real cost of coding each match or literal is
- * unknown until the parse is fully determined. It can be approximated based
- * on iterative parses, but the end result is not guaranteed to be globally
- * optimal.
- *
- * - Although this function allows @get_match_cost() and
- * @get_prev_literal_cost() to take into account adaptive state, coding
- * decisions made with respect to the adaptive state will be locally optimal
- * but will not necessarily be globally optimal. This is because the
- * algorithm only keeps the least-costly path to get to a given location and
- * does not take into account that a slightly more costly path could result in
- * a different adaptive state that ultimately results in a lower global cost.
- *
- * - The array space used by this function is bounded, so in degenerate cases it
- * is forced to start returning matches/literals before the algorithm has
- * really finished.
- *
- * Each call to this function does one of two things:
- *
- * 1. Build a sequence of near-optimal matches/literals, up to some point, that
- * will be returned by subsequent calls to this function, then return the
- * first one.
- *
- * OR
- *
- * 2. Return the next match/literal previously computed by a call to this
- * function.
- *
- * The return value is a (length, offset) pair specifying the match or literal
- * chosen. For literals, the length is 0 or 1 and the offset is meaningless.
- *
- * NOTE: this code has been factored out of the LZX compressor so that it can be
- * shared by other formats such as LZMS. It is inlined so there is no loss of
- * performance, especially with the different implementations of match-finding,
- * cost evaluation, and adaptive state.
- */
-static _always_inline_attribute struct raw_match
-lz_get_near_optimal_match(struct lz_match_chooser *mc,
- lz_get_matches_t get_matches,
- lz_skip_bytes_t skip_bytes,
- lz_get_prev_literal_cost_t get_prev_literal_cost,
- lz_get_match_cost_t get_match_cost,
- LZ_COMPRESSOR *ctx,
- const LZ_ADAPTIVE_STATE *initial_state)
-{
- u32 num_possible_matches;
- struct raw_match *possible_matches;
- struct raw_match match;
- input_idx_t longest_match_len;
-
- if (mc->optimum_cur_idx != mc->optimum_end_idx) {
- /* Case 2: Return the next match/literal already found. */
- match.len = mc->optimum[mc->optimum_cur_idx].next.link -
- mc->optimum_cur_idx;
- match.offset = mc->optimum[mc->optimum_cur_idx].next.match_offset;
-
- mc->optimum_cur_idx = mc->optimum[mc->optimum_cur_idx].next.link;
- return match;
- }
-
- /* Case 1: Compute a new list of matches/literals to return. */
-
- mc->optimum_cur_idx = 0;
- mc->optimum_end_idx = 0;
-
- /* Get matches at this position. */
- num_possible_matches = (*get_matches)(ctx,
- initial_state,
- &possible_matches);
-
- /* If no matches found, return literal. */
- if (num_possible_matches == 0)
- return (struct raw_match){ .len = 0 };
-
- /* The matches that were found are sorted in decreasing order by length.
- * Get the length of the longest one. */
- longest_match_len = possible_matches[0].len;
-
- /* Greedy heuristic: if the longest match that was found is greater
- * than nice_len, return it immediately; don't both doing more work. */
- if (longest_match_len >= mc->nice_len) {
- (*skip_bytes)(ctx, longest_match_len - 1);
- return possible_matches[0];
- }
-
- /* Calculate the cost to reach the next position by coding a literal.
- */
- mc->optimum[1].state = *initial_state;
- mc->optimum[1].cost = (*get_prev_literal_cost)(ctx, &mc->optimum[1].state);
- mc->optimum[1].prev.link = 0;
-
- /* Calculate the cost to reach any position up to and including that
- * reached by the longest match. Use the shortest available match that
- * reaches each position, assuming that @get_matches() only returned
- * shorter matches because their estimated costs were less than that of
- * the longest match. */
- for (input_idx_t len = 2, match_idx = num_possible_matches - 1;
- len <= longest_match_len; len++)
- {
-
- LZ_ASSERT(match_idx < num_possible_matches);
- LZ_ASSERT(len <= possible_matches[match_idx].len);
-
- mc->optimum[len].state = *initial_state;
- mc->optimum[len].prev.link = 0;
- mc->optimum[len].prev.match_offset = possible_matches[match_idx].offset;
- mc->optimum[len].cost = (*get_match_cost)(ctx,
- &mc->optimum[len].state,
- len,
- possible_matches[match_idx].offset);
- if (len == possible_matches[match_idx].len)
- match_idx--;
- }
-
- /* Step forward, calculating the estimated minimum cost to reach each
- * 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 @mc->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 @len_end 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 @len_end are reachable by at least one path.)
- *
- * The loop terminates when any one of the following conditions occurs:
- *
- * 1. A match with length greater than or equal to @nice_len is found.
- * When this occurs, the algorithm chooses this match
- * unconditionally, and consequently the near-optimal match/literal
- * sequence up to and including that match is fully determined and it
- * can begin returning the match/literal list.
- *
- * 2. @cur_pos reaches a position not overlapped by a preceding match.
- * In such cases, the near-optimal match/literal sequence up to
- * @cur_pos is fully determined and it can begin returning the
- * match/literal list.
- *
- * 3. Failing either of the above in a degenerate case, the loop
- * terminates when space in the @mc->optimum array is exhausted.
- * This terminates the algorithm and forces it to start returning
- * matches/literals even though they may not be globally optimal.
- *
- * Upon loop termination, a nonempty list of matches/literals will have
- * been produced and stored in the @optimum array. These
- * matches/literals are linked in reverse order, so the last thing this
- * function does is reverse this list and return the first
- * match/literal, leaving the rest to be returned immediately by
- * subsequent calls to this function.
- */
- input_idx_t cur_pos = 0;
- input_idx_t len_end = longest_match_len;
- for (;;) {
- /* Advance to next position. */
- cur_pos++;
-
- /* Check termination conditions (2) and (3) noted above. */
- if (cur_pos == len_end || cur_pos == mc->array_space)
- return lz_match_chooser_reverse_list(mc, cur_pos);
-
- /* Retrieve a (possibly empty) list of potentially useful
- * matches available at this position. */
- num_possible_matches = (*get_matches)(ctx,
- &mc->optimum[cur_pos].state,
- &possible_matches);
-
- if (num_possible_matches == 0)
- longest_match_len = 0;
- else
- longest_match_len = possible_matches[0].len;
-
- /* Greedy heuristic and termination condition (1) noted above:
- * if we found a match greater than @nice_len, choose it
- * unconditionally and begin returning matches/literals. */
- if (longest_match_len >= mc->nice_len) {
- /* Build the list of matches to return and get
- * the first one. */
- match = lz_match_chooser_reverse_list(mc, cur_pos);
-
- /* Append the long match to the end of the list. */
- mc->optimum[cur_pos].next.match_offset =
- possible_matches[0].offset;
- mc->optimum[cur_pos].next.link = cur_pos + longest_match_len;
- mc->optimum_end_idx = cur_pos + longest_match_len;
-
- /* Skip over the remaining bytes of the long match. */
- (*skip_bytes)(ctx, longest_match_len - 1);
-
- /* Return first match in the list. */
- return match;
- }
-
- /* Load minimum cost to reach the current position. */
- input_idx_t cur_cost = mc->optimum[cur_pos].cost;
-
- /* Consider proceeding with a literal byte. */
- {
- LZ_ADAPTIVE_STATE state;
- lz_mc_cost_t cost;
-
- state = mc->optimum[cur_pos].state;
- cost = cur_cost + (*get_prev_literal_cost)(ctx, &state);
-
- if (cost < mc->optimum[cur_pos + 1].cost) {
- mc->optimum[cur_pos + 1].cost = cost;
- mc->optimum[cur_pos + 1].prev.link = cur_pos;
- mc->optimum[cur_pos + 1].state = state;
- }
- }
-
- /* If no matches were found, continue to the next position.
- * Otherwise, consider proceeding with a match. */
-
- if (num_possible_matches == 0)
- continue;
-
- /* Initialize any uninitialized costs up to the length of the
- * longest match found. */
- while (len_end < cur_pos + longest_match_len)
- mc->optimum[++len_end].cost = LZ_MC_INFINITE_COST;
-
- /* Calculate the minimum cost to reach any position up to and
- * including that reached by the longest match. Use the
- * shortest available match that reaches each position, assuming
- * that @get_matches() only returned shorter matches because
- * their estimated costs were less than that of the longest
- * match. */
- for (input_idx_t len = 2, match_idx = num_possible_matches - 1;
- len <= longest_match_len; len++)
- {
- LZ_ASSERT(match_idx < num_possible_matches);
- LZ_ASSERT(len <= possible_matches[match_idx].len);
-
- LZ_ADAPTIVE_STATE state;
- lz_mc_cost_t cost;
-
- state = mc->optimum[cur_pos].state;
- cost = cur_cost + (*get_match_cost)(ctx,
- &state,
- len,
- possible_matches[match_idx].offset);
-
- if (cost < mc->optimum[cur_pos + len].cost) {
- mc->optimum[cur_pos + len].cost = cost;
- mc->optimum[cur_pos + len].prev.link = cur_pos;
- mc->optimum[cur_pos + len].prev.match_offset =
- possible_matches[match_idx].offset;
- mc->optimum[cur_pos + len].state = state;
- }
-
- if (len == possible_matches[match_idx].len)
- match_idx--;
- }
- }
-}
-
-#endif /* _LZ_OPTIMAL_H */
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