lzx-compress.c: Don't compute match/literal array before actually needed

[wimlib] / src / lzx-compress.c

/*
 * lzx-compress.c
 */

/*
 * Copyright (C) 2012, 2013, 2014 Eric Biggers
 *
 * This file is part of wimlib, a library for working with WIM files.
 *
 * wimlib is free software; you can redistribute it and/or modify it under the
 * terms of the GNU General Public License as published by the Free
 * Software Foundation; either version 3 of the License, or (at your option)
 * any later version.
 *
 * wimlib is distributed in the hope that it will be useful, but WITHOUT ANY
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
 * A PARTICULAR PURPOSE. See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License
 * along with wimlib; if not, see http://www.gnu.org/licenses/.
 */


/*
 * This file contains a compressor for the LZX ("Lempel-Ziv eXtended"?)
 * compression format, as used in the WIM (Windows IMaging) file format.  This
 * code may need some slight modifications to be used outside of the WIM format.
 * In particular, in other situations the LZX block header might be slightly
 * different, and a sliding window rather than a fixed-size window might be
 * required.
 *
 * ----------------------------------------------------------------------------
 *
 *                               Format Overview
 *
 * The primary reference for LZX is the specification released by Microsoft.
 * However, the comments in lzx-decompress.c provide more information about LZX
 * and note some errors in the Microsoft specification.
 *
 * LZX shares many similarities with DEFLATE, the format used by zlib and gzip.
 * Both LZX and DEFLATE use LZ77 matching and Huffman coding.  Certain details
 * are quite similar, such as the method for storing Huffman codes.  However,
 * the main differences are:
 *
 * - LZX preprocesses the data to attempt to make x86 machine code slightly more
 *   compressible before attempting to compress it further.
 *
 * - LZX uses a "main" alphabet which combines literals and matches, with the
 *   match symbols containing a "length header" (giving all or part of the match
 *   length) and a "position slot" (giving, roughly speaking, the order of
 *   magnitude of the match offset).
 *
 * - LZX does not have static Huffman blocks (that is, the kind with preset
 *   Huffman codes); however it does have two types of dynamic Huffman blocks
 *   ("verbatim" and "aligned").
 *
 * - LZX has a minimum match length of 2 rather than 3.
 *
 * - In LZX, match offsets 0 through 2 actually represent entries in an LRU
 *   queue of match offsets.  This is very useful for certain types of files,
 *   such as binary files that have repeating records.
 *
 * ----------------------------------------------------------------------------
 *
 *                            Algorithmic Overview
 *
 * At a high level, any implementation of LZX compression must operate as
 * follows:
 *
 * 1. Preprocess the input data to translate the targets of 32-bit x86 call
 *    instructions to absolute offsets.  (Actually, this is required for WIM,
 *    but might not be in other places LZX is used.)
 *
 * 2. Find a sequence of LZ77-style matches and literal bytes that expands to
 *    the preprocessed data.
 *
 * 3. Divide the match/literal sequence into one or more LZX blocks, each of
 *    which may be "uncompressed", "verbatim", or "aligned".
 *
 * 4. Output each LZX block.
 *
 * Step (1) is fairly straightforward.  It requires looking for 0xe8 bytes in
 * the input data and performing a translation on the 4 bytes following each
 * one.
 *
 * Step (4) is complicated, but it is mostly determined by the LZX format.  The
 * only real choice we have is what algorithm to use to build the length-limited
 * canonical Huffman codes.  See lzx_write_all_blocks() for details.
 *
 * That leaves steps (2) and (3) as where all the hard stuff happens.  Focusing
 * on step (2), we need to do LZ77-style parsing on the input data, or "window",
 * to divide it into a sequence of matches and literals.  Each position in the
 * window might have multiple matches associated with it, and we need to choose
 * which one, if any, to actually use.  Therefore, the problem can really be
 * divided into two areas of concern: (a) finding matches at a given position,
 * which we shall call "match-finding", and (b) choosing whether to use a
 * match or a literal at a given position, and if using a match, which one (if
 * there is more than one available).  We shall call this "match-choosing".  We
 * first consider match-finding, then match-choosing.
 *
 * ----------------------------------------------------------------------------
 *
 *                               Match-finding
 *
 * Given a position in the window, we want to find LZ77-style "matches" with
 * 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.
 *
 * Depending on how good a compression ratio we want (see the "Match-choosing"
 * section), we may want to find: (a) all matches, or (b) just the longest
 * match, or (c) just some "promising" matches that we are able to find quickly,
 * or (d) just the longest match that we're able to find quickly.  Below we
 * introduce the match-finding methods that the code currently uses or has
 * previously used:
 *
 * - Hash chains.  Maintain a table that maps hash codes, computed from
 *   fixed-length byte sequences, to linked lists containing previous window
 *   positions.  To search for matches, compute the hash for the current
 *   position in the window and search the appropriate hash chain.  When
 *   advancing to the next position, prepend the current position to the
 *   appropriate hash list.  This is a good approach for producing matches with
 *   stategy (d) and is useful for fast compression.  Therefore, we provide an
 *   option to use this method for LZX compression.  See lz_hash.c for the
 *   implementation.
 *
 * - Binary trees.  Similar to hash chains, but each hash bucket contains a
 *   binary tree of previous window positions rather than a linked list.  This
 *   is a good approach for producing matches with stategy (c) and is useful for
 *   achieving a good compression ratio.  Therefore, we provide an option to use
 *   this method; see lz_bt.c for the implementation.
 *
 * - Suffix arrays.  This code previously used this method to produce matches
 *   with stategy (c), but I've dropped it because it was slower than the binary
 *   trees approach, used more memory, and did not improve the compression ratio
 *   enough to compensate.  Download wimlib v1.6.2 if you want the code.
 *   However, the suffix array method was basically as follows.  Build the
 *   suffix array for the entire window.  The suffix array contains each
 *   possible window position, sorted by the lexicographic order of the strings
 *   that begin at those positions.  Find the matches at a given position by
 *   searching the suffix array outwards, in both directions, from the suffix
 *   array slot for that position.  This produces the longest matches first, but
 *   "matches" that actually occur at later positions in the window must be
 *   skipped.  To do this skipping, use an auxiliary array with dynamically
 *   constructed linked lists.  Also, use the inverse suffix array to quickly
 *   find the suffix array slot for a given position without doing a binary
 *   search.
 *
 * ----------------------------------------------------------------------------
 *
 *                               Match-choosing
 *
 * Usually, choosing the longest match is best because it encodes the most data
 * in that one item.  However, sometimes the longest match is not optimal
 * because (a) choosing a long match now might prevent using an even longer
 * match later, or (b) more generally, what we actually care about is the number
 * of bits it will ultimately take to output each match or literal, which is
 * actually dependent on the entropy encoding using by the underlying
 * compression format.  Consequently, a longer match usually, but not always,
 * takes fewer bits to encode than multiple shorter matches or literals that
 * cover the same data.
 *
 * This problem of choosing the truly best match/literal sequence is probably
 * impossible to solve efficiently when combined with entropy encoding.  If we
 * knew how many bits it takes to output each match/literal, then we could
 * choose the optimal sequence using shortest-path search a la Dijkstra's
 * algorithm.  However, with entropy encoding, the chosen match/literal sequence
 * affects its own encoding.  Therefore, we can't know how many bits it will
 * take to actually output any one match or literal until we have actually
 * chosen the full sequence of matches and literals.
 *
 * Notwithstanding the entropy encoding problem, we also aren't guaranteed to
 * choose the optimal match/literal sequence unless the match-finder (see
 * section "Match-finder") provides the match-chooser with all possible matches
 * at each position.  However, this is not computationally efficient.  For
 * example, there might be many matches of the same length, and usually (but not
 * always) the best choice is the one with the smallest offset.  So in practice,
 * it's fine to only consider the smallest offset for a given match length at a
 * given position.  (Actually, for LZX, it's also worth considering repeat
 * offsets.)
 *
 * In addition, as mentioned earlier, in LZX we have the choice of using
 * multiple blocks, each of which resets the Huffman codes.  This expands the
 * search space even further.  Therefore, to simplify the problem, we currently
 * we don't attempt to actually choose the LZX blocks based on the data.
 * Instead, we just divide the data into fixed-size blocks of LZX_DIV_BLOCK_SIZE
 * bytes each, and always use verbatim or aligned blocks (never uncompressed).
 * A previous version of this code recursively split the input data into
 * equal-sized blocks, up to a maximum depth, and chose the lowest-cost block
 * divisions.  However, this made compression much slower and did not actually
 * help very much.  It remains an open question whether a sufficiently fast and
 * useful block-splitting algorithm is possible for LZX.  Essentially the same
 * problem also applies to DEFLATE.  The Microsoft LZX compressor seemingly does
 * do block splitting, although I don't know how fast or useful it is,
 * specifically.
 *
 * Now, back to the entropy encoding problem.  The "solution" is to use an
 * iterative approach to compute a good, but not necessarily optimal,
 * match/literal sequence.  Start with a fixed assignment of symbol costs and
 * choose an "optimal" match/literal sequence based on those costs, using
 * shortest-path seach a la Dijkstra's algorithm.  Then, for each iteration of
 * the optimization, update the costs based on the entropy encoding of the
 * current match/literal sequence, then choose a new match/literal sequence
 * based on the updated costs.  Usually, the actual cost to output the current
 * match/literal sequence will decrease in each iteration until it converges on
 * a fixed point.  This result may not be the truly optimal match/literal
 * sequence, but it usually is much better than one chosen by doing a "greedy"
 * parse where we always chooe the longest match.
 *
 * An alternative to both greedy parsing and iterative, near-optimal parsing is
 * "lazy" parsing.  Briefly, "lazy" parsing considers just the longest match at
 * each position, but it waits to choose that match until it has also examined
 * the next position.  This is actually a useful approach; it's used by zlib,
 * for example.  Therefore, for fast compression we combine lazy parsing with
 * the hash chain max-finder.  For normal/high compression we combine
 * near-optimal parsing with the binary tree match-finder.
 *
 * Anyway, if you've read through this comment, you hopefully should have a
 * better idea of why things are done in a certain way in this LZX compressor,
 * as well as in other compressors for LZ77-based formats (including third-party
 * ones).  In my opinion, the phrase "compression algorithm" is often mis-used
 * in place of "compression format",  since there can be many different
 * algorithms that all generate compressed data in the same format.  The
 * challenge is to design an algorithm that is efficient but still gives a good
 * compression ratio.
 */

#ifdef HAVE_CONFIG_H
#  include "config.h"
#endif

#include "wimlib.h"
#include "wimlib/compressor_ops.h"
#include "wimlib/compress_common.h"
#include "wimlib/endianness.h"
#include "wimlib/error.h"
#include "wimlib/lz.h"
#include "wimlib/lz_hash.h"
#include "wimlib/lz_bt.h"
#include "wimlib/lzx.h"
#include "wimlib/util.h"
#include <string.h>

#ifdef ENABLE_LZX_DEBUG
#  include "wimlib/decompress_common.h"
#endif

#define LZX_OPTIM_ARRAY_SIZE    4096

#define LZX_DIV_BLOCK_SIZE      32768

#define LZX_CACHE_PER_POS       8

#define LZX_CACHE_LEN (LZX_DIV_BLOCK_SIZE * (LZX_CACHE_PER_POS + 1))
#define LZX_CACHE_SIZE (LZX_CACHE_LEN * sizeof(struct raw_match))
#define LZX_MAX_MATCHES_PER_POS (LZX_MAX_MATCH_LEN - LZX_MIN_MATCH_LEN + 1)

/* Codewords for the LZX main, length, and aligned offset Huffman codes  */
struct lzx_codewords {
        u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
        u32 len[LZX_LENCODE_NUM_SYMBOLS];
        u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};

/* Codeword lengths (in bits) for the LZX main, length, and aligned offset
 * Huffman codes.
 *
 * A 0 length means the codeword has zero frequency.
 */
struct lzx_lens {
        u8 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
        u8 len[LZX_LENCODE_NUM_SYMBOLS];
        u8 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};

/* Costs for the LZX main, length, and aligned offset Huffman symbols.
 *
 * If a codeword has zero frequency, it must still be assigned some nonzero cost
 * --- generally a high cost, since even if it gets used in the next iteration,
 * it probably will not be used very times.  */
struct lzx_costs {
        u8 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
        u8 len[LZX_LENCODE_NUM_SYMBOLS];
        u8 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};

/* The LZX main, length, and aligned offset Huffman codes  */
struct lzx_codes {
        struct lzx_codewords codewords;
        struct lzx_lens lens;
};

/* Tables for tallying symbol frequencies in the three LZX alphabets  */
struct lzx_freqs {
        input_idx_t main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
        input_idx_t len[LZX_LENCODE_NUM_SYMBOLS];
        input_idx_t aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};

/* LZX intermediate match/literal format  */
struct lzx_match {
        /* Bit     Description
         *
         * 31      1 if a match, 0 if a literal.
         *
         * 30-25   position slot.  This can be at most 50, so it will fit in 6
         *         bits.
         *
         * 8-24    position footer.  This is the offset of the real formatted
         *         offset from the position base.  This can be at most 17 bits
         *         (since lzx_extra_bits[LZX_MAX_POSITION_SLOTS - 1] is 17).
         *
         * 0-7     length of match, minus 2.  This can be at most
         *         (LZX_MAX_MATCH_LEN - 2) == 255, so it will fit in 8 bits.  */
        u32 data;
};

/* Specification for an LZX block.  */
struct lzx_block_spec {

        /* One of the LZX_BLOCKTYPE_* constants indicating which type of this
         * block.  */
        int block_type;

        /* 0-based position in the window at which this block starts.  */
        input_idx_t window_pos;

        /* The number of bytes of uncompressed data this block represents.  */
        input_idx_t block_size;

        /* The match/literal sequence for this block.  */
        struct lzx_match *chosen_matches;

        /* The length of the @chosen_matches sequence.  */
        input_idx_t num_chosen_matches;

        /* Huffman codes for this block.  */
        struct lzx_codes codes;
};

/* State of the LZX compressor.  */
struct lzx_compressor {

        /* The parameters that were used to create the compressor.  */
        struct wimlib_lzx_compressor_params params;

        /* The buffer of data to be compressed.
         *
         * 0xe8 byte preprocessing is done directly on the data here before
         * further compression.
         *
         * Note that this compressor does *not* use a real sliding window!!!!
         * It's not needed in the WIM format, since every chunk is compressed
         * independently.  This is by design, to allow random access to the
         * chunks.
         *
         * We reserve a few extra bytes to potentially allow reading off the end
         * of the array in the match-finding code for optimization purposes
         * (currently only needed for the hash chain match-finder).  */
        u8 *window;

        /* Number of bytes of data to be compressed, which is the number of
         * bytes of data in @window that are actually valid.  */
        input_idx_t window_size;

        /* Allocated size of the @window.  */
        input_idx_t max_window_size;

        /* Number of symbols in the main alphabet (depends on the
         * @max_window_size since it determines the maximum allowed offset).  */
        unsigned num_main_syms;

        /* The current match offset LRU queue.  */
        struct lzx_lru_queue queue;

        /* Space for the sequences of matches/literals that were chosen for each
         * block.  */
        struct lzx_match *chosen_matches;

        /* Information about the LZX blocks the preprocessed input was divided
         * into.  */
        struct lzx_block_spec *block_specs;

        /* Number of LZX blocks the input was divided into; a.k.a. the number of
         * elements of @block_specs that are valid.  */
        unsigned num_blocks;

        /* This is simply filled in with zeroes and used to avoid special-casing
         * the output of the first compressed Huffman code, which conceptually
         * has a delta taken from a code with all symbols having zero-length
         * codewords.  */
        struct lzx_codes zero_codes;

        /* The current cost model.  */
        struct lzx_costs costs;

        /* Fast algorithm only:  Array of hash table links.  */
        input_idx_t *prev_tab;

        /* Slow algorithm only: Binary tree match-finder.  */
        struct lz_bt mf;

        /* Position in window of next match to return.  */
        input_idx_t match_window_pos;

        /* The end-of-block position.  We can't allow any matches to span this
         * position.  */
        input_idx_t match_window_end;

        /* Matches found by the match-finder are cached in the following array
         * to achieve a slight speedup when the same matches are needed on
         * subsequent passes.  This is suboptimal because different matches may
         * be preferred with different cost models, but seems to be a worthwhile
         * speedup.  */
        struct raw_match *cached_matches;
        struct raw_match *cache_ptr;
        bool matches_cached;
        struct raw_match *cache_limit;

        /* Match-chooser state.
         * 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.  */
        struct lzx_mc_pos_data *optimum;
        unsigned optimum_cur_idx;
        unsigned 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 lzx_mc_pos_data {
        /* The approximate minimum cost, in bits, to reach this position in the
         * window which has been found so far.  */
        u32 cost;
#define MC_INFINITE_COST ((u32)~0UL)

        /* 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;
        };

        /* Adaptive state that exists after an approximate minimum-cost path to
         * reach this position is taken.  */
        struct lzx_lru_queue queue;
};

/* Returns the LZX position slot that corresponds to a given match offset,
 * taking into account the recent offset queue and updating it if the offset is
 * found in it.  */
static unsigned
lzx_get_position_slot(u32 offset, struct lzx_lru_queue *queue)
{
        unsigned position_slot;

        /* See if the offset was recently used.  */
        for (int i = 0; i < LZX_NUM_RECENT_OFFSETS; i++) {
                if (offset == queue->R[i]) {
                        /* Found it.  */

                        /* Bring the repeat offset to the front of the
                         * queue.  Note: this is, in fact, not a real
                         * LRU queue because repeat matches are simply
                         * swapped to the front.  */
                        swap(queue->R[0], queue->R[i]);

                        /* The resulting position slot is simply the first index
                         * at which the offset was found in the queue.  */
                        return i;
                }
        }

        /* The offset was not recently used; look up its real position slot.  */
        position_slot = lzx_get_position_slot_raw(offset + LZX_OFFSET_OFFSET);

        /* Bring the new offset to the front of the queue.  */
        for (int i = LZX_NUM_RECENT_OFFSETS - 1; i > 0; i--)
                queue->R[i] = queue->R[i - 1];
        queue->R[0] = offset;

        return position_slot;
}

/* Build the main, length, and aligned offset Huffman codes used in LZX.
 *
 * This takes as input the frequency tables for each code and produces as output
 * a set of tables that map symbols to codewords and codeword lengths.  */
static void
lzx_make_huffman_codes(const struct lzx_freqs *freqs,
                       struct lzx_codes *codes,
                       unsigned num_main_syms)
{
        make_canonical_huffman_code(num_main_syms,
                                    LZX_MAX_MAIN_CODEWORD_LEN,
                                    freqs->main,
                                    codes->lens.main,
                                    codes->codewords.main);

        make_canonical_huffman_code(LZX_LENCODE_NUM_SYMBOLS,
                                    LZX_MAX_LEN_CODEWORD_LEN,
                                    freqs->len,
                                    codes->lens.len,
                                    codes->codewords.len);

        make_canonical_huffman_code(LZX_ALIGNEDCODE_NUM_SYMBOLS,
                                    LZX_MAX_ALIGNED_CODEWORD_LEN,
                                    freqs->aligned,
                                    codes->lens.aligned,
                                    codes->codewords.aligned);
}

/*
 * Output a precomputed LZX match.
 *
 * @out:
 *      The bitstream to which to write the match.
 * @block_type:
 *      The type of the LZX block (LZX_BLOCKTYPE_ALIGNED or
 *      LZX_BLOCKTYPE_VERBATIM)
 * @match:
 *      The match, as a (length, offset) pair.
 * @codes:
 *      Pointer to a structure that contains the codewords for the main, length,
 *      and aligned offset Huffman codes for the current LZX compressed block.
 */
static void
lzx_write_match(struct output_bitstream *out, int block_type,
                struct lzx_match match, const struct lzx_codes *codes)
{
        /* low 8 bits are the match length minus 2 */
        unsigned match_len_minus_2 = match.data & 0xff;
        /* Next 17 bits are the position footer */
        unsigned position_footer = (match.data >> 8) & 0x1ffff; /* 17 bits */
        /* Next 6 bits are the position slot. */
        unsigned position_slot = (match.data >> 25) & 0x3f;     /* 6 bits */
        unsigned len_header;
        unsigned len_footer;
        unsigned main_symbol;
        unsigned num_extra_bits;
        unsigned verbatim_bits;
        unsigned aligned_bits;

        /* If the match length is less than MIN_MATCH_LEN (= 2) +
         * NUM_PRIMARY_LENS (= 7), the length header contains
         * the match length minus MIN_MATCH_LEN, and there is no
         * length footer.
         *
         * Otherwise, the length header contains
         * NUM_PRIMARY_LENS, and the length footer contains
         * the match length minus NUM_PRIMARY_LENS minus
         * MIN_MATCH_LEN. */
        if (match_len_minus_2 < LZX_NUM_PRIMARY_LENS) {
                len_header = match_len_minus_2;
        } else {
                len_header = LZX_NUM_PRIMARY_LENS;
                len_footer = match_len_minus_2 - LZX_NUM_PRIMARY_LENS;
        }

        /* Combine the position slot with the length header into a single symbol
         * that will be encoded with the main code.
         *
         * The actual main symbol is offset by LZX_NUM_CHARS because values
         * under LZX_NUM_CHARS are used to indicate a literal byte rather than a
         * match.  */
        main_symbol = ((position_slot << 3) | len_header) + LZX_NUM_CHARS;

        /* Output main symbol. */
        bitstream_put_bits(out, codes->codewords.main[main_symbol],
                           codes->lens.main[main_symbol]);

        /* If there is a length footer, output it using the
         * length Huffman code. */
        if (len_header == LZX_NUM_PRIMARY_LENS)
                bitstream_put_bits(out, codes->codewords.len[len_footer],
                                   codes->lens.len[len_footer]);

        num_extra_bits = lzx_get_num_extra_bits(position_slot);

        /* For aligned offset blocks with at least 3 extra bits, output the
         * verbatim bits literally, then the aligned bits encoded using the
         * aligned offset code.  Otherwise, only the verbatim bits need to be
         * output. */
        if ((block_type == LZX_BLOCKTYPE_ALIGNED) && (num_extra_bits >= 3)) {

                verbatim_bits = position_footer >> 3;
                bitstream_put_bits(out, verbatim_bits,
                                   num_extra_bits - 3);

                aligned_bits = (position_footer & 7);
                bitstream_put_bits(out,
                                   codes->codewords.aligned[aligned_bits],
                                   codes->lens.aligned[aligned_bits]);
        } else {
                /* verbatim bits is the same as the position
                 * footer, in this case. */
                bitstream_put_bits(out, position_footer, num_extra_bits);
        }
}

/* Output an LZX literal (encoded with the main Huffman code).  */
static void
lzx_write_literal(struct output_bitstream *out, u8 literal,
                  const struct lzx_codes *codes)
{
        bitstream_put_bits(out,
                           codes->codewords.main[literal],
                           codes->lens.main[literal]);
}

static unsigned
lzx_build_precode(const u8 lens[restrict],
                  const u8 prev_lens[restrict],
                  const unsigned num_syms,
                  input_idx_t precode_freqs[restrict LZX_PRECODE_NUM_SYMBOLS],
                  u8 output_syms[restrict num_syms],
                  u8 precode_lens[restrict LZX_PRECODE_NUM_SYMBOLS],
                  u32 precode_codewords[restrict LZX_PRECODE_NUM_SYMBOLS],
                  unsigned *num_additional_bits_ret)
{
        memset(precode_freqs, 0,
               LZX_PRECODE_NUM_SYMBOLS * sizeof(precode_freqs[0]));

        /* Since the code word lengths use a form of RLE encoding, the goal here
         * is to find each run of identical lengths when going through them in
         * symbol order (including runs of length 1).  For each run, as many
         * lengths are encoded using RLE as possible, and the rest are output
         * literally.
         *
         * output_syms[] will be filled in with the length symbols that will be
         * output, including RLE codes, not yet encoded using the precode.
         *
         * cur_run_len keeps track of how many code word lengths are in the
         * current run of identical lengths.  */
        unsigned output_syms_idx = 0;
        unsigned cur_run_len = 1;
        unsigned num_additional_bits = 0;
        for (unsigned i = 1; i <= num_syms; i++) {

                if (i != num_syms && lens[i] == lens[i - 1]) {
                        /* Still in a run--- keep going. */
                        cur_run_len++;
                        continue;
                }

                /* Run ended! Check if it is a run of zeroes or a run of
                 * nonzeroes. */

                /* The symbol that was repeated in the run--- not to be confused
                 * with the length *of* the run (cur_run_len) */
                unsigned len_in_run = lens[i - 1];

                if (len_in_run == 0) {
                        /* A run of 0's.  Encode it in as few length
                         * codes as we can. */

                        /* The magic length 18 indicates a run of 20 + n zeroes,
                         * where n is an uncompressed literal 5-bit integer that
                         * follows the magic length. */
                        while (cur_run_len >= 20) {
                                unsigned additional_bits;

                                additional_bits = min(cur_run_len - 20, 0x1f);
                                num_additional_bits += 5;
                                precode_freqs[18]++;
                                output_syms[output_syms_idx++] = 18;
                                output_syms[output_syms_idx++] = additional_bits;
                                cur_run_len -= 20 + additional_bits;
                        }

                        /* The magic length 17 indicates a run of 4 + n zeroes,
                         * where n is an uncompressed literal 4-bit integer that
                         * follows the magic length. */
                        while (cur_run_len >= 4) {
                                unsigned additional_bits;

                                additional_bits = min(cur_run_len - 4, 0xf);
                                num_additional_bits += 4;
                                precode_freqs[17]++;
                                output_syms[output_syms_idx++] = 17;
                                output_syms[output_syms_idx++] = additional_bits;
                                cur_run_len -= 4 + additional_bits;
                        }

                } else {

                        /* A run of nonzero lengths. */

                        /* The magic length 19 indicates a run of 4 + n
                         * nonzeroes, where n is a literal bit that follows the
                         * magic length, and where the value of the lengths in
                         * the run is given by an extra length symbol, encoded
                         * with the precode, that follows the literal bit.
                         *
                         * The extra length symbol is encoded as a difference
                         * from the length of the codeword for the first symbol
                         * in the run in the previous code.
                         * */
                        while (cur_run_len >= 4) {
                                unsigned additional_bits;
                                signed char delta;

                                additional_bits = (cur_run_len > 4);
                                num_additional_bits += 1;
                                delta = (signed char)prev_lens[i - cur_run_len] -
                                        (signed char)len_in_run;
                                if (delta < 0)
                                        delta += 17;
                                precode_freqs[19]++;
                                precode_freqs[(unsigned char)delta]++;
                                output_syms[output_syms_idx++] = 19;
                                output_syms[output_syms_idx++] = additional_bits;
                                output_syms[output_syms_idx++] = delta;
                                cur_run_len -= 4 + additional_bits;
                        }
                }

                /* Any remaining lengths in the run are outputted without RLE,
                 * as a difference from the length of that codeword in the
                 * previous code. */
                while (cur_run_len > 0) {
                        signed char delta;

                        delta = (signed char)prev_lens[i - cur_run_len] -
                                (signed char)len_in_run;
                        if (delta < 0)
                                delta += 17;

                        precode_freqs[(unsigned char)delta]++;
                        output_syms[output_syms_idx++] = delta;
                        cur_run_len--;
                }

                cur_run_len = 1;
        }

        /* Build the precode from the frequencies of the length symbols. */

        make_canonical_huffman_code(LZX_PRECODE_NUM_SYMBOLS,
                                    LZX_MAX_PRE_CODEWORD_LEN,
                                    precode_freqs, precode_lens,
                                    precode_codewords);

        *num_additional_bits_ret = num_additional_bits;

        return output_syms_idx;
}

/*
 * Output a Huffman code in the compressed form used in LZX.
 *
 * The Huffman code is represented in the output as a logical series of codeword
 * lengths from which the Huffman code, which must be in canonical form, can be
 * reconstructed.
 *
 * The codeword lengths are themselves compressed using a separate Huffman code,
 * the "precode", which contains a symbol for each possible codeword length in
 * the larger code as well as several special symbols to represent repeated
 * codeword lengths (a form of run-length encoding).  The precode is itself
 * constructed in canonical form, and its codeword lengths are represented
 * literally in 20 4-bit fields that immediately precede the compressed codeword
 * lengths of the larger code.
 *
 * Furthermore, the codeword lengths of the larger code are actually represented
 * as deltas from the codeword lengths of the corresponding code in the previous
 * block.
 *
 * @out:
 *      Bitstream to which to write the compressed Huffman code.
 * @lens:
 *      The codeword lengths, indexed by symbol, in the Huffman code.
 * @prev_lens:
 *      The codeword lengths, indexed by symbol, in the corresponding Huffman
 *      code in the previous block, or all zeroes if this is the first block.
 * @num_syms:
 *      The number of symbols in the Huffman code.
 */
static void
lzx_write_compressed_code(struct output_bitstream *out,
                          const u8 lens[restrict],
                          const u8 prev_lens[restrict],
                          unsigned num_syms)
{
        input_idx_t precode_freqs[LZX_PRECODE_NUM_SYMBOLS];
        u8 output_syms[num_syms];
        u8 precode_lens[LZX_PRECODE_NUM_SYMBOLS];
        u32 precode_codewords[LZX_PRECODE_NUM_SYMBOLS];
        unsigned i;
        unsigned num_output_syms;
        u8 precode_sym;
        unsigned dummy;

        num_output_syms = lzx_build_precode(lens,
                                            prev_lens,
                                            num_syms,
                                            precode_freqs,
                                            output_syms,
                                            precode_lens,
                                            precode_codewords,
                                            &dummy);

        /* Write the lengths of the precode codes to the output. */
        for (i = 0; i < LZX_PRECODE_NUM_SYMBOLS; i++)
                bitstream_put_bits(out, precode_lens[i],
                                   LZX_PRECODE_ELEMENT_SIZE);

        /* Write the length symbols, encoded with the precode, to the output. */

        for (i = 0; i < num_output_syms; ) {
                precode_sym = output_syms[i++];

                bitstream_put_bits(out, precode_codewords[precode_sym],
                                   precode_lens[precode_sym]);
                switch (precode_sym) {
                case 17:
                        bitstream_put_bits(out, output_syms[i++], 4);
                        break;
                case 18:
                        bitstream_put_bits(out, output_syms[i++], 5);
                        break;
                case 19:
                        bitstream_put_bits(out, output_syms[i++], 1);
                        bitstream_put_bits(out,
                                           precode_codewords[output_syms[i]],
                                           precode_lens[output_syms[i]]);
                        i++;
                        break;
                default:
                        break;
                }
        }
}

/*
 * Write all matches and literal bytes (which were precomputed) in an LZX
 * compressed block to the output bitstream in the final compressed
 * representation.
 *
 * @ostream
 *      The output bitstream.
 * @block_type
 *      The chosen type of the LZX compressed block (LZX_BLOCKTYPE_ALIGNED or
 *      LZX_BLOCKTYPE_VERBATIM).
 * @match_tab
 *      The array of matches/literals to output.
 * @match_count
 *      Number of matches/literals to output (length of @match_tab).
 * @codes
 *      The main, length, and aligned offset Huffman codes for the current
 *      LZX compressed block.
 */
static void
lzx_write_matches_and_literals(struct output_bitstream *ostream,
                               int block_type,
                               const struct lzx_match match_tab[],
                               unsigned match_count,
                               const struct lzx_codes *codes)
{
        for (unsigned i = 0; i < match_count; i++) {
                struct lzx_match match = match_tab[i];

                /* The high bit of the 32-bit intermediate representation
                 * indicates whether the item is an actual LZ-style match (1) or
                 * a literal byte (0).  */
                if (match.data & 0x80000000)
                        lzx_write_match(ostream, block_type, match, codes);
                else
                        lzx_write_literal(ostream, match.data, codes);
        }
}

static void
lzx_assert_codes_valid(const struct lzx_codes * codes, unsigned num_main_syms)
{
#ifdef ENABLE_LZX_DEBUG
        unsigned i;

        for (i = 0; i < num_main_syms; i++)
                LZX_ASSERT(codes->lens.main[i] <= LZX_MAX_MAIN_CODEWORD_LEN);

        for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++)
                LZX_ASSERT(codes->lens.len[i] <= LZX_MAX_LEN_CODEWORD_LEN);

        for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++)
                LZX_ASSERT(codes->lens.aligned[i] <= LZX_MAX_ALIGNED_CODEWORD_LEN);

        const unsigned tablebits = 10;
        u16 decode_table[(1 << tablebits) +
                         (2 * max(num_main_syms, LZX_LENCODE_NUM_SYMBOLS))]
                         _aligned_attribute(DECODE_TABLE_ALIGNMENT);
        LZX_ASSERT(0 == make_huffman_decode_table(decode_table,
                                                  num_main_syms,
                                                  min(tablebits, LZX_MAINCODE_TABLEBITS),
                                                  codes->lens.main,
                                                  LZX_MAX_MAIN_CODEWORD_LEN));
        LZX_ASSERT(0 == make_huffman_decode_table(decode_table,
                                                  LZX_LENCODE_NUM_SYMBOLS,
                                                  min(tablebits, LZX_LENCODE_TABLEBITS),
                                                  codes->lens.len,
                                                  LZX_MAX_LEN_CODEWORD_LEN));
        LZX_ASSERT(0 == make_huffman_decode_table(decode_table,
                                                  LZX_ALIGNEDCODE_NUM_SYMBOLS,
                                                  min(tablebits, LZX_ALIGNEDCODE_TABLEBITS),
                                                  codes->lens.aligned,
                                                  LZX_MAX_ALIGNED_CODEWORD_LEN));
#endif /* ENABLE_LZX_DEBUG */
}

/* Write an LZX aligned offset or verbatim block to the output.  */
static void
lzx_write_compressed_block(int block_type,
                           unsigned block_size,
                           unsigned max_window_size,
                           unsigned num_main_syms,
                           struct lzx_match * chosen_matches,
                           unsigned num_chosen_matches,
                           const struct lzx_codes * codes,
                           const struct lzx_codes * prev_codes,
                           struct output_bitstream * ostream)
{
        unsigned i;

        LZX_ASSERT(block_type == LZX_BLOCKTYPE_ALIGNED ||
                   block_type == LZX_BLOCKTYPE_VERBATIM);
        lzx_assert_codes_valid(codes, num_main_syms);

        /* The first three bits indicate the type of block and are one of the
         * LZX_BLOCKTYPE_* constants.  */
        bitstream_put_bits(ostream, block_type, 3);

        /* Output the block size.
         *
         * The original LZX format seemed to always encode the block size in 3
         * bytes.  However, the implementation in WIMGAPI, as used in WIM files,
         * uses the first bit to indicate whether the block is the default size
         * (32768) or a different size given explicitly by the next 16 bits.
         *
         * By default, this compressor uses a window size of 32768 and therefore
         * follows the WIMGAPI behavior.  However, this compressor also supports
         * window sizes greater than 32768 bytes, which do not appear to be
         * supported by WIMGAPI.  In such cases, we retain the default size bit
         * to mean a size of 32768 bytes but output non-default block size in 24
         * bits rather than 16.  The compatibility of this behavior is unknown
         * because WIMs created with chunk size greater than 32768 can seemingly
         * only be opened by wimlib anyway.  */
        if (block_size == LZX_DEFAULT_BLOCK_SIZE) {
                bitstream_put_bits(ostream, 1, 1);
        } else {
                bitstream_put_bits(ostream, 0, 1);

                if (max_window_size >= 65536)
                        bitstream_put_bits(ostream, block_size >> 16, 8);

                bitstream_put_bits(ostream, block_size, 16);
        }

        /* Write out lengths of the main code. Note that the LZX specification
         * incorrectly states that the aligned offset code comes after the
         * length code, but in fact it is the very first code to be written
         * (before the main code).  */
        if (block_type == LZX_BLOCKTYPE_ALIGNED)
                for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++)
                        bitstream_put_bits(ostream, codes->lens.aligned[i],
                                           LZX_ALIGNEDCODE_ELEMENT_SIZE);

        LZX_DEBUG("Writing main code...");

        /* Write the precode and lengths for the first LZX_NUM_CHARS symbols in
         * the main code, which are the codewords for literal bytes.  */
        lzx_write_compressed_code(ostream,
                                  codes->lens.main,
                                  prev_codes->lens.main,
                                  LZX_NUM_CHARS);

        /* Write the precode and lengths for the rest of the main code, which
         * are the codewords for match headers.  */
        lzx_write_compressed_code(ostream,
                                  codes->lens.main + LZX_NUM_CHARS,
                                  prev_codes->lens.main + LZX_NUM_CHARS,
                                  num_main_syms - LZX_NUM_CHARS);

        LZX_DEBUG("Writing length code...");

        /* Write the precode and lengths for the length code.  */
        lzx_write_compressed_code(ostream,
                                  codes->lens.len,
                                  prev_codes->lens.len,
                                  LZX_LENCODE_NUM_SYMBOLS);

        LZX_DEBUG("Writing matches and literals...");

        /* Write the actual matches and literals.  */
        lzx_write_matches_and_literals(ostream, block_type,
                                       chosen_matches, num_chosen_matches,
                                       codes);

        LZX_DEBUG("Done writing block.");
}

/* Write out the LZX blocks that were computed.  */
static void
lzx_write_all_blocks(struct lzx_compressor *ctx, struct output_bitstream *ostream)
{

        const struct lzx_codes *prev_codes = &ctx->zero_codes;
        for (unsigned i = 0; i < ctx->num_blocks; i++) {
                const struct lzx_block_spec *spec = &ctx->block_specs[i];

                LZX_DEBUG("Writing block %u/%u (type=%d, size=%u, num_chosen_matches=%u)...",
                          i + 1, ctx->num_blocks,
                          spec->block_type, spec->block_size,
                          spec->num_chosen_matches);

                lzx_write_compressed_block(spec->block_type,
                                           spec->block_size,
                                           ctx->max_window_size,
                                           ctx->num_main_syms,
                                           spec->chosen_matches,
                                           spec->num_chosen_matches,
                                           &spec->codes,
                                           prev_codes,
                                           ostream);

                prev_codes = &spec->codes;
        }
}

/* Constructs an LZX match from a literal byte and updates the main code symbol
 * frequencies.  */
static inline u32
lzx_tally_literal(u8 lit, struct lzx_freqs *freqs)
{
        freqs->main[lit]++;
        return (u32)lit;
}

/* Constructs an LZX match from an offset and a length, and updates the LRU
 * queue and the frequency of symbols in the main, length, and aligned offset
 * alphabets.  The return value is a 32-bit number that provides the match in an
 * intermediate representation documented below.  */
static inline u32
lzx_tally_match(unsigned match_len, u32 match_offset,
                struct lzx_freqs *freqs, struct lzx_lru_queue *queue)
{
        unsigned position_slot;
        unsigned position_footer;
        u32 len_header;
        unsigned main_symbol;
        unsigned len_footer;
        unsigned adjusted_match_len;

        LZX_ASSERT(match_len >= LZX_MIN_MATCH_LEN && match_len <= LZX_MAX_MATCH_LEN);

        /* The match offset shall be encoded as a position slot (itself encoded
         * as part of the main symbol) and a position footer.  */
        position_slot = lzx_get_position_slot(match_offset, queue);
        position_footer = (match_offset + LZX_OFFSET_OFFSET) &
                                ((1U << lzx_get_num_extra_bits(position_slot)) - 1);

        /* The match length shall be encoded as a length header (itself encoded
         * as part of the main symbol) and an optional length footer.  */
        adjusted_match_len = match_len - LZX_MIN_MATCH_LEN;
        if (adjusted_match_len < LZX_NUM_PRIMARY_LENS) {
                /* No length footer needed.  */
                len_header = adjusted_match_len;
        } else {
                /* Length footer needed.  It will be encoded using the length
                 * code.  */
                len_header = LZX_NUM_PRIMARY_LENS;
                len_footer = adjusted_match_len - LZX_NUM_PRIMARY_LENS;
                freqs->len[len_footer]++;
        }

        /* Account for the main symbol.  */
        main_symbol = ((position_slot << 3) | len_header) + LZX_NUM_CHARS;

        freqs->main[main_symbol]++;

        /* In an aligned offset block, 3 bits of the position footer are output
         * as an aligned offset symbol.  Account for this, although we may
         * ultimately decide to output the block as verbatim.  */

        /* The following check is equivalent to:
         *
         * if (lzx_extra_bits[position_slot] >= 3)
         *
         * Note that this correctly excludes position slots that correspond to
         * recent offsets.  */
        if (position_slot >= 8)
                freqs->aligned[position_footer & 7]++;

        /* Pack the position slot, position footer, and match length into an
         * intermediate representation.  See `struct lzx_match' for details.
         */
        LZX_ASSERT(LZX_MAX_POSITION_SLOTS <= 64);
        LZX_ASSERT(lzx_get_num_extra_bits(LZX_MAX_POSITION_SLOTS - 1) <= 17);
        LZX_ASSERT(LZX_MAX_MATCH_LEN - LZX_MIN_MATCH_LEN + 1 <= 256);

        LZX_ASSERT(position_slot      <= (1U << (31 - 25)) - 1);
        LZX_ASSERT(position_footer    <= (1U << (25 -  8)) - 1);
        LZX_ASSERT(adjusted_match_len <= (1U << (8  -  0)) - 1);
        return 0x80000000 |
                (position_slot << 25) |
                (position_footer << 8) |
                (adjusted_match_len);
}

struct lzx_record_ctx {
        struct lzx_freqs freqs;
        struct lzx_lru_queue queue;
        struct lzx_match *matches;
};

static void
lzx_record_match(unsigned len, unsigned offset, void *_ctx)
{
        struct lzx_record_ctx *ctx = _ctx;

        (ctx->matches++)->data = lzx_tally_match(len, offset, &ctx->freqs, &ctx->queue);
}

static void
lzx_record_literal(u8 lit, void *_ctx)
{
        struct lzx_record_ctx *ctx = _ctx;

        (ctx->matches++)->data = lzx_tally_literal(lit, &ctx->freqs);
}

/* Returns the cost, in bits, to output a literal byte using the specified cost
 * model.  */
static u32
lzx_literal_cost(u8 c, const struct lzx_costs * costs)
{
        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.  */
static u32
lzx_match_cost(unsigned length, u32 offset, const struct lzx_costs *costs,
               struct lzx_lru_queue *queue)
{
        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);
        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];

        return cost;

}

/* Set the cost model @ctx->costs from the Huffman codeword lengths specified in
 * @lens.
 *
 * The cost model and codeword lengths are almost the same thing, but the
 * Huffman codewords with length 0 correspond to symbols with zero frequency
 * that still need to be assigned actual costs.  The specific values assigned
 * are arbitrary, but they should be fairly high (near the maximum codeword
 * length) to take into account the fact that uses of these symbols are expected
 * to be rare.  */
static void
lzx_set_costs(struct lzx_compressor * ctx, const struct lzx_lens * lens)
{
        unsigned i;
        unsigned num_main_syms = ctx->num_main_syms;

        /* Main code  */
        for (i = 0; i < num_main_syms; i++) {
                ctx->costs.main[i] = lens->main[i];
                if (ctx->costs.main[i] == 0)
                        ctx->costs.main[i] = ctx->params.alg_params.slow.main_nostat_cost;
        }

        /* Length code  */
        for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++) {
                ctx->costs.len[i] = lens->len[i];
                if (ctx->costs.len[i] == 0)
                        ctx->costs.len[i] = ctx->params.alg_params.slow.len_nostat_cost;
        }

        /* Aligned offset code  */
        for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
                ctx->costs.aligned[i] = lens->aligned[i];
                if (ctx->costs.aligned[i] == 0)
                        ctx->costs.aligned[i] = ctx->params.alg_params.slow.aligned_nostat_cost;
        }
}

/* Retrieve a list of matches available at the next position in the input.
 *
 * A pointer to the matches array is written into @matches_ret, and the return
 * value is the number of matches found.  */
static unsigned
lzx_get_matches(struct lzx_compressor *ctx,
                const struct raw_match **matches_ret)
{
        struct raw_match *cache_ptr;
        struct raw_match *matches;
        unsigned num_matches;

        LZX_ASSERT(ctx->match_window_pos < ctx->match_window_end);

        cache_ptr = ctx->cache_ptr;
        matches = cache_ptr + 1;
        if (likely(cache_ptr <= ctx->cache_limit)) {
                if (ctx->matches_cached) {
                        num_matches = cache_ptr->len;
                } else {
                        num_matches = lz_bt_get_matches(&ctx->mf, matches);
                        cache_ptr->len = num_matches;
                }
        } else {
                num_matches = 0;
        }

        /* Don't allow matches to span the end of an LZX block.  */
        if (ctx->match_window_end < ctx->window_size && num_matches != 0) {
                unsigned limit = ctx->match_window_end - ctx->match_window_pos;

                if (limit >= LZX_MIN_MATCH_LEN) {

                        unsigned i = num_matches - 1;
                        do {
                                if (matches[i].len >= limit) {
                                        matches[i].len = limit;

                                        /* Truncation might produce multiple
                                         * matches with length 'limit'.  Keep at
                                         * most 1.  */
                                        num_matches = i + 1;
                                }
                        } while (i--);
                } else {
                        num_matches = 0;
                }
                cache_ptr->len = num_matches;
        }

#if 0
        fprintf(stderr, "Pos %u/%u: %u matches\n",
                ctx->match_window_pos, ctx->window_size, num_matches);
        for (unsigned i = 0; i < num_matches; i++)
                fprintf(stderr, "\tLen %u Offset %u\n", matches[i].len, matches[i].offset);
#endif

#ifdef ENABLE_LZX_DEBUG
        for (unsigned i = 0; i < num_matches; i++) {
                LZX_ASSERT(matches[i].len >= LZX_MIN_MATCH_LEN);
                LZX_ASSERT(matches[i].len <= LZX_MAX_MATCH_LEN);
                LZX_ASSERT(matches[i].len <= ctx->match_window_end - ctx->match_window_pos);
                LZX_ASSERT(matches[i].offset > 0);
                LZX_ASSERT(matches[i].offset <= ctx->match_window_pos);
                LZX_ASSERT(!memcmp(&ctx->window[ctx->match_window_pos],
                                   &ctx->window[ctx->match_window_pos - matches[i].offset],
                                   matches[i].len));
                if (i) {
                        LZX_ASSERT(matches[i].len > matches[i - 1].len);
                        LZX_ASSERT(matches[i].offset > matches[i - 1].offset);
                }
        }
#endif
        ctx->match_window_pos++;
        ctx->cache_ptr = matches + num_matches;
        *matches_ret = matches;
        return num_matches;
}

static void
lzx_skip_bytes(struct lzx_compressor *ctx, unsigned n)
{
        struct raw_match *cache_ptr;

        LZX_ASSERT(n <= ctx->match_window_end - ctx->match_window_pos);

        cache_ptr = ctx->cache_ptr;
        ctx->match_window_pos += n;
        if (ctx->matches_cached) {
                while (n-- && cache_ptr <= ctx->cache_limit)
                        cache_ptr += 1 + cache_ptr->len;
        } else {
                lz_bt_skip_positions(&ctx->mf, n);
                while (n-- && cache_ptr <= ctx->cache_limit) {
                        cache_ptr->len = 0;
                        cache_ptr += 1;
                }
        }
        ctx->cache_ptr = cache_ptr;
}

/*
 * 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 struct raw_match
lzx_match_chooser_reverse_list(struct lzx_compressor *ctx, unsigned cur_pos)
{
        unsigned prev_link, saved_prev_link;
        unsigned prev_match_offset, saved_prev_match_offset;

        ctx->optimum_end_idx = cur_pos;

        saved_prev_link = ctx->optimum[cur_pos].prev.link;
        saved_prev_match_offset = ctx->optimum[cur_pos].prev.match_offset;

        do {
                prev_link = saved_prev_link;
                prev_match_offset = saved_prev_match_offset;

                saved_prev_link = ctx->optimum[prev_link].prev.link;
                saved_prev_match_offset = ctx->optimum[prev_link].prev.match_offset;

                ctx->optimum[prev_link].next.link = cur_pos;
                ctx->optimum[prev_link].next.match_offset = prev_match_offset;

                cur_pos = prev_link;
        } while (cur_pos != 0);

        ctx->optimum_cur_idx = ctx->optimum[0].next.link;

        return (struct raw_match)
                { .len = ctx->optimum_cur_idx,
                  .offset = ctx->optimum[0].next.match_offset,
                };
}

/*
 * lzx_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 algorithms used in 7-Zip, including the DEFLATE encoder
 * and the LZMA encoder, written by Igor Pavlov.
 *
 * 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:
 *
 * - 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.
 *
 * - Very long matches are chosen 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 because the
 *   underlying match-finder limits the number and type of matches produced at
 *   each position.  For example, for a given match length it's usually not
 *   worth it to only consider matches other than the lowest-offset match,
 *   except in the case of a repeat offset.
 *
 * - Although we take into account the adaptive state (in LZX, the recent offset
 *   queue), 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.
 */
static struct raw_match
lzx_get_near_optimal_match(struct lzx_compressor *ctx)
{
        unsigned num_matches;
        const struct raw_match *matches;
        struct raw_match match;
        unsigned longest_len;
        unsigned longest_rep_len;
        u32 longest_rep_offset;
        unsigned cur_pos;
        unsigned end_pos;

        if (ctx->optimum_cur_idx != ctx->optimum_end_idx) {
                /* Case 2: Return the next match/literal already found.  */
                match.len = ctx->optimum[ctx->optimum_cur_idx].next.link -
                                    ctx->optimum_cur_idx;
                match.offset = ctx->optimum[ctx->optimum_cur_idx].next.match_offset;

                ctx->optimum_cur_idx = ctx->optimum[ctx->optimum_cur_idx].next.link;
                return match;
        }

        /* Case 1:  Compute a new list of matches/literals to return.  */

        ctx->optimum_cur_idx = 0;
        ctx->optimum_end_idx = 0;

        /* Search for matches at recent offsets.  Only keep the one with the
         * longest match length.  */
        longest_rep_len = LZX_MIN_MATCH_LEN - 1;
        if (ctx->match_window_pos >= 1) {
                unsigned limit = min(LZX_MAX_MATCH_LEN,
                                     ctx->match_window_end - ctx->match_window_pos);
                for (int i = 0; i < LZX_NUM_RECENT_OFFSETS; i++) {
                        u32 offset = ctx->queue.R[i];
                        const u8 *strptr = &ctx->window[ctx->match_window_pos];
                        const u8 *matchptr = strptr - offset;
                        unsigned len = 0;
                        while (len < limit && strptr[len] == matchptr[len])
                                len++;
                        if (len > longest_rep_len) {
                                longest_rep_len = len;
                                longest_rep_offset = offset;
                        }
                }
        }

        /* If there's a long match with a recent offset, take it.  */
        if (longest_rep_len >= ctx->params.alg_params.slow.nice_match_length) {
                lzx_skip_bytes(ctx, longest_rep_len);
                return (struct raw_match) {
                        .len = longest_rep_len,
                        .offset = longest_rep_offset,
                };
        }

        /* Search other matches.  */
        num_matches = lzx_get_matches(ctx, &matches);

        /* If there's a long match, take it.  */
        if (num_matches) {
                longest_len = matches[num_matches - 1].len;
                if (longest_len >= ctx->params.alg_params.slow.nice_match_length) {
                        lzx_skip_bytes(ctx, longest_len - 1);
                        return matches[num_matches - 1];
                }
        } else {
                longest_len = 1;
        }

        /* Calculate the cost to reach the next position by coding a literal.
         */
        ctx->optimum[1].queue = ctx->queue;
        ctx->optimum[1].cost = lzx_literal_cost(ctx->window[ctx->match_window_pos - 1],
                                                &ctx->costs);
        ctx->optimum[1].prev.link = 0;

        /* Calculate the cost to reach any position up to and including that
         * reached by the longest match.
         *
         * Note: We consider only the lowest-offset match that reaches each
         * position.
         *
         * Note: Some of the cost calculation stays the same for each offset,
         * regardless of how many lengths it gets used for.  Therefore, to
         * improve performance, we hand-code the cost calculation instead of
         * calling lzx_match_cost() to do a from-scratch cost evaluation at each
         * 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 = ctx->queue;
                position_cost = 0;

                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 += ctx->costs.aligned[(offset + LZX_OFFSET_OFFSET) & 7];
                } else {
                        position_cost += num_extra_bits;
                }

                do {
                        unsigned len_header;
                        unsigned main_symbol;
                        u32 cost;

                        cost = position_cost;

                        len_header = min(len - LZX_MIN_MATCH_LEN, LZX_NUM_PRIMARY_LENS);
                        main_symbol = ((position_slot << 3) | len_header) + LZX_NUM_CHARS;
                        cost += ctx->costs.main[main_symbol];
                        if (len_header == LZX_NUM_PRIMARY_LENS)
                                cost += ctx->costs.len[len - LZX_MIN_MATCH_LEN - LZX_NUM_PRIMARY_LENS];

                        ctx->optimum[len].queue = queue;
                        ctx->optimum[len].prev.link = 0;
                        ctx->optimum[len].prev.match_offset = offset;
                        ctx->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)
                        ctx->optimum[++end_pos].cost = MC_INFINITE_COST;

                queue = ctx->queue;
                cost = lzx_match_cost(longest_rep_len, longest_rep_offset,
                                      &ctx->costs, &queue);
                if (cost <= ctx->optimum[longest_rep_len].cost) {
                        ctx->optimum[longest_rep_len].queue = queue;
                        ctx->optimum[longest_rep_len].prev.link = 0;
                        ctx->optimum[longest_rep_len].prev.match_offset = longest_rep_offset;
                        ctx->optimum[longest_rep_len].cost = cost;
                }
        }

        /* 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 @ctx->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.)
         *
         * The loop terminates when any one of the following conditions occurs:
         *
         * 1. A match with length greater than or equal to @nice_match_length 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 @ctx->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.
         */
        cur_pos = 0;
        for (;;) {
                u32 cost;

                /* Advance to next position.  */
                cur_pos++;

                /* Check termination conditions (2) and (3) noted above.  */
                if (cur_pos == end_pos || cur_pos == LZX_OPTIM_ARRAY_SIZE)
                        return lzx_match_chooser_reverse_list(ctx, cur_pos);

                /* Search for matches at recent offsets.  */
                longest_rep_len = LZX_MIN_MATCH_LEN - 1;
                unsigned limit = min(LZX_MAX_MATCH_LEN,
                                     ctx->match_window_end - ctx->match_window_pos);
                for (int i = 0; i < LZX_NUM_RECENT_OFFSETS; i++) {
                        u32 offset = ctx->optimum[cur_pos].queue.R[i];
                        const u8 *strptr = &ctx->window[ctx->match_window_pos];
                        const u8 *matchptr = strptr - offset;
                        unsigned len = 0;
                        while (len < limit && strptr[len] == matchptr[len])
                                len++;
                        if (len > longest_rep_len) {
                                longest_rep_len = len;
                                longest_rep_offset = offset;
                        }
                }

                /* If we found a long match at a recent offset, choose it
                 * immediately.  */
                if (longest_rep_len >= ctx->params.alg_params.slow.nice_match_length) {
                        /* Build the list of matches to return and get
                         * the first one.  */
                        match = lzx_match_chooser_reverse_list(ctx, cur_pos);

                        /* Append the long match to the end of the list.  */
                        ctx->optimum[cur_pos].next.match_offset = longest_rep_offset;
                        ctx->optimum[cur_pos].next.link = cur_pos + longest_rep_len;
                        ctx->optimum_end_idx = cur_pos + longest_rep_len;

                        /* Skip over the remaining bytes of the long match.  */
                        lzx_skip_bytes(ctx, longest_rep_len);

                        /* Return first match in the list.  */
                        return match;
                }

                /* Search other matches.  */
                num_matches = lzx_get_matches(ctx, &matches);

                /* If there's a long match, take it.  */
                if (num_matches) {
                        longest_len = matches[num_matches - 1].len;
                        if (longest_len >= ctx->params.alg_params.slow.nice_match_length) {
                                /* Build the list of matches to return and get
                                 * the first one.  */
                                match = lzx_match_chooser_reverse_list(ctx, cur_pos);

                                /* Append the long match to the end of the list.  */
                                ctx->optimum[cur_pos].next.match_offset =
                                        matches[num_matches - 1].offset;
                                ctx->optimum[cur_pos].next.link = cur_pos + longest_len;
                                ctx->optimum_end_idx = cur_pos + longest_len;

                                /* Skip over the remaining bytes of the long match.  */
                                lzx_skip_bytes(ctx, longest_len - 1);

                                /* Return first match in the list.  */
                                return match;
                        }
                } else {
                        longest_len = 1;
                }

                while (end_pos < cur_pos + longest_len)
                        ctx->optimum[++end_pos].cost = MC_INFINITE_COST;

                /* Consider coding a literal.  */
                cost = ctx->optimum[cur_pos].cost +
                        lzx_literal_cost(ctx->window[ctx->match_window_pos - 1],
                                         &ctx->costs);
                if (cost < ctx->optimum[cur_pos + 1].cost) {
                        ctx->optimum[cur_pos + 1].queue = ctx->optimum[cur_pos].queue;
                        ctx->optimum[cur_pos + 1].cost = cost;
                        ctx->optimum[cur_pos + 1].prev.link = cur_pos;
                }

                /* Consider coding a match.
                 *
                 * The hard-coded cost calculation is done for the same reason
                 * stated in the comment for the similar loop earlier.
                 * Actually, it is *this* one that has the biggest effect on
                 * performance; overall LZX compression is > 10% faster with
                 * this code compared to calling lzx_match_cost() with each
                 * 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 = ctx->optimum[cur_pos].queue;
                        position_cost = ctx->optimum[cur_pos].cost;

                        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 += ctx->costs.aligned[
                                                (offset + LZX_OFFSET_OFFSET) & 7];
                        } else {
                                position_cost += num_extra_bits;
                        }

                        do {
                                unsigned len_header;
                                unsigned main_symbol;
                                u32 cost;

                                cost = position_cost;

                                len_header = min(len - LZX_MIN_MATCH_LEN,
                                                 LZX_NUM_PRIMARY_LENS);
                                main_symbol = ((position_slot << 3) | len_header) +
                                                LZX_NUM_CHARS;
                                cost += ctx->costs.main[main_symbol];
                                if (len_header == LZX_NUM_PRIMARY_LENS) {
                                        cost += ctx->costs.len[len -
                                                        LZX_MIN_MATCH_LEN -
                                                        LZX_NUM_PRIMARY_LENS];
                                }
                                if (cost < ctx->optimum[cur_pos + len].cost) {
                                        ctx->optimum[cur_pos + len].queue = queue;
                                        ctx->optimum[cur_pos + len].prev.link = cur_pos;
                                        ctx->optimum[cur_pos + len].prev.match_offset = offset;
                                        ctx->optimum[cur_pos + len].cost = cost;
                                }
                        } while (++len <= matches[i].len);
                }

                if (longest_rep_len >= LZX_MIN_MATCH_LEN) {
                        struct lzx_lru_queue queue;

                        while (end_pos < cur_pos + longest_rep_len)
                                ctx->optimum[++end_pos].cost = MC_INFINITE_COST;

                        queue = ctx->optimum[cur_pos].queue;

                        cost = ctx->optimum[cur_pos].cost +
                                lzx_match_cost(longest_rep_len, longest_rep_offset,
                                               &ctx->costs, &queue);
                        if (cost <= ctx->optimum[cur_pos + longest_rep_len].cost) {
                                ctx->optimum[cur_pos + longest_rep_len].queue =
                                        queue;
                                ctx->optimum[cur_pos + longest_rep_len].prev.link =
                                        cur_pos;
                                ctx->optimum[cur_pos + longest_rep_len].prev.match_offset =
                                        longest_rep_offset;
                                ctx->optimum[cur_pos + longest_rep_len].cost =
                                        cost;
                        }
                }
        }
}

/* Set default symbol costs for the LZX Huffman codes.  */
static void
lzx_set_default_costs(struct lzx_costs * costs, unsigned num_main_syms)
{
        unsigned i;

        /* Main code (part 1): Literal symbols  */
        for (i = 0; i < LZX_NUM_CHARS; i++)
                costs->main[i] = 8;

        /* Main code (part 2): Match header symbols  */
        for (; i < num_main_syms; i++)
                costs->main[i] = 10;

        /* Length code  */
        for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++)
                costs->len[i] = 8;

        /* Aligned offset code  */
        for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++)
                costs->aligned[i] = 3;
}

/* Given the frequencies of symbols in an LZX-compressed block and the
 * corresponding Huffman codes, return LZX_BLOCKTYPE_ALIGNED or
 * LZX_BLOCKTYPE_VERBATIM if an aligned offset or verbatim block, respectively,
 * will take fewer bits to output.  */
static int
lzx_choose_verbatim_or_aligned(const struct lzx_freqs * freqs,
                               const struct lzx_codes * codes)
{
        unsigned aligned_cost = 0;
        unsigned verbatim_cost = 0;

        /* Verbatim blocks have a constant 3 bits per position footer.  Aligned
         * offset blocks have an aligned offset symbol per position footer, plus
         * an extra 24 bits per block to output the lengths necessary to
         * reconstruct the aligned offset code itself.  */
        for (unsigned i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
                verbatim_cost += 3 * freqs->aligned[i];
                aligned_cost += codes->lens.aligned[i] * freqs->aligned[i];
        }
        aligned_cost += LZX_ALIGNEDCODE_ELEMENT_SIZE * LZX_ALIGNEDCODE_NUM_SYMBOLS;
        if (aligned_cost < verbatim_cost)
                return LZX_BLOCKTYPE_ALIGNED;
        else
                return LZX_BLOCKTYPE_VERBATIM;
}

/* Find a near-optimal sequence of matches/literals with which to output the
 * specified LZX block, then set the block's type to that which has the minimum
 * cost to output (either verbatim or aligned).  */
static void
lzx_optimize_block(struct lzx_compressor *ctx, struct lzx_block_spec *spec,
                   unsigned num_passes)
{
        const struct lzx_lru_queue orig_queue = ctx->queue;
        unsigned num_passes_remaining = num_passes;
        struct lzx_freqs freqs;
        const u8 *window_ptr;
        const u8 *window_end;
        struct lzx_match *next_chosen_match;
        struct raw_match raw_match;
        struct lzx_match lzx_match;

        LZX_ASSERT(num_passes >= 1);
        LZX_ASSERT(lz_bt_get_position(&ctx->mf) == spec->window_pos);

        ctx->match_window_end = spec->window_pos + spec->block_size;
        ctx->matches_cached = false;

        /* The first optimal parsing pass is done using the cost model already
         * set in ctx->costs.  Each later pass is done using a cost model
         * computed from the previous pass.
         *
         * To improve performance we only generate the array containing the
         * matches and literals in intermediate form on the final pass.  */

        while (--num_passes_remaining) {
                ctx->match_window_pos = spec->window_pos;
                ctx->cache_ptr = ctx->cached_matches;
                memset(&freqs, 0, sizeof(freqs));
                window_ptr = &ctx->window[spec->window_pos];
                window_end = window_ptr + spec->block_size;

                while (window_ptr != window_end) {

                        raw_match = lzx_get_near_optimal_match(ctx);

                        LZX_ASSERT(!(raw_match.len == LZX_MIN_MATCH_LEN &&
                                     raw_match.offset == ctx->max_window_size -
                                                         LZX_MIN_MATCH_LEN));
                        if (raw_match.len >= LZX_MIN_MATCH_LEN) {
                                lzx_tally_match(raw_match.len, raw_match.offset,
                                                &freqs, &ctx->queue);
                                window_ptr += raw_match.len;
                        } else {
                                lzx_tally_literal(*window_ptr, &freqs);
                                window_ptr += 1;
                        }
                }
                lzx_make_huffman_codes(&freqs, &spec->codes, ctx->num_main_syms);
                lzx_set_costs(ctx, &spec->codes.lens);
                ctx->queue = orig_queue;
                ctx->matches_cached = true;
        }

        ctx->match_window_pos = spec->window_pos;
        ctx->cache_ptr = ctx->cached_matches;
        memset(&freqs, 0, sizeof(freqs));
        window_ptr = &ctx->window[spec->window_pos];
        window_end = window_ptr + spec->block_size;

        spec->chosen_matches = &ctx->chosen_matches[spec->window_pos];
        next_chosen_match = spec->chosen_matches;

        while (window_ptr != window_end) {
                raw_match = lzx_get_near_optimal_match(ctx);

                LZX_ASSERT(!(raw_match.len == LZX_MIN_MATCH_LEN &&
                             raw_match.offset == ctx->max_window_size -
                                                 LZX_MIN_MATCH_LEN));
                if (raw_match.len >= LZX_MIN_MATCH_LEN) {
                        lzx_match.data = lzx_tally_match(raw_match.len,
                                                         raw_match.offset,
                                                         &freqs, &ctx->queue);
                        window_ptr += raw_match.len;
                } else {
                        lzx_match.data = lzx_tally_literal(*window_ptr, &freqs);
                        window_ptr += 1;
                }
                *next_chosen_match++ = lzx_match;
        }
        spec->num_chosen_matches = next_chosen_match - spec->chosen_matches;
        lzx_make_huffman_codes(&freqs, &spec->codes, ctx->num_main_syms);
        spec->block_type = lzx_choose_verbatim_or_aligned(&freqs, &spec->codes);
}

/* Prepare the input window into one or more LZX blocks ready to be output.  */
static void
lzx_prepare_blocks(struct lzx_compressor * ctx)
{
        /* Set up a default cost model.  */
        lzx_set_default_costs(&ctx->costs, ctx->num_main_syms);

        /* Set up the block specifications.
         * TODO: The compression ratio could be slightly improved by performing
         * data-dependent block splitting instead of using fixed-size blocks.
         * Doing so well is a computationally hard problem, however.  */
        ctx->num_blocks = DIV_ROUND_UP(ctx->window_size, LZX_DIV_BLOCK_SIZE);
        for (unsigned i = 0; i < ctx->num_blocks; i++) {
                unsigned pos = LZX_DIV_BLOCK_SIZE * i;
                ctx->block_specs[i].window_pos = pos;
                ctx->block_specs[i].block_size = min(ctx->window_size - pos,
                                                     LZX_DIV_BLOCK_SIZE);
        }

        /* Load the window into the match-finder.  */
        lz_bt_load_window(&ctx->mf, ctx->window, ctx->window_size);

        /* Determine sequence of matches/literals to output for each block.  */
        lzx_lru_queue_init(&ctx->queue);
        ctx->optimum_cur_idx = 0;
        ctx->optimum_end_idx = 0;
        for (unsigned i = 0; i < ctx->num_blocks; i++) {
                lzx_optimize_block(ctx, &ctx->block_specs[i],
                                   ctx->params.alg_params.slow.num_optim_passes);
        }
}

/*
 * This is the fast version of lzx_prepare_blocks().  This version "quickly"
 * prepares a single compressed block containing the entire input.  See the
 * description of the "Fast algorithm" at the beginning of this file for more
 * information.
 *
 * Input ---  the preprocessed data:
 *
 *      ctx->window[]
 *      ctx->window_size
 *
 * Output --- the block specification and the corresponding match/literal data:
 *
 *      ctx->block_specs[]
 *      ctx->num_blocks
 *      ctx->chosen_matches[]
 */
static void
lzx_prepare_block_fast(struct lzx_compressor * ctx)
{
        struct lzx_record_ctx record_ctx;
        struct lzx_block_spec *spec;

        /* Parameters to hash chain LZ match finder
         * (lazy with 1 match lookahead)  */
        static const struct lz_params lzx_lz_params = {
                /* Although LZX_MIN_MATCH_LEN == 2, length 2 matches typically
                 * aren't worth choosing when using greedy or lazy parsing.  */
                .min_match      = 3,
                .max_match      = LZX_MAX_MATCH_LEN,
                .max_offset     = LZX_MAX_WINDOW_SIZE,
                .good_match     = LZX_MAX_MATCH_LEN,
                .nice_match     = LZX_MAX_MATCH_LEN,
                .max_chain_len  = LZX_MAX_MATCH_LEN,
                .max_lazy_match = LZX_MAX_MATCH_LEN,
                .too_far        = 4096,
        };

        /* Initialize symbol frequencies and match offset LRU queue.  */
        memset(&record_ctx.freqs, 0, sizeof(struct lzx_freqs));
        lzx_lru_queue_init(&record_ctx.queue);
        record_ctx.matches = ctx->chosen_matches;

        /* Determine series of matches/literals to output.  */
        lz_analyze_block(ctx->window,
                         ctx->window_size,
                         lzx_record_match,
                         lzx_record_literal,
                         &record_ctx,
                         &lzx_lz_params,
                         ctx->prev_tab);

        /* Set up block specification.  */
        spec = &ctx->block_specs[0];
        spec->block_type = LZX_BLOCKTYPE_ALIGNED;
        spec->window_pos = 0;
        spec->block_size = ctx->window_size;
        spec->num_chosen_matches = (record_ctx.matches - ctx->chosen_matches);
        spec->chosen_matches = ctx->chosen_matches;
        lzx_make_huffman_codes(&record_ctx.freqs, &spec->codes,
                               ctx->num_main_syms);
        ctx->num_blocks = 1;
}

static size_t
lzx_compress(const void *uncompressed_data, size_t uncompressed_size,
             void *compressed_data, size_t compressed_size_avail, void *_ctx)
{
        struct lzx_compressor *ctx = _ctx;
        struct output_bitstream ostream;
        size_t compressed_size;

        if (uncompressed_size < 100) {
                LZX_DEBUG("Too small to bother compressing.");
                return 0;
        }

        if (uncompressed_size > ctx->max_window_size) {
                LZX_DEBUG("Can't compress %zu bytes using window of %u bytes!",
                          uncompressed_size, ctx->max_window_size);
                return 0;
        }

        LZX_DEBUG("Attempting to compress %zu bytes...",
                  uncompressed_size);

        /* The input data must be preprocessed.  To avoid changing the original
         * input, copy it to a temporary buffer.  */
        memcpy(ctx->window, uncompressed_data, uncompressed_size);
        ctx->window_size = uncompressed_size;

        /* This line is unnecessary; it just avoids inconsequential accesses of
         * uninitialized memory that would show up in memory-checking tools such
         * as valgrind.  */
        memset(&ctx->window[ctx->window_size], 0, 12);

        LZX_DEBUG("Preprocessing data...");

        /* Before doing any actual compression, do the call instruction (0xe8
         * byte) translation on the uncompressed data.  */
        lzx_do_e8_preprocessing(ctx->window, ctx->window_size);

        LZX_DEBUG("Preparing blocks...");

        /* Prepare the compressed data.  */
        if (ctx->params.algorithm == WIMLIB_LZX_ALGORITHM_FAST)
                lzx_prepare_block_fast(ctx);
        else
                lzx_prepare_blocks(ctx);

        LZX_DEBUG("Writing compressed blocks...");

        /* Generate the compressed data.  */
        init_output_bitstream(&ostream, compressed_data, compressed_size_avail);
        lzx_write_all_blocks(ctx, &ostream);

        LZX_DEBUG("Flushing bitstream...");
        compressed_size = flush_output_bitstream(&ostream);
        if (compressed_size == ~(input_idx_t)0) {
                LZX_DEBUG("Data did not compress to %zu bytes or less!",
                          compressed_size_avail);
                return 0;
        }

        LZX_DEBUG("Done: compressed %zu => %zu bytes.",
                  uncompressed_size, compressed_size);

        /* Verify that we really get the same thing back when decompressing.
         * Although this could be disabled by default in all cases, it only
         * takes around 2-3% of the running time of the slow algorithm to do the
         * verification.  */
        if (ctx->params.algorithm == WIMLIB_LZX_ALGORITHM_SLOW
        #if defined(ENABLE_LZX_DEBUG) || defined(ENABLE_VERIFY_COMPRESSION)
            || 1
        #endif
            )
        {
                struct wimlib_decompressor *decompressor;

                if (0 == wimlib_create_decompressor(WIMLIB_COMPRESSION_TYPE_LZX,
                                                    ctx->max_window_size,
                                                    NULL,
                                                    &decompressor))
                {
                        int ret;
                        ret = wimlib_decompress(compressed_data,
                                                compressed_size,
                                                ctx->window,
                                                uncompressed_size,
                                                decompressor);
                        wimlib_free_decompressor(decompressor);

                        if (ret) {
                                ERROR("Failed to decompress data we "
                                      "compressed using LZX algorithm");
                                wimlib_assert(0);
                                return 0;
                        }
                        if (memcmp(uncompressed_data, ctx->window, uncompressed_size)) {
                                ERROR("Data we compressed using LZX algorithm "
                                      "didn't decompress to original");
                                wimlib_assert(0);
                                return 0;
                        }
                } else {
                        WARNING("Failed to create decompressor for "
                                "data verification!");
                }
        }
        return compressed_size;
}

static void
lzx_free_compressor(void *_ctx)
{
        struct lzx_compressor *ctx = _ctx;

        if (ctx) {
                FREE(ctx->chosen_matches);
                FREE(ctx->cached_matches);
                FREE(ctx->optimum);
                lz_bt_destroy(&ctx->mf);
                FREE(ctx->block_specs);
                FREE(ctx->prev_tab);
                FREE(ctx->window);
                FREE(ctx);
        }
}

static const struct wimlib_lzx_compressor_params lzx_fast_default = {
        .hdr = {
                .size = sizeof(struct wimlib_lzx_compressor_params),
        },
        .algorithm = WIMLIB_LZX_ALGORITHM_FAST,
        .use_defaults = 0,
        .alg_params = {
                .fast = {
                },
        },
};
static const struct wimlib_lzx_compressor_params lzx_slow_default = {
        .hdr = {
                .size = sizeof(struct wimlib_lzx_compressor_params),
        },
        .algorithm = WIMLIB_LZX_ALGORITHM_SLOW,
        .use_defaults = 0,
        .alg_params = {
                .slow = {
                        .use_len2_matches = 1,
                        .nice_match_length = 32,
                        .num_optim_passes = 2,
                        .max_search_depth = 50,
                        .main_nostat_cost = 15,
                        .len_nostat_cost = 15,
                        .aligned_nostat_cost = 7,
                },
        },
};

static const struct wimlib_lzx_compressor_params *
lzx_get_params(const struct wimlib_compressor_params_header *_params)
{
        const struct wimlib_lzx_compressor_params *params =
                (const struct wimlib_lzx_compressor_params*)_params;

        if (params == NULL) {
                LZX_DEBUG("Using default algorithm and parameters.");
                params = &lzx_slow_default;
        } else {
                if (params->use_defaults) {
                        if (params->algorithm == WIMLIB_LZX_ALGORITHM_SLOW)
                                params = &lzx_slow_default;
                        else
                                params = &lzx_fast_default;
                }
        }
        return params;
}

static int
lzx_create_compressor(size_t window_size,
                      const struct wimlib_compressor_params_header *_params,
                      void **ctx_ret)
{
        const struct wimlib_lzx_compressor_params *params = lzx_get_params(_params);
        struct lzx_compressor *ctx;

        LZX_DEBUG("Allocating LZX context...");

        if (!lzx_window_size_valid(window_size))
                return WIMLIB_ERR_INVALID_PARAM;

        LZX_DEBUG("Allocating memory.");

        ctx = CALLOC(1, sizeof(struct lzx_compressor));
        if (ctx == NULL)
                goto oom;

        ctx->num_main_syms = lzx_get_num_main_syms(window_size);
        ctx->max_window_size = window_size;
        ctx->window = MALLOC(window_size + 12);
        if (ctx->window == NULL)
                goto oom;

        if (params->algorithm == WIMLIB_LZX_ALGORITHM_FAST) {
                ctx->prev_tab = MALLOC(window_size * sizeof(ctx->prev_tab[0]));
                if (ctx->prev_tab == NULL)
                        goto oom;
        }

        size_t block_specs_length = DIV_ROUND_UP(window_size, LZX_DIV_BLOCK_SIZE);
        ctx->block_specs = MALLOC(block_specs_length * sizeof(ctx->block_specs[0]));
        if (ctx->block_specs == NULL)
                goto oom;

        if (params->algorithm == WIMLIB_LZX_ALGORITHM_SLOW) {
                unsigned min_match_len = LZX_MIN_MATCH_LEN;
                if (!params->alg_params.slow.use_len2_matches)
                        min_match_len = max(min_match_len, 3);

                if (!lz_bt_init(&ctx->mf,
                                window_size,
                                min_match_len,
                                LZX_MAX_MATCH_LEN,
                                params->alg_params.slow.nice_match_length,
                                params->alg_params.slow.max_search_depth))
                        goto oom;
        }

        if (params->algorithm == WIMLIB_LZX_ALGORITHM_SLOW) {
                ctx->optimum = MALLOC((LZX_OPTIM_ARRAY_SIZE +
                                       min(params->alg_params.slow.nice_match_length,
                                           LZX_MAX_MATCH_LEN)) *
                                                sizeof(ctx->optimum[0]));
                if (!ctx->optimum)
                        goto oom;
        }

        if (params->algorithm == WIMLIB_LZX_ALGORITHM_SLOW) {
                ctx->cached_matches = MALLOC(LZX_CACHE_SIZE);
                if (ctx->cached_matches == NULL)
                        goto oom;
                ctx->cache_limit = ctx->cached_matches +
                                   LZX_CACHE_LEN - (LZX_MAX_MATCHES_PER_POS + 1);
        }

        ctx->chosen_matches = MALLOC(window_size * sizeof(ctx->chosen_matches[0]));
        if (ctx->chosen_matches == NULL)
                goto oom;

        memcpy(&ctx->params, params, sizeof(struct wimlib_lzx_compressor_params));
        memset(&ctx->zero_codes, 0, sizeof(ctx->zero_codes));

        LZX_DEBUG("Successfully allocated new LZX context.");

        *ctx_ret = ctx;
        return 0;

oom:
        lzx_free_compressor(ctx);
        return WIMLIB_ERR_NOMEM;
}

static u64
lzx_get_needed_memory(size_t max_block_size,
                      const struct wimlib_compressor_params_header *_params)
{
        const struct wimlib_lzx_compressor_params *params = lzx_get_params(_params);

        u64 size = 0;

        size += sizeof(struct lzx_compressor);

        size += max_block_size + 12;

        size += DIV_ROUND_UP(max_block_size, LZX_DIV_BLOCK_SIZE) *
                sizeof(((struct lzx_compressor*)0)->block_specs[0]);

        if (params->algorithm == WIMLIB_LZX_ALGORITHM_SLOW) {
                size += max_block_size * sizeof(((struct lzx_compressor*)0)->chosen_matches[0]);
                size += lz_bt_get_needed_memory(max_block_size);
                size += (LZX_OPTIM_ARRAY_SIZE +
                         min(params->alg_params.slow.nice_match_length,
                             LZX_MAX_MATCH_LEN)) *
                                sizeof(((struct lzx_compressor *)0)->optimum[0]);
                size += LZX_CACHE_SIZE;
        } else {
                size += max_block_size * sizeof(((struct lzx_compressor*)0)->prev_tab[0]);
        }
        return size;
}

static bool
lzx_params_valid(const struct wimlib_compressor_params_header *_params)
{
        const struct wimlib_lzx_compressor_params *params =
                (const struct wimlib_lzx_compressor_params*)_params;

        if (params->hdr.size != sizeof(struct wimlib_lzx_compressor_params)) {
                LZX_DEBUG("Invalid parameter structure size!");
                return false;
        }

        if (params->algorithm != WIMLIB_LZX_ALGORITHM_SLOW &&
            params->algorithm != WIMLIB_LZX_ALGORITHM_FAST)
        {
                LZX_DEBUG("Invalid algorithm.");
                return false;
        }

        if (params->algorithm == WIMLIB_LZX_ALGORITHM_SLOW &&
            !params->use_defaults)
        {
                if (params->alg_params.slow.num_optim_passes < 1)
                {
                        LZX_DEBUG("Invalid number of optimization passes!");
                        return false;
                }

                if (params->alg_params.slow.main_nostat_cost < 1 ||
                    params->alg_params.slow.main_nostat_cost > 16)
                {
                        LZX_DEBUG("Invalid main_nostat_cost!");
                        return false;
                }

                if (params->alg_params.slow.len_nostat_cost < 1 ||
                    params->alg_params.slow.len_nostat_cost > 16)
                {
                        LZX_DEBUG("Invalid len_nostat_cost!");
                        return false;
                }

                if (params->alg_params.slow.aligned_nostat_cost < 1 ||
                    params->alg_params.slow.aligned_nostat_cost > 8)
                {
                        LZX_DEBUG("Invalid aligned_nostat_cost!");
                        return false;
                }
        }
        return true;
}

const struct compressor_ops lzx_compressor_ops = {
        .params_valid       = lzx_params_valid,
        .get_needed_memory  = lzx_get_needed_memory,
        .create_compressor  = lzx_create_compressor,
        .compress           = lzx_compress,
        .free_compressor    = lzx_free_compressor,
};