LZX compressor improvements

[wimlib] / src / lzx_compress.c

/*
 * lzx_compress.c
 *
 * A compressor for the LZX compression format, as used in WIM archives.
 */

/*
 * Copyright (C) 2012-2016 Eric Biggers
 *
 * This file is free software; you can redistribute it and/or modify it under
 * the terms of the GNU Lesser General Public License as published by the Free
 * Software Foundation; either version 3 of the License, or (at your option) any
 * later version.
 *
 * This file 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 Lesser General Public License for more
 * details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this file; 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.
 *
 * Two different LZX-compatible algorithms are implemented: "near-optimal" and
 * "lazy".  "Near-optimal" is significantly slower than "lazy", but results in a
 * better compression ratio.  The "near-optimal" algorithm is used at the
 * default compression level.
 *
 * This file 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 sliding window support might be required.
 *
 * LZX is a compression format derived from 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 an "offset 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.  Length 2 matches can be
 *   useful, but generally only if the compressor is smart about choosing them.
 *
 * - In LZX, offset slots 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.
 */

/******************************************************************************/
/*                            General parameters                              */
/*----------------------------------------------------------------------------*/

/*
 * The compressor uses the faster algorithm at levels <= MAX_FAST_LEVEL.  It
 * uses the slower algorithm at levels > MAX_FAST_LEVEL.
 */
#define MAX_FAST_LEVEL                          34

/*
 * The compressor-side limits on the codeword lengths (in bits) for each Huffman
 * code.  To make outputting bits slightly faster, some of these limits are
 * lower than the limits defined by the LZX format.  This does not significantly
 * affect the compression ratio.
 */
#define MAIN_CODEWORD_LIMIT                     16
#define LENGTH_CODEWORD_LIMIT                   12
#define ALIGNED_CODEWORD_LIMIT                  7
#define PRE_CODEWORD_LIMIT                      7


/******************************************************************************/
/*                         Block splitting parameters                         */
/*----------------------------------------------------------------------------*/

/*
 * The compressor always outputs blocks of at least this size in bytes, except
 * for the last block which may need to be smaller.
 */
#define MIN_BLOCK_SIZE                          6500

/*
 * The compressor attempts to end a block when it reaches this size in bytes.
 * The final size might be slightly larger due to matches extending beyond the
 * end of the block.  Specifically:
 *
 *  - The near-optimal compressor may choose a match of up to LZX_MAX_MATCH_LEN
 *    bytes starting at the SOFT_MAX_BLOCK_SIZE'th byte.
 *
 *  - The lazy compressor may choose a sequence of literals starting at the
 *    SOFT_MAX_BLOCK_SIZE'th byte when it sees a sequence of increasingly better
 *    matches.  The final match may be up to LZX_MAX_MATCH_LEN bytes.  The
 *    length of the literal sequence is approximately limited by the "nice match
 *    length" parameter.
 */
#define SOFT_MAX_BLOCK_SIZE                     100000

/*
 * The number of observed items (matches and literals) that represents
 * sufficient data for the compressor to decide whether the current block should
 * be ended or not.
 */
#define NUM_OBSERVATIONS_PER_BLOCK_CHECK        400


/******************************************************************************/
/*                      Parameters for slower algorithm                       */
/*----------------------------------------------------------------------------*/

/*
 * The log base 2 of the number of entries in the hash table for finding length
 * 2 matches.  This could be as high as 16, but using a smaller hash table
 * speeds up compression due to reduced cache pressure.
 */
#define BT_MATCHFINDER_HASH2_ORDER              12

/*
 * The number of lz_match structures in the match cache, excluding the extra
 * "overflow" entries.  This value should be high enough so that nearly the
 * time, all matches found in a given block can fit in the match cache.
 * However, fallback behavior (immediately terminating the block) on cache
 * overflow is still required.
 */
#define CACHE_LENGTH                            (SOFT_MAX_BLOCK_SIZE * 5)

/*
 * An upper bound on the number of matches that can ever be saved in the match
 * cache for a single position.  Since each match we save for a single position
 * has a distinct length, we can use the number of possible match lengths in LZX
 * as this bound.  This bound is guaranteed to be valid in all cases, although
 * if 'nice_match_length < LZX_MAX_MATCH_LEN', then it will never actually be
 * reached.
 */
#define MAX_MATCHES_PER_POS                     LZX_NUM_LENS

/*
 * A scaling factor that makes it possible to consider fractional bit costs.  A
 * single bit has a cost of (1 << COST_SHIFT).
 *
 * Note: this is only useful as a statistical trick for when the true costs are
 * unknown.  In reality, each token in LZX requires a whole number of bits to
 * output.
 */
#define COST_SHIFT                              6

/*
 * Should the compressor take into account the costs of aligned offset symbols?
 */
#define CONSIDER_ALIGNED_COSTS                  1

/*
 * Should the "minimum" cost path search algorithm consider "gap" matches, where
 * a normal match is followed by a literal, then by a match with the same
 * offset?  This is one specific, somewhat common situation in which the true
 * minimum cost path is often different from the path found by looking only one
 * edge ahead.
 */
#define CONSIDER_GAP_MATCHES                    1

/******************************************************************************/
/*                                  Includes                                  */
/*----------------------------------------------------------------------------*/

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

#include "wimlib/compress_common.h"
#include "wimlib/compressor_ops.h"
#include "wimlib/error.h"
#include "wimlib/lz_extend.h"
#include "wimlib/lzx_common.h"
#include "wimlib/unaligned.h"
#include "wimlib/util.h"

/* Note: BT_MATCHFINDER_HASH2_ORDER must be defined *before* including
 * bt_matchfinder.h. */

/* Matchfinders with 16-bit positions  */
#define mf_pos_t        u16
#define MF_SUFFIX       _16
#include "wimlib/bt_matchfinder.h"
#include "wimlib/hc_matchfinder.h"

/* Matchfinders with 32-bit positions  */
#undef mf_pos_t
#undef MF_SUFFIX
#define mf_pos_t        u32
#define MF_SUFFIX       _32
#include "wimlib/bt_matchfinder.h"
#include "wimlib/hc_matchfinder.h"

/******************************************************************************/
/*                            Compressor structure                            */
/*----------------------------------------------------------------------------*/

/* Codewords for the 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 Huffman codes.
 *
 * A codeword length of 0 means the corresponding codeword has zero frequency.
 *
 * The main and length codes each have one extra entry for use as a sentinel.
 * See lzx_write_compressed_code().
 */
struct lzx_lens {
        u8 main[LZX_MAINCODE_MAX_NUM_SYMBOLS + 1];
        u8 len[LZX_LENCODE_NUM_SYMBOLS + 1];
        u8 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};

/* Codewords and lengths for the Huffman codes */
struct lzx_codes {
        struct lzx_codewords codewords;
        struct lzx_lens lens;
};

/* Symbol frequency counters for the Huffman-encoded alphabets */
struct lzx_freqs {
        u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
        u32 len[LZX_LENCODE_NUM_SYMBOLS];
        u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};

/* Block split statistics.  See the "Block splitting algorithm" section later in
 * this file for details. */
#define NUM_LITERAL_OBSERVATION_TYPES 8
#define NUM_MATCH_OBSERVATION_TYPES 2
#define NUM_OBSERVATION_TYPES (NUM_LITERAL_OBSERVATION_TYPES + \
                               NUM_MATCH_OBSERVATION_TYPES)
struct lzx_block_split_stats {
        u32 new_observations[NUM_OBSERVATION_TYPES];
        u32 observations[NUM_OBSERVATION_TYPES];
        u32 num_new_observations;
        u32 num_observations;
};

/*
 * Represents a run of literals followed by a match or end-of-block.  This
 * structure is needed to temporarily store items chosen by the compressor,
 * since items cannot be written until all items for the block have been chosen
 * and the block's Huffman codes have been computed.
 */
struct lzx_sequence {

        /* The number of literals in the run.  This may be 0.  The literals are
         * not stored explicitly in this structure; instead, they are read
         * directly from the uncompressed data.  */
        u16 litrunlen;

        /* If the next field doesn't indicate end-of-block, then this is the
         * match length minus LZX_MIN_MATCH_LEN.  */
        u16 adjusted_length;

        /* If bit 31 is clear, then this field contains the match header in bits
         * 0-8, and either the match offset plus LZX_OFFSET_ADJUSTMENT or a
         * recent offset code in bits 9-30.  Otherwise (if bit 31 is set), this
         * sequence's literal run was the last literal run in the block, so
         * there is no match that follows it.  */
        u32 adjusted_offset_and_match_hdr;
};

/*
 * This structure represents a byte position in the input buffer and a node in
 * the graph of possible match/literal choices.
 *
 * Logically, each incoming edge to this node is labeled with a literal or a
 * match that can be taken to reach this position from an earlier position; and
 * each outgoing edge from this node is labeled with a literal or a match that
 * can be taken to advance from this position to a later position.
 */
struct lzx_optimum_node {

        /* The cost, in bits, of the lowest-cost path that has been found to
         * reach this position.  This can change as progressively lower cost
         * paths are found to reach this position.  */
        u32 cost;

        /*
         * The match or literal that was taken to reach this position.  This can
         * change as progressively lower cost paths are found to reach this
         * position.
         *
         * For non-gap matches, this variable is divided into two bitfields
         * whose meanings depend on the item type:
         *
         * Literals:
         *      Low bits are 0, high bits are the literal.
         *
         * Explicit offset matches:
         *      Low bits are the match length, high bits are the offset plus 2.
         *
         * Repeat offset matches:
         *      Low bits are the match length, high bits are the queue index.
         *
         * For gap matches, identified by OPTIMUM_GAP_MATCH set, special
         * behavior applies --- see the code.
         */
        u32 item;
#define OPTIMUM_OFFSET_SHIFT 9
#define OPTIMUM_LEN_MASK ((1 << OPTIMUM_OFFSET_SHIFT) - 1)
#if CONSIDER_GAP_MATCHES
#  define OPTIMUM_GAP_MATCH 0x80000000
#endif

} _aligned_attribute(8);

/* The cost model for near-optimal parsing */
struct lzx_costs {

        /* 'match_cost[offset_slot][len - LZX_MIN_MATCH_LEN]' is the cost for a
         * length 'len' match that has an offset belonging to 'offset_slot'. */
        u16 match_cost[LZX_MAX_OFFSET_SLOTS][LZX_NUM_LENS];

        /* Cost for each symbol in the main code */
        u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];

        /* Cost for each symbol in the length code */
        u32 len[LZX_LENCODE_NUM_SYMBOLS];

#if CONSIDER_ALIGNED_COSTS
        /* Cost for each symbol in the aligned code */
        u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
#endif
};

struct lzx_output_bitstream;

/* The main LZX compressor structure  */
struct lzx_compressor {

        /* The buffer for preprocessed input data, if not using destructive
         * compression */
        void *in_buffer;

        /* If true, the compressor need not preserve the input buffer if it
         * compresses the data successfully.  */
        bool destructive;

        /* Pointer to the compress() implementation chosen at allocation time */
        void (*impl)(struct lzx_compressor *, const u8 *, size_t,
                     struct lzx_output_bitstream *);

        /* The log base 2 of the window size for LZ match offset encoding
         * purposes.  This will be >= LZX_MIN_WINDOW_ORDER and <=
         * LZX_MAX_WINDOW_ORDER. */
        unsigned window_order;

        /* The number of symbols in the main alphabet.  This depends on the
         * window order, since the window order determines the maximum possible
         * match offset. */
        unsigned num_main_syms;

        /* The "nice" match length: if a match of this length is found, then it
         * is chosen immediately without further consideration. */
        unsigned nice_match_length;

        /* The maximum search depth: at most this many potential matches are
         * considered at each position. */
        unsigned max_search_depth;

        /* The number of optimization passes per block */
        unsigned num_optim_passes;

        /* The symbol frequency counters for the current block */
        struct lzx_freqs freqs;

        /* Block split statistics */
        struct lzx_block_split_stats split_stats;

        /* The Huffman codes for the current and previous blocks.  The one with
         * index 'codes_index' is for the current block, and the other one is
         * for the previous block. */
        struct lzx_codes codes[2];
        unsigned codes_index;

        /* The matches and literals that the compressor has chosen for the
         * current block.  The required length of this array is limited by the
         * maximum number of matches that can ever be chosen for a single block,
         * plus one for the special entry at the end. */
        struct lzx_sequence chosen_sequences[
                       DIV_ROUND_UP(SOFT_MAX_BLOCK_SIZE, LZX_MIN_MATCH_LEN) + 1];

        /* Tables for mapping adjusted offsets to offset slots */
        u8 offset_slot_tab_1[32768]; /* offset slots [0, 29] */
        u8 offset_slot_tab_2[128]; /* offset slots [30, 49] */

        union {
                /* Data for lzx_compress_lazy() */
                struct {
                        /* Hash chains matchfinder (MUST BE LAST!!!) */
                        union {
                                struct hc_matchfinder_16 hc_mf_16;
                                struct hc_matchfinder_32 hc_mf_32;
                        };
                };

                /* Data for lzx_compress_near_optimal() */
                struct {
                        /*
                         * Array of nodes, one per position, for running the
                         * minimum-cost path algorithm.
                         *
                         * This array must be large enough to accommodate the
                         * worst-case number of nodes, which occurs if the
                         * compressor finds a match of length LZX_MAX_MATCH_LEN
                         * at position 'SOFT_MAX_BLOCK_SIZE - 1', producing a
                         * block of size 'SOFT_MAX_BLOCK_SIZE - 1 +
                         * LZX_MAX_MATCH_LEN'.  Add one for the end-of-block
                         * node.
                         */
                        struct lzx_optimum_node optimum_nodes[
                                                    SOFT_MAX_BLOCK_SIZE - 1 +
                                                    LZX_MAX_MATCH_LEN + 1];

                        /* The cost model for the current optimization pass */
                        struct lzx_costs costs;

                        /*
                         * Cached matches for the current block.  This array
                         * contains the matches that were found at each position
                         * in the block.  Specifically, for each position, there
                         * is a special 'struct lz_match' whose 'length' field
                         * contains the number of matches that were found at
                         * that position; this is followed by the matches
                         * themselves, if any, sorted by strictly increasing
                         * length.
                         *
                         * Note: in rare cases, there will be a very high number
                         * of matches in the block and this array will overflow.
                         * If this happens, we force the end of the current
                         * block.  CACHE_LENGTH is the length at which we
                         * actually check for overflow.  The extra slots beyond
                         * this are enough to absorb the worst case overflow,
                         * which occurs if starting at &match_cache[CACHE_LENGTH
                         * - 1], we write the match count header, then write
                         * MAX_MATCHES_PER_POS matches, then skip searching for
                         * matches at 'LZX_MAX_MATCH_LEN - 1' positions and
                         * write the match count header for each.
                         */
                        struct lz_match match_cache[CACHE_LENGTH +
                                                    MAX_MATCHES_PER_POS +
                                                    LZX_MAX_MATCH_LEN - 1];

                        /* Binary trees matchfinder (MUST BE LAST!!!) */
                        union {
                                struct bt_matchfinder_16 bt_mf_16;
                                struct bt_matchfinder_32 bt_mf_32;
                        };
                };
        };
};

/******************************************************************************/
/*                            Matchfinder utilities                           */
/*----------------------------------------------------------------------------*/

/*
 * Will a matchfinder using 16-bit positions be sufficient for compressing
 * buffers of up to the specified size?  The limit could be 65536 bytes, but we
 * also want to optimize out the use of offset_slot_tab_2 in the 16-bit case.
 * This requires that the limit be no more than the length of offset_slot_tab_1
 * (currently 32768).
 */
static inline bool
lzx_is_16_bit(size_t max_bufsize)
{
        STATIC_ASSERT(ARRAY_LEN(((struct lzx_compressor *)0)->offset_slot_tab_1) == 32768);
        return max_bufsize <= 32768;
}

/*
 * Return the offset slot for the specified adjusted match offset.
 */
static inline unsigned
lzx_get_offset_slot(struct lzx_compressor *c, u32 adjusted_offset,
                    bool is_16_bit)
{
        if (is_16_bit || adjusted_offset < ARRAY_LEN(c->offset_slot_tab_1))
                return c->offset_slot_tab_1[adjusted_offset];
        return c->offset_slot_tab_2[adjusted_offset >> 14];
}

/*
 * The following macros call either the 16-bit or the 32-bit version of a
 * matchfinder function based on the value of 'is_16_bit', which will be known
 * at compilation time.
 */

#define CALL_HC_MF(is_16_bit, c, funcname, ...)                               \
        ((is_16_bit) ? CONCAT(funcname, _16)(&(c)->hc_mf_16, ##__VA_ARGS__) : \
                       CONCAT(funcname, _32)(&(c)->hc_mf_32, ##__VA_ARGS__));

#define CALL_BT_MF(is_16_bit, c, funcname, ...)                               \
        ((is_16_bit) ? CONCAT(funcname, _16)(&(c)->bt_mf_16, ##__VA_ARGS__) : \
                       CONCAT(funcname, _32)(&(c)->bt_mf_32, ##__VA_ARGS__));

/******************************************************************************/
/*                             Output bitstream                               */
/*----------------------------------------------------------------------------*/

/*
 * The LZX bitstream is encoded as a sequence of little endian 16-bit coding
 * units.  Bits are ordered from most significant to least significant within
 * each coding unit.
 */

/*
 * Structure to keep track of the current state of sending bits to the
 * compressed output buffer.
 */
struct lzx_output_bitstream {

        /* Bits that haven't yet been written to the output buffer */
        machine_word_t bitbuf;

        /* Number of bits currently held in @bitbuf */
        machine_word_t bitcount;

        /* Pointer to the start of the output buffer */
        u8 *start;

        /* Pointer to the position in the output buffer at which the next coding
         * unit should be written */
        u8 *next;

        /* Pointer just past the end of the output buffer, rounded down to a
         * 2-byte boundary */
        u8 *end;
};

/* Can the specified number of bits always be added to 'bitbuf' after any
 * pending 16-bit coding units have been flushed?  */
#define CAN_BUFFER(n)   ((n) <= WORDBITS - 15)

/* Initialize the output bitstream to write to the specified buffer. */
static void
lzx_init_output(struct lzx_output_bitstream *os, void *buffer, size_t size)
{
        os->bitbuf = 0;
        os->bitcount = 0;
        os->start = buffer;
        os->next = os->start;
        os->end = os->start + (size & ~1);
}

/*
 * Add some bits to the bitbuffer variable of the output bitstream.  The caller
 * must make sure there is enough room.
 */
static inline void
lzx_add_bits(struct lzx_output_bitstream *os, u32 bits, unsigned num_bits)
{
        os->bitbuf = (os->bitbuf << num_bits) | bits;
        os->bitcount += num_bits;
}

/*
 * Flush bits from the bitbuffer variable to the output buffer.  'max_num_bits'
 * specifies the maximum number of bits that may have been added since the last
 * flush.
 */
static inline void
lzx_flush_bits(struct lzx_output_bitstream *os, unsigned max_num_bits)
{
        /* Masking the number of bits to shift is only needed to avoid undefined
         * behavior; we don't actually care about the results of bad shifts.  On
         * x86, the explicit masking generates no extra code.  */
        const u32 shift_mask = WORDBITS - 1;

        if (os->end - os->next < 6)
                return;
        put_unaligned_le16(os->bitbuf >> ((os->bitcount - 16) &
                                            shift_mask), os->next + 0);
        if (max_num_bits > 16)
                put_unaligned_le16(os->bitbuf >> ((os->bitcount - 32) &
                                                shift_mask), os->next + 2);
        if (max_num_bits > 32)
                put_unaligned_le16(os->bitbuf >> ((os->bitcount - 48) &
                                                shift_mask), os->next + 4);
        os->next += (os->bitcount >> 4) << 1;
        os->bitcount &= 15;
}

/* Add at most 16 bits to the bitbuffer and flush it.  */
static inline void
lzx_write_bits(struct lzx_output_bitstream *os, u32 bits, unsigned num_bits)
{
        lzx_add_bits(os, bits, num_bits);
        lzx_flush_bits(os, 16);
}

/*
 * Flush the last coding unit to the output buffer if needed.  Return the total
 * number of bytes written to the output buffer, or 0 if an overflow occurred.
 */
static size_t
lzx_flush_output(struct lzx_output_bitstream *os)
{
        if (os->end - os->next < 6)
                return 0;

        if (os->bitcount != 0) {
                put_unaligned_le16(os->bitbuf << (16 - os->bitcount), os->next);
                os->next += 2;
        }

        return os->next - os->start;
}

/******************************************************************************/
/*                           Preparing Huffman codes                          */
/*----------------------------------------------------------------------------*/

/*
 * Build the Huffman codes.  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_build_huffman_codes(struct lzx_compressor *c)
{
        const struct lzx_freqs *freqs = &c->freqs;
        struct lzx_codes *codes = &c->codes[c->codes_index];

        STATIC_ASSERT(MAIN_CODEWORD_LIMIT >= 9 &&
                      MAIN_CODEWORD_LIMIT <= LZX_MAX_MAIN_CODEWORD_LEN);
        make_canonical_huffman_code(c->num_main_syms,
                                    MAIN_CODEWORD_LIMIT,
                                    freqs->main,
                                    codes->lens.main,
                                    codes->codewords.main);

        STATIC_ASSERT(LENGTH_CODEWORD_LIMIT >= 8 &&
                      LENGTH_CODEWORD_LIMIT <= LZX_MAX_LEN_CODEWORD_LEN);
        make_canonical_huffman_code(LZX_LENCODE_NUM_SYMBOLS,
                                    LENGTH_CODEWORD_LIMIT,
                                    freqs->len,
                                    codes->lens.len,
                                    codes->codewords.len);

        STATIC_ASSERT(ALIGNED_CODEWORD_LIMIT >= LZX_NUM_ALIGNED_OFFSET_BITS &&
                      ALIGNED_CODEWORD_LIMIT <= LZX_MAX_ALIGNED_CODEWORD_LEN);
        make_canonical_huffman_code(LZX_ALIGNEDCODE_NUM_SYMBOLS,
                                    ALIGNED_CODEWORD_LIMIT,
                                    freqs->aligned,
                                    codes->lens.aligned,
                                    codes->codewords.aligned);
}

/* Reset the symbol frequencies for the current block. */
static void
lzx_reset_symbol_frequencies(struct lzx_compressor *c)
{
        memset(&c->freqs, 0, sizeof(c->freqs));
}

static unsigned
lzx_compute_precode_items(const u8 lens[restrict],
                          const u8 prev_lens[restrict],
                          u32 precode_freqs[restrict],
                          unsigned precode_items[restrict])
{
        unsigned *itemptr;
        unsigned run_start;
        unsigned run_end;
        unsigned extra_bits;
        int delta;
        u8 len;

        itemptr = precode_items;
        run_start = 0;

        while (!((len = lens[run_start]) & 0x80)) {

                /* len = the length being repeated  */

                /* Find the next run of codeword lengths.  */

                run_end = run_start + 1;

                /* Fast case for a single length.  */
                if (likely(len != lens[run_end])) {
                        delta = prev_lens[run_start] - len;
                        if (delta < 0)
                                delta += 17;
                        precode_freqs[delta]++;
                        *itemptr++ = delta;
                        run_start++;
                        continue;
                }

                /* Extend the run.  */
                do {
                        run_end++;
                } while (len == lens[run_end]);

                if (len == 0) {
                        /* Run of zeroes.  */

                        /* Symbol 18: RLE 20 to 51 zeroes at a time.  */
                        while ((run_end - run_start) >= 20) {
                                extra_bits = min((run_end - run_start) - 20, 0x1f);
                                precode_freqs[18]++;
                                *itemptr++ = 18 | (extra_bits << 5);
                                run_start += 20 + extra_bits;
                        }

                        /* Symbol 17: RLE 4 to 19 zeroes at a time.  */
                        if ((run_end - run_start) >= 4) {
                                extra_bits = min((run_end - run_start) - 4, 0xf);
                                precode_freqs[17]++;
                                *itemptr++ = 17 | (extra_bits << 5);
                                run_start += 4 + extra_bits;
                        }
                } else {

                        /* A run of nonzero lengths. */

                        /* Symbol 19: RLE 4 to 5 of any length at a time.  */
                        while ((run_end - run_start) >= 4) {
                                extra_bits = (run_end - run_start) > 4;
                                delta = prev_lens[run_start] - len;
                                if (delta < 0)
                                        delta += 17;
                                precode_freqs[19]++;
                                precode_freqs[delta]++;
                                *itemptr++ = 19 | (extra_bits << 5) | (delta << 6);
                                run_start += 4 + extra_bits;
                        }
                }

                /* Output any remaining lengths without RLE.  */
                while (run_start != run_end) {
                        delta = prev_lens[run_start] - len;
                        if (delta < 0)
                                delta += 17;
                        precode_freqs[delta]++;
                        *itemptr++ = delta;
                        run_start++;
                }
        }

        return itemptr - precode_items;
}

/******************************************************************************/
/*                          Outputting compressed data                        */
/*----------------------------------------------------------------------------*/

/*
 * 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.
 *
 * @os:
 *      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_lens:
 *      The number of symbols in the Huffman code.
 */
static void
lzx_write_compressed_code(struct lzx_output_bitstream *os,
                          const u8 lens[restrict],
                          const u8 prev_lens[restrict],
                          unsigned num_lens)
{
        u32 precode_freqs[LZX_PRECODE_NUM_SYMBOLS];
        u8 precode_lens[LZX_PRECODE_NUM_SYMBOLS];
        u32 precode_codewords[LZX_PRECODE_NUM_SYMBOLS];
        unsigned precode_items[num_lens];
        unsigned num_precode_items;
        unsigned precode_item;
        unsigned precode_sym;
        unsigned i;
        u8 saved = lens[num_lens];
        *(u8 *)(lens + num_lens) = 0x80;

        for (i = 0; i < LZX_PRECODE_NUM_SYMBOLS; i++)
                precode_freqs[i] = 0;

        /* Compute the "items" (RLE / literal tokens and extra bits) with which
         * the codeword lengths in the larger code will be output.  */
        num_precode_items = lzx_compute_precode_items(lens,
                                                      prev_lens,
                                                      precode_freqs,
                                                      precode_items);

        /* Build the precode.  */
        STATIC_ASSERT(PRE_CODEWORD_LIMIT >= 5 &&
                      PRE_CODEWORD_LIMIT <= LZX_MAX_PRE_CODEWORD_LEN);
        make_canonical_huffman_code(LZX_PRECODE_NUM_SYMBOLS,
                                    PRE_CODEWORD_LIMIT,
                                    precode_freqs, precode_lens,
                                    precode_codewords);

        /* Output the lengths of the codewords in the precode.  */
        for (i = 0; i < LZX_PRECODE_NUM_SYMBOLS; i++)
                lzx_write_bits(os, precode_lens[i], LZX_PRECODE_ELEMENT_SIZE);

        /* Output the encoded lengths of the codewords in the larger code.  */
        for (i = 0; i < num_precode_items; i++) {
                precode_item = precode_items[i];
                precode_sym = precode_item & 0x1F;
                lzx_add_bits(os, precode_codewords[precode_sym],
                             precode_lens[precode_sym]);
                if (precode_sym >= 17) {
                        if (precode_sym == 17) {
                                lzx_add_bits(os, precode_item >> 5, 4);
                        } else if (precode_sym == 18) {
                                lzx_add_bits(os, precode_item >> 5, 5);
                        } else {
                                lzx_add_bits(os, (precode_item >> 5) & 1, 1);
                                precode_sym = precode_item >> 6;
                                lzx_add_bits(os, precode_codewords[precode_sym],
                                             precode_lens[precode_sym]);
                        }
                }
                STATIC_ASSERT(CAN_BUFFER(2 * PRE_CODEWORD_LIMIT + 1));
                lzx_flush_bits(os, 2 * PRE_CODEWORD_LIMIT + 1);
        }

        *(u8 *)(lens + num_lens) = saved;
}

/*
 * Write all matches and literal bytes (which were precomputed) in an LZX
 * compressed block to the output bitstream in the final compressed
 * representation.
 *
 * @os
 *      The output bitstream.
 * @block_type
 *      The chosen type of the LZX compressed block (LZX_BLOCKTYPE_ALIGNED or
 *      LZX_BLOCKTYPE_VERBATIM).
 * @block_data
 *      The uncompressed data of the block.
 * @sequences
 *      The matches and literals to output, given as a series of sequences.
 * @codes
 *      The main, length, and aligned offset Huffman codes for the current
 *      LZX compressed block.
 */
static void
lzx_write_sequences(struct lzx_output_bitstream *os, int block_type,
                    const u8 *block_data, const struct lzx_sequence sequences[],
                    const struct lzx_codes *codes)
{
        const struct lzx_sequence *seq = sequences;
        u32 ones_if_aligned = 0 - (block_type == LZX_BLOCKTYPE_ALIGNED);

        for (;;) {
                /* Output the next sequence.  */

                unsigned litrunlen = seq->litrunlen;
                unsigned match_hdr;
                unsigned main_symbol;
                unsigned adjusted_length;
                u32 adjusted_offset;
                unsigned offset_slot;
                unsigned num_extra_bits;
                u32 extra_bits;

                /* Output the literal run of the sequence.  */

                if (litrunlen) {  /* Is the literal run nonempty?  */

                        /* Verify optimization is enabled on 64-bit  */
                        STATIC_ASSERT(WORDBITS < 64 ||
                                      CAN_BUFFER(3 * MAIN_CODEWORD_LIMIT));

                        if (CAN_BUFFER(3 * MAIN_CODEWORD_LIMIT)) {

                                /* 64-bit: write 3 literals at a time.  */
                                while (litrunlen >= 3) {
                                        unsigned lit0 = block_data[0];
                                        unsigned lit1 = block_data[1];
                                        unsigned lit2 = block_data[2];
                                        lzx_add_bits(os, codes->codewords.main[lit0],
                                                     codes->lens.main[lit0]);
                                        lzx_add_bits(os, codes->codewords.main[lit1],
                                                     codes->lens.main[lit1]);
                                        lzx_add_bits(os, codes->codewords.main[lit2],
                                                     codes->lens.main[lit2]);
                                        lzx_flush_bits(os, 3 * MAIN_CODEWORD_LIMIT);
                                        block_data += 3;
                                        litrunlen -= 3;
                                }
                                if (litrunlen--) {
                                        unsigned lit = *block_data++;
                                        lzx_add_bits(os, codes->codewords.main[lit],
                                                     codes->lens.main[lit]);
                                        if (litrunlen--) {
                                                unsigned lit = *block_data++;
                                                lzx_add_bits(os, codes->codewords.main[lit],
                                                             codes->lens.main[lit]);
                                                lzx_flush_bits(os, 2 * MAIN_CODEWORD_LIMIT);
                                        } else {
                                                lzx_flush_bits(os, 1 * MAIN_CODEWORD_LIMIT);
                                        }
                                }
                        } else {
                                /* 32-bit: write 1 literal at a time.  */
                                do {
                                        unsigned lit = *block_data++;
                                        lzx_add_bits(os, codes->codewords.main[lit],
                                                     codes->lens.main[lit]);
                                        lzx_flush_bits(os, MAIN_CODEWORD_LIMIT);
                                } while (--litrunlen);
                        }
                }

                /* Was this the last literal run?  */
                if (seq->adjusted_offset_and_match_hdr & 0x80000000)
                        return;

                /* Nope; output the match.  */

                match_hdr = seq->adjusted_offset_and_match_hdr & 0x1FF;
                main_symbol = LZX_NUM_CHARS + match_hdr;
                adjusted_length = seq->adjusted_length;

                block_data += adjusted_length + LZX_MIN_MATCH_LEN;

                offset_slot = match_hdr / LZX_NUM_LEN_HEADERS;
                adjusted_offset = seq->adjusted_offset_and_match_hdr >> 9;

                num_extra_bits = lzx_extra_offset_bits[offset_slot];
                extra_bits = adjusted_offset - lzx_offset_slot_base[offset_slot];

        #define MAX_MATCH_BITS  (MAIN_CODEWORD_LIMIT + LENGTH_CODEWORD_LIMIT + \
                                 14 + ALIGNED_CODEWORD_LIMIT)

                /* Verify optimization is enabled on 64-bit  */
                STATIC_ASSERT(WORDBITS < 64 || CAN_BUFFER(MAX_MATCH_BITS));

                /* Output the main symbol for the match.  */

                lzx_add_bits(os, codes->codewords.main[main_symbol],
                             codes->lens.main[main_symbol]);
                if (!CAN_BUFFER(MAX_MATCH_BITS))
                        lzx_flush_bits(os, MAIN_CODEWORD_LIMIT);

                /* If needed, output the length symbol for the match.  */

                if (adjusted_length >= LZX_NUM_PRIMARY_LENS) {
                        lzx_add_bits(os, codes->codewords.len[adjusted_length -
                                                              LZX_NUM_PRIMARY_LENS],
                                     codes->lens.len[adjusted_length -
                                                     LZX_NUM_PRIMARY_LENS]);
                        if (!CAN_BUFFER(MAX_MATCH_BITS))
                                lzx_flush_bits(os, LENGTH_CODEWORD_LIMIT);
                }

                /* Output the extra offset bits for the match.  In aligned
                 * offset blocks, the lowest 3 bits of the adjusted offset are
                 * Huffman-encoded using the aligned offset code, provided that
                 * there are at least extra 3 offset bits required.  All other
                 * extra offset bits are output verbatim.  */

                if ((adjusted_offset & ones_if_aligned) >= 16) {

                        lzx_add_bits(os, extra_bits >> LZX_NUM_ALIGNED_OFFSET_BITS,
                                     num_extra_bits - LZX_NUM_ALIGNED_OFFSET_BITS);
                        if (!CAN_BUFFER(MAX_MATCH_BITS))
                                lzx_flush_bits(os, 14);

                        lzx_add_bits(os, codes->codewords.aligned[adjusted_offset &
                                                                  LZX_ALIGNED_OFFSET_BITMASK],
                                     codes->lens.aligned[adjusted_offset &
                                                         LZX_ALIGNED_OFFSET_BITMASK]);
                        if (!CAN_BUFFER(MAX_MATCH_BITS))
                                lzx_flush_bits(os, ALIGNED_CODEWORD_LIMIT);
                } else {
                        STATIC_ASSERT(CAN_BUFFER(17));

                        lzx_add_bits(os, extra_bits, num_extra_bits);
                        if (!CAN_BUFFER(MAX_MATCH_BITS))
                                lzx_flush_bits(os, 17);
                }

                if (CAN_BUFFER(MAX_MATCH_BITS))
                        lzx_flush_bits(os, MAX_MATCH_BITS);

                /* Advance to the next sequence.  */
                seq++;
        }
}

static void
lzx_write_compressed_block(const u8 *block_begin,
                           int block_type,
                           u32 block_size,
                           unsigned window_order,
                           unsigned num_main_syms,
                           const struct lzx_sequence sequences[],
                           const struct lzx_codes * codes,
                           const struct lzx_lens * prev_lens,
                           struct lzx_output_bitstream * os)
{
        /* The first three bits indicate the type of block and are one of the
         * LZX_BLOCKTYPE_* constants.  */
        lzx_write_bits(os, block_type, 3);

        /*
         * Output the block size.
         *
         * The original LZX format encoded the block size in 24 bits.  However,
         * the LZX format used in WIM archives uses 1 bit to specify whether the
         * block has the default size of 32768 bytes, then optionally 16 bits to
         * specify a non-default size.  This works fine for Microsoft's WIM
         * software (WIMGAPI), which never compresses more than 32768 bytes at a
         * time with LZX.  However, as an extension, our LZX compressor supports
         * compressing up to 2097152 bytes, with a corresponding increase in
         * window size.  It is possible for blocks in these larger buffers to
         * exceed 65535 bytes; such blocks cannot have their size represented in
         * 16 bits.
         *
         * The chosen solution was to use 24 bits for the block size when
         * possibly required --- specifically, when the compressor has been
         * allocated to be capable of compressing more than 32768 bytes at once.
         */
        if (block_size == LZX_DEFAULT_BLOCK_SIZE) {
                lzx_write_bits(os, 1, 1);
        } else {
                lzx_write_bits(os, 0, 1);

                if (window_order >= 16)
                        lzx_write_bits(os, block_size >> 16, 8);

                lzx_write_bits(os, block_size & 0xFFFF, 16);
        }

        /* If it's an aligned offset block, output the aligned offset code.  */
        if (block_type == LZX_BLOCKTYPE_ALIGNED) {
                for (int i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
                        lzx_write_bits(os, codes->lens.aligned[i],
                                       LZX_ALIGNEDCODE_ELEMENT_SIZE);
                }
        }

        /* Output the main code (two parts).  */
        lzx_write_compressed_code(os, codes->lens.main,
                                  prev_lens->main,
                                  LZX_NUM_CHARS);
        lzx_write_compressed_code(os, codes->lens.main + LZX_NUM_CHARS,
                                  prev_lens->main + LZX_NUM_CHARS,
                                  num_main_syms - LZX_NUM_CHARS);

        /* Output the length code.  */
        lzx_write_compressed_code(os, codes->lens.len,
                                  prev_lens->len,
                                  LZX_LENCODE_NUM_SYMBOLS);

        /* Output the compressed matches and literals.  */
        lzx_write_sequences(os, block_type, block_begin, sequences, codes);
}

/*
 * 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)
{
        u32 aligned_cost = 0;
        u32 verbatim_cost = 0;

        /* A verbatim block requires 3 bits in each place that an aligned offset
         * symbol would be used in an aligned offset block.  */
        for (unsigned i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
                verbatim_cost += LZX_NUM_ALIGNED_OFFSET_BITS * freqs->aligned[i];
                aligned_cost += codes->lens.aligned[i] * freqs->aligned[i];
        }

        /* Account for the cost of sending the codeword lengths of the aligned
         * offset code.  */
        aligned_cost += LZX_ALIGNEDCODE_ELEMENT_SIZE *
                        LZX_ALIGNEDCODE_NUM_SYMBOLS;

        if (aligned_cost < verbatim_cost)
                return LZX_BLOCKTYPE_ALIGNED;
        else
                return LZX_BLOCKTYPE_VERBATIM;
}

/*
 * Flush an LZX block:
 *
 * 1. Build the Huffman codes.
 * 2. Decide whether to output the block as VERBATIM or ALIGNED.
 * 3. Write the block.
 * 4. Swap the indices of the current and previous Huffman codes.
 */
static void
lzx_flush_block(struct lzx_compressor *c, struct lzx_output_bitstream *os,
                const u8 *block_begin, u32 block_size, u32 seq_idx)
{
        int block_type;

        lzx_build_huffman_codes(c);

        block_type = lzx_choose_verbatim_or_aligned(&c->freqs,
                                                    &c->codes[c->codes_index]);
        lzx_write_compressed_block(block_begin,
                                   block_type,
                                   block_size,
                                   c->window_order,
                                   c->num_main_syms,
                                   &c->chosen_sequences[seq_idx],
                                   &c->codes[c->codes_index],
                                   &c->codes[c->codes_index ^ 1].lens,
                                   os);
        c->codes_index ^= 1;
}

/******************************************************************************/
/*                          Block splitting algorithm                         */
/*----------------------------------------------------------------------------*/

/*
 * The problem of block splitting is to decide when it is worthwhile to start a
 * new block with new entropy codes.  There is a theoretically optimal solution:
 * recursively consider every possible block split, considering the exact cost
 * of each block, and choose the minimum cost approach.  But this is far too
 * slow.  Instead, as an approximation, we can count symbols and after every N
 * symbols, compare the expected distribution of symbols based on the previous
 * data with the actual distribution.  If they differ "by enough", then start a
 * new block.
 *
 * As an optimization and heuristic, we don't distinguish between every symbol
 * but rather we combine many symbols into a single "observation type".  For
 * literals we only look at the high bits and low bits, and for matches we only
 * look at whether the match is long or not.  The assumption is that for typical
 * "real" data, places that are good block boundaries will tend to be noticable
 * based only on changes in these aggregate frequencies, without looking for
 * subtle differences in individual symbols.  For example, a change from ASCII
 * bytes to non-ASCII bytes, or from few matches (generally less compressible)
 * to many matches (generally more compressible), would be easily noticed based
 * on the aggregates.
 *
 * For determining whether the frequency distributions are "different enough" to
 * start a new block, the simply heuristic of splitting when the sum of absolute
 * differences exceeds a constant seems to be good enough.
 *
 * Finally, for an approximation, it is not strictly necessary that the exact
 * symbols being used are considered.  With "near-optimal parsing", for example,
 * the actual symbols that will be used are unknown until after the block
 * boundary is chosen and the block has been optimized.  Since the final choices
 * cannot be used, we can use preliminary "greedy" choices instead.
 */

/* Initialize the block split statistics when starting a new block. */
static void
lzx_init_block_split_stats(struct lzx_block_split_stats *stats)
{
        for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
                stats->new_observations[i] = 0;
                stats->observations[i] = 0;
        }
        stats->num_new_observations = 0;
        stats->num_observations = 0;
}

/* Literal observation.  Heuristic: use the top 2 bits and low 1 bits of the
 * literal, for 8 possible literal observation types.  */
static inline void
lzx_observe_literal(struct lzx_block_split_stats *stats, u8 lit)
{
        stats->new_observations[((lit >> 5) & 0x6) | (lit & 1)]++;
        stats->num_new_observations++;
}

/* Match observation.  Heuristic: use one observation type for "short match" and
 * one observation type for "long match".  */
static inline void
lzx_observe_match(struct lzx_block_split_stats *stats, unsigned length)
{
        stats->new_observations[NUM_LITERAL_OBSERVATION_TYPES + (length >= 5)]++;
        stats->num_new_observations++;
}

static bool
lzx_end_block_check(struct lzx_block_split_stats *stats)
{
        if (stats->num_observations > 0) {

                /* Note: to avoid slow divisions, we do not divide by
                 * 'num_observations', but rather do all math with the numbers
                 * multiplied by 'num_observations'. */
                u32 total_delta = 0;
                for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
                        u32 expected = stats->observations[i] *
                                        stats->num_new_observations;
                        u32 actual = stats->new_observations[i] *
                                        stats->num_observations;
                        u32 delta = (actual > expected) ? actual - expected :
                                                          expected - actual;
                        total_delta += delta;
                }

                /* Ready to end the block? */
                if (total_delta >=
                    stats->num_new_observations * 7 / 8 * stats->num_observations)
                        return true;
        }

        for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
                stats->num_observations += stats->new_observations[i];
                stats->observations[i] += stats->new_observations[i];
                stats->new_observations[i] = 0;
        }
        stats->num_new_observations = 0;
        return false;
}

static inline bool
lzx_should_end_block(struct lzx_block_split_stats *stats,
                     const u8 *in_block_begin, const u8 *in_next, const u8 *in_end)
{
        /* Ready to check block split statistics? */
        if (stats->num_new_observations < NUM_OBSERVATIONS_PER_BLOCK_CHECK ||
            in_next - in_block_begin < MIN_BLOCK_SIZE ||
            in_end - in_next < MIN_BLOCK_SIZE)
                return false;

        return lzx_end_block_check(stats);
}

/******************************************************************************/
/*                   Slower ("near-optimal") compression algorithm            */
/*----------------------------------------------------------------------------*/

/*
 * Least-recently-used queue for match offsets.
 *
 * This is represented as a 64-bit integer for efficiency.  There are three
 * offsets of 21 bits each.  Bit 64 is garbage.
 */
struct lzx_lru_queue {
        u64 R;
};

#define LZX_QUEUE_OFFSET_SHIFT  21
#define LZX_QUEUE_OFFSET_MASK   (((u64)1 << LZX_QUEUE_OFFSET_SHIFT) - 1)

#define LZX_QUEUE_R0_SHIFT (0 * LZX_QUEUE_OFFSET_SHIFT)
#define LZX_QUEUE_R1_SHIFT (1 * LZX_QUEUE_OFFSET_SHIFT)
#define LZX_QUEUE_R2_SHIFT (2 * LZX_QUEUE_OFFSET_SHIFT)

#define LZX_QUEUE_R0_MASK (LZX_QUEUE_OFFSET_MASK << LZX_QUEUE_R0_SHIFT)
#define LZX_QUEUE_R1_MASK (LZX_QUEUE_OFFSET_MASK << LZX_QUEUE_R1_SHIFT)
#define LZX_QUEUE_R2_MASK (LZX_QUEUE_OFFSET_MASK << LZX_QUEUE_R2_SHIFT)

#define LZX_QUEUE_INITIALIZER {         \
        ((u64)1 << LZX_QUEUE_R0_SHIFT) |        \
        ((u64)1 << LZX_QUEUE_R1_SHIFT) |        \
        ((u64)1 << LZX_QUEUE_R2_SHIFT) }

static inline u64
lzx_lru_queue_R0(struct lzx_lru_queue queue)
{
        return (queue.R >> LZX_QUEUE_R0_SHIFT) & LZX_QUEUE_OFFSET_MASK;
}

static inline u64
lzx_lru_queue_R1(struct lzx_lru_queue queue)
{
        return (queue.R >> LZX_QUEUE_R1_SHIFT) & LZX_QUEUE_OFFSET_MASK;
}

static inline u64
lzx_lru_queue_R2(struct lzx_lru_queue queue)
{
        return (queue.R >> LZX_QUEUE_R2_SHIFT) & LZX_QUEUE_OFFSET_MASK;
}

/* Push a match offset onto the front (most recently used) end of the queue.  */
static inline struct lzx_lru_queue
lzx_lru_queue_push(struct lzx_lru_queue queue, u32 offset)
{
        return (struct lzx_lru_queue) {
                .R = (queue.R << LZX_QUEUE_OFFSET_SHIFT) | offset,
        };
}

/* Swap a match offset to the front of the queue.  */
static inline struct lzx_lru_queue
lzx_lru_queue_swap(struct lzx_lru_queue queue, unsigned idx)
{
        unsigned shift = idx * 21;
        const u64 mask = LZX_QUEUE_R0_MASK;
        const u64 mask_high = mask << shift;

        return (struct lzx_lru_queue) {
                .R = (queue.R & ~(mask | mask_high)) |
                     ((queue.R & mask_high) >> shift) |
                     ((queue.R & mask) << shift),
        };
}

static inline u32
lzx_walk_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit,
                   bool record)
{
        u32 node_idx = block_size;
        u32 seq_idx = ARRAY_LEN(c->chosen_sequences) - 1;
        u32 lit_start_node;

        if (record) {
                /* Special value to mark last sequence  */
                c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr = 0x80000000;
                lit_start_node = node_idx;
        }

        for (;;) {
                u32 item;
                u32 len;
                u32 offset_data;
                unsigned v;
                unsigned offset_slot;

                /* Tally literals until either a match or the beginning of the
                 * block is reached.  Note: the item in the node at the
                 * beginning of the block has all bits set, causing this loop to
                 * end when it is reached. */
                for (;;) {
                        item = c->optimum_nodes[node_idx].item;
                        if (item & OPTIMUM_LEN_MASK)
                                break;
                        c->freqs.main[item >> OPTIMUM_OFFSET_SHIFT]++;
                        node_idx--;
                }

        #if CONSIDER_GAP_MATCHES
                if (item & OPTIMUM_GAP_MATCH) {

                        if (node_idx == 0)
                                break;

                        /* Save the literal run length for the next sequence
                         * (the "previous sequence" when walking backwards). */
                        len = item & OPTIMUM_LEN_MASK;
                        if (record) {
                                c->chosen_sequences[seq_idx--].litrunlen =
                                                lit_start_node - node_idx;
                                lit_start_node = node_idx - len;
                        }

                        /* Tally the rep0 match after the gap. */
                        v = len - LZX_MIN_MATCH_LEN;
                        if (record)
                                c->chosen_sequences[seq_idx].adjusted_length = v;
                        if (v >= LZX_NUM_PRIMARY_LENS) {
                                c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
                                v = LZX_NUM_PRIMARY_LENS;
                        }
                        c->freqs.main[LZX_NUM_CHARS + v]++;
                        if (record)
                                c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr = v;

                        /* Tally the literal in the gap. */
                        c->freqs.main[(u8)(item >> OPTIMUM_OFFSET_SHIFT)]++;

                        /* Fall through and tally the match before the gap.
                         * (It was temporarily saved in the 'cost' field of the
                         * previous node, which was free to reuse.) */
                        item = c->optimum_nodes[--node_idx].cost;
                        node_idx -= len;
                }
        #else /* CONSIDER_GAP_MATCHES */
                if (node_idx == 0)
                        break;
        #endif /* !CONSIDER_GAP_MATCHES */

                len = item & OPTIMUM_LEN_MASK;
                offset_data = item >> OPTIMUM_OFFSET_SHIFT;

                /* Save the literal run length for the next sequence (the
                 * "previous sequence" when walking backwards). */
                if (record) {
                        c->chosen_sequences[seq_idx--].litrunlen =
                                        lit_start_node - node_idx;
                        node_idx -= len;
                        lit_start_node = node_idx;
                } else {
                        node_idx -= len;
                }

                /* Record a match. */

                /* Tally the aligned offset symbol if needed. */
                if (offset_data >= 16)
                        c->freqs.aligned[offset_data & LZX_ALIGNED_OFFSET_BITMASK]++;

                /* Save the adjusted length. */
                v = len - LZX_MIN_MATCH_LEN;
                if (record)
                        c->chosen_sequences[seq_idx].adjusted_length = v;

                /* Tally the length symbol if needed. */
                if (v >= LZX_NUM_PRIMARY_LENS) {
                        c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
                        v = LZX_NUM_PRIMARY_LENS;
                }

                /* Tally the main symbol. */
                offset_slot = lzx_get_offset_slot(c, offset_data, is_16_bit);
                v += offset_slot * LZX_NUM_LEN_HEADERS;
                c->freqs.main[LZX_NUM_CHARS + v]++;

                /* Save the adjusted offset and match header. */
                if (record) {
                        c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr =
                                        (offset_data << 9) | v;
                }
        }

        /* Save the literal run length for the first sequence. */
        if (record)
                c->chosen_sequences[seq_idx].litrunlen = lit_start_node - node_idx;

        /* Return the index in c->chosen_sequences at which the lzx_sequences
         * begin. */
        return seq_idx;
}

/*
 * Given the minimum-cost path computed through the item graph for the current
 * block, walk the path and count how many of each symbol in each Huffman-coded
 * alphabet would be required to output the items (matches and literals) along
 * the path.
 *
 * Note that the path will be walked backwards (from the end of the block to the
 * beginning of the block), but this doesn't matter because this function only
 * computes frequencies.
 */
static inline void
lzx_tally_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit)
{
        lzx_walk_item_list(c, block_size, is_16_bit, false);
}

/*
 * Like lzx_tally_item_list(), but this function also generates the list of
 * lzx_sequences for the minimum-cost path and writes it to c->chosen_sequences,
 * ready to be output to the bitstream after the Huffman codes are computed.
 * The lzx_sequences will be written to decreasing memory addresses as the path
 * is walked backwards, which means they will end up in the expected
 * first-to-last order.  The return value is the index in c->chosen_sequences at
 * which the lzx_sequences begin.
 */
static inline u32
lzx_record_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit)
{
        return lzx_walk_item_list(c, block_size, is_16_bit, true);
}

/*
 * Find an inexpensive path through the graph of possible match/literal choices
 * for the current block.  The nodes of the graph are
 * c->optimum_nodes[0...block_size].  They correspond directly to the bytes in
 * the current block, plus one extra node for end-of-block.  The edges of the
 * graph are matches and literals.  The goal is to find the minimum cost path
 * from 'c->optimum_nodes[0]' to 'c->optimum_nodes[block_size]', given the cost
 * model 'c->costs'.
 *
 * The algorithm works forwards, starting at 'c->optimum_nodes[0]' and
 * proceeding forwards one node at a time.  At each node, a selection of matches
 * (len >= 2), as well as the literal byte (len = 1), is considered.  An item of
 * length 'len' provides a new path to reach the node 'len' bytes later.  If
 * such a path is the lowest cost found so far to reach that later node, then
 * that later node is updated with the new path.
 *
 * Note that although this algorithm is based on minimum cost path search, due
 * to various simplifying assumptions the result is not guaranteed to be the
 * true minimum cost, or "optimal", path over the graph of all valid LZX
 * representations of this block.
 *
 * Also, note that because of the presence of the recent offsets queue (which is
 * a type of adaptive state), the algorithm cannot work backwards and compute
 * "cost to end" instead of "cost to beginning".  Furthermore, the way the
 * algorithm handles this adaptive state in the "minimum cost" parse is actually
 * only an approximation.  It's possible for the globally optimal, minimum cost
 * path to contain a prefix, ending at a position, where that path prefix is
 * *not* the minimum cost path to that position.  This can happen if such a path
 * prefix results in a different adaptive state which results in lower costs
 * later.  The algorithm does not solve this problem; it only considers the
 * lowest cost to reach each individual position.
 */
static inline struct lzx_lru_queue
lzx_find_min_cost_path(struct lzx_compressor * const restrict c,
                       const u8 * const restrict block_begin,
                       const u32 block_size,
                       const struct lzx_lru_queue initial_queue,
                       bool is_16_bit)
{
        struct lzx_optimum_node *cur_node = c->optimum_nodes;
        struct lzx_optimum_node * const end_node = cur_node + block_size;
        struct lz_match *cache_ptr = c->match_cache;
        const u8 *in_next = block_begin;
        const u8 * const block_end = block_begin + block_size;

        /*
         * Instead of storing the match offset LRU queues in the
         * 'lzx_optimum_node' structures, we save memory (and cache lines) by
         * storing them in a smaller array.  This works because the algorithm
         * only requires a limited history of the adaptive state.  Once a given
         * state is more than LZX_MAX_MATCH_LEN bytes behind the current node
         * (more if gap match consideration is enabled; we just round up to 512
         * so it's a power of 2), it is no longer needed.
         */
        struct lzx_lru_queue queues[512];
        STATIC_ASSERT(ARRAY_LEN(queues) >= LZX_MAX_MATCH_LEN + 1);
        STATIC_ASSERT(sizeof(c->optimum_nodes[0]) == sizeof(queues[0]));
#define QUEUE(node) \
        (*(struct lzx_lru_queue *)((char *)queues + \
                        ((uintptr_t)(node) % (ARRAY_LEN(queues) * sizeof(*(node))))))
        /*(queues[(uintptr_t)(node) / sizeof(*(node)) % ARRAY_LEN(queues)])*/

#if CONSIDER_GAP_MATCHES
        u32 matches_before_gap[ARRAY_LEN(queues)];
#define MATCH_BEFORE_GAP(node) \
        (matches_before_gap[(uintptr_t)(node) / sizeof(*(node)) % \
                            ARRAY_LEN(matches_before_gap)])
#endif

        /*
         * Initially, the cost to reach each node is "infinity".
         *
         * The first node actually should have cost 0, but "infinity"
         * (0xFFFFFFFF) works just as well because it immediately overflows.
         *
         * This statement also intentionally sets the 'item' of the first node,
         * which would otherwise have no meaning, to 0xFFFFFFFF for use as a
         * sentinel.  See lzx_walk_item_list().
         */
        memset(c->optimum_nodes, 0xFF,
               (block_size + 1) * sizeof(c->optimum_nodes[0]));

        /* Initialize the recent offsets queue for the first node. */
        QUEUE(cur_node) = initial_queue;

        do { /* For each node in the block in position order... */

                unsigned num_matches;
                unsigned literal;
                u32 cost;

                /*
                 * A selection of matches for the block was already saved in
                 * memory so that we don't have to run the uncompressed data
                 * through the matchfinder on every optimization pass.  However,
                 * we still search for repeat offset matches during each
                 * optimization pass because we cannot predict the state of the
                 * recent offsets queue.  But as a heuristic, we don't bother
                 * searching for repeat offset matches if the general-purpose
                 * matchfinder failed to find any matches.
                 *
                 * Note that a match of length n at some offset implies there is
                 * also a match of length l for LZX_MIN_MATCH_LEN <= l <= n at
                 * that same offset.  In other words, we don't necessarily need
                 * to use the full length of a match.  The key heuristic that
                 * saves a significicant amount of time is that for each
                 * distinct length, we only consider the smallest offset for
                 * which that length is available.  This heuristic also applies
                 * to repeat offsets, which we order specially: R0 < R1 < R2 <
                 * any explicit offset.  Of course, this heuristic may be
                 * produce suboptimal results because offset slots in LZX are
                 * subject to entropy encoding, but in practice this is a useful
                 * heuristic.
                 */

                num_matches = cache_ptr->length;
                cache_ptr++;

                if (num_matches) {
                        struct lz_match *end_matches = cache_ptr + num_matches;
                        unsigned next_len = LZX_MIN_MATCH_LEN;
                        unsigned max_len = min(block_end - in_next, LZX_MAX_MATCH_LEN);
                        const u8 *matchptr;

                        /* Consider rep0 matches. */
                        matchptr = in_next - lzx_lru_queue_R0(QUEUE(cur_node));
                        if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next))
                                goto rep0_done;
                        STATIC_ASSERT(LZX_MIN_MATCH_LEN == 2);
                        do {
                                u32 cost = cur_node->cost +
                                           c->costs.match_cost[0][
                                                        next_len - LZX_MIN_MATCH_LEN];
                                if (cost <= (cur_node + next_len)->cost) {
                                        (cur_node + next_len)->cost = cost;
                                        (cur_node + next_len)->item =
                                                (0 << OPTIMUM_OFFSET_SHIFT) | next_len;
                                }
                                if (unlikely(++next_len > max_len)) {
                                        cache_ptr = end_matches;
                                        goto done_matches;
                                }
                        } while (in_next[next_len - 1] == matchptr[next_len - 1]);

                rep0_done:

                        /* Consider rep1 matches. */
                        matchptr = in_next - lzx_lru_queue_R1(QUEUE(cur_node));
                        if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next))
                                goto rep1_done;
                        if (matchptr[next_len - 1] != in_next[next_len - 1])
                                goto rep1_done;
                        for (unsigned len = 2; len < next_len - 1; len++)
                                if (matchptr[len] != in_next[len])
                                        goto rep1_done;
                        do {
                                u32 cost = cur_node->cost +
                                           c->costs.match_cost[1][
                                                        next_len - LZX_MIN_MATCH_LEN];
                                if (cost <= (cur_node + next_len)->cost) {
                                        (cur_node + next_len)->cost = cost;
                                        (cur_node + next_len)->item =
                                                (1 << OPTIMUM_OFFSET_SHIFT) | next_len;
                                }
                                if (unlikely(++next_len > max_len)) {
                                        cache_ptr = end_matches;
                                        goto done_matches;
                                }
                        } while (in_next[next_len - 1] == matchptr[next_len - 1]);

                rep1_done:

                        /* Consider rep2 matches. */
                        matchptr = in_next - lzx_lru_queue_R2(QUEUE(cur_node));
                        if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next))
                                goto rep2_done;
                        if (matchptr[next_len - 1] != in_next[next_len - 1])
                                goto rep2_done;
                        for (unsigned len = 2; len < next_len - 1; len++)
                                if (matchptr[len] != in_next[len])
                                        goto rep2_done;
                        do {
                                u32 cost = cur_node->cost +
                                           c->costs.match_cost[2][
                                                        next_len - LZX_MIN_MATCH_LEN];
                                if (cost <= (cur_node + next_len)->cost) {
                                        (cur_node + next_len)->cost = cost;
                                        (cur_node + next_len)->item =
                                                (2 << OPTIMUM_OFFSET_SHIFT) | next_len;
                                }
                                if (unlikely(++next_len > max_len)) {
                                        cache_ptr = end_matches;
                                        goto done_matches;
                                }
                        } while (in_next[next_len - 1] == matchptr[next_len - 1]);

                rep2_done:

                        while (next_len > cache_ptr->length)
                                if (++cache_ptr == end_matches)
                                        goto done_matches;

                        /* Consider explicit offset matches. */
                        for (;;) {
                                u32 offset = cache_ptr->offset;
                                u32 offset_data = offset + LZX_OFFSET_ADJUSTMENT;
                                unsigned offset_slot = lzx_get_offset_slot(c, offset_data, is_16_bit);
                                u32 base_cost = cur_node->cost;
                                u32 cost;

                        #if CONSIDER_ALIGNED_COSTS
                                if (offset >= 16 - LZX_OFFSET_ADJUSTMENT)
                                        base_cost += c->costs.aligned[offset_data &
                                                                      LZX_ALIGNED_OFFSET_BITMASK];
                        #endif
                                do {
                                        cost = base_cost +
                                               c->costs.match_cost[offset_slot][
                                                                next_len - LZX_MIN_MATCH_LEN];
                                        if (cost < (cur_node + next_len)->cost) {
                                                (cur_node + next_len)->cost = cost;
                                                (cur_node + next_len)->item =
                                                        (offset_data << OPTIMUM_OFFSET_SHIFT) | next_len;
                                        }
                                } while (++next_len <= cache_ptr->length);

                                if (++cache_ptr == end_matches) {
                                #if CONSIDER_GAP_MATCHES
                                        /* Also consider the longest explicit
                                         * offset match as a "gap match": match
                                         * + lit + rep0. */
                                        s32 remaining = (block_end - in_next) - next_len;
                                        if (likely(remaining >= 2)) {
                                                const u8 *strptr = in_next + next_len;
                                                const u8 *matchptr = strptr - offset;
                                                if (load_u16_unaligned(strptr) == load_u16_unaligned(matchptr)) {
                                                        STATIC_ASSERT(ARRAY_LEN(queues) - LZX_MAX_MATCH_LEN - 2 >= 250);
                                                        STATIC_ASSERT(ARRAY_LEN(queues) == ARRAY_LEN(matches_before_gap));
                                                        u32 limit = min(remaining,
                                                                        min(ARRAY_LEN(queues) - LZX_MAX_MATCH_LEN - 2,
                                                                            LZX_MAX_MATCH_LEN));
                                                        u32 rep0_len = lz_extend(strptr, matchptr, 2, limit);
                                                        u8 lit = strptr[-1];
                                                        cost += c->costs.main[lit] +
                                                                c->costs.match_cost[0][rep0_len - LZX_MIN_MATCH_LEN];
                                                        u32 total_len = next_len + rep0_len;
                                                        if (cost < (cur_node + total_len)->cost) {
                                                                (cur_node + total_len)->cost = cost;
                                                                (cur_node + total_len)->item =
                                                                        OPTIMUM_GAP_MATCH |
                                                                        ((u32)lit << OPTIMUM_OFFSET_SHIFT) |
                                                                        rep0_len;
                                                                MATCH_BEFORE_GAP(cur_node + total_len) =
                                                                        (offset_data << OPTIMUM_OFFSET_SHIFT) |
                                                                        (next_len - 1);
                                                        }
                                                }
                                        }
                                #endif /* CONSIDER_GAP_MATCHES */
                                        break;
                                }
                        }
                }

        done_matches:

                /* Consider coding a literal.

                 * To avoid an extra branch, actually checking the preferability
                 * of coding the literal is integrated into the queue update
                 * code below. */
                literal = *in_next++;
                cost = cur_node->cost + c->costs.main[literal];

                /* Advance to the next position. */
                cur_node++;

                /* The lowest-cost path to the current position is now known.
                 * Finalize the recent offsets queue that results from taking
                 * this lowest-cost path. */

                if (cost <= cur_node->cost) {
                        /* Literal: queue remains unchanged. */
                        cur_node->cost = cost;
                        cur_node->item = (u32)literal << OPTIMUM_OFFSET_SHIFT;
                        QUEUE(cur_node) = QUEUE(cur_node - 1);
                } else {
                        /* Match: queue update is needed. */
                        unsigned len = cur_node->item & OPTIMUM_LEN_MASK;
                #if CONSIDER_GAP_MATCHES
                        s32 offset_data = (s32)cur_node->item >> OPTIMUM_OFFSET_SHIFT;
                        STATIC_ASSERT(OPTIMUM_GAP_MATCH == 0x80000000); /* assuming sign extension */
                #else
                        u32 offset_data = cur_node->item >> OPTIMUM_OFFSET_SHIFT;
                #endif

                        if (offset_data >= LZX_NUM_RECENT_OFFSETS) {
                                /* Explicit offset match: insert offset at front. */
                                QUEUE(cur_node) =
                                        lzx_lru_queue_push(QUEUE(cur_node - len),
                                                           offset_data - LZX_OFFSET_ADJUSTMENT);
                        } else
                #if CONSIDER_GAP_MATCHES
                        if (offset_data < 0) {
                                /* "Gap match": Explicit offset match, then a
                                 * literal, then rep0 match.  Save the explicit
                                 * offset match information in the cost field of
                                 * the previous node, which isn't needed
                                 * anymore.  Then insert the offset at the front
                                 * of the queue. */
                                u32 match_before_gap = MATCH_BEFORE_GAP(cur_node);
                                (cur_node - 1)->cost = match_before_gap;
                                QUEUE(cur_node) =
                                        lzx_lru_queue_push(QUEUE(cur_node - len - 1 -
                                                                 (match_before_gap & OPTIMUM_LEN_MASK)),
                                                           (match_before_gap >> OPTIMUM_OFFSET_SHIFT) -
                                                           LZX_OFFSET_ADJUSTMENT);
                        } else
                #endif
                        {
                                /* Repeat offset match: swap offset to front. */
                                QUEUE(cur_node) =
                                        lzx_lru_queue_swap(QUEUE(cur_node - len),
                                                           offset_data);
                        }
                }
        } while (cur_node != end_node);

        /* Return the recent offsets queue at the end of the path. */
        return QUEUE(cur_node);
}

/* Given the costs for the main and length codewords, compute 'match_costs'.  */
static void
lzx_compute_match_costs(struct lzx_compressor *c)
{
        unsigned num_offset_slots = (c->num_main_syms - LZX_NUM_CHARS) /
                                        LZX_NUM_LEN_HEADERS;
        struct lzx_costs *costs = &c->costs;

        for (unsigned offset_slot = 0; offset_slot < num_offset_slots;
             offset_slot++)
        {
                u32 extra_cost = (u32)lzx_extra_offset_bits[offset_slot] <<
                                                                COST_SHIFT;
                unsigned main_symbol = LZX_NUM_CHARS + (offset_slot *
                                                        LZX_NUM_LEN_HEADERS);
                unsigned i;

        #if CONSIDER_ALIGNED_COSTS
                if (offset_slot >= 8)
                        extra_cost -= LZX_NUM_ALIGNED_OFFSET_BITS << COST_SHIFT;
        #endif

                for (i = 0; i < LZX_NUM_PRIMARY_LENS; i++) {
                        costs->match_cost[offset_slot][i] =
                                costs->main[main_symbol++] + extra_cost;
                }

                extra_cost += costs->main[main_symbol];

                for (; i < LZX_NUM_LENS; i++) {
                        costs->match_cost[offset_slot][i] =
                                costs->len[i - LZX_NUM_PRIMARY_LENS] +
                                extra_cost;
                }
        }
}

/* Set default LZX Huffman symbol costs to bootstrap the iterative optimization
 * algorithm.  */
static void
lzx_set_default_costs(struct lzx_compressor *c, const u8 *block, u32 block_size)
{
        u32 i;
        bool have_byte[256];
        unsigned num_used_bytes;

        /* The costs below are hard coded to use a COST_SHIFT of 6.  */
        STATIC_ASSERT(COST_SHIFT == 6);

        /*
         * Heuristics:
         *
         * - Use smaller initial costs for literal symbols when the input buffer
         *   contains fewer distinct bytes.
         *
         * - Assume that match symbols are more costly than literal symbols.
         *
         * - Assume that length symbols for shorter lengths are less costly than
         *   length symbols for longer lengths.
         */

        for (i = 0; i < 256; i++)
                have_byte[i] = false;

        for (i = 0; i < block_size; i++)
                have_byte[block[i]] = true;

        num_used_bytes = 0;
        for (i = 0; i < 256; i++)
                num_used_bytes += have_byte[i];

        for (i = 0; i < 256; i++)
                c->costs.main[i] = 560 - (256 - num_used_bytes);

        for (; i < c->num_main_syms; i++)
                c->costs.main[i] = 680;

        for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++)
                c->costs.len[i] = 412 + i;

#if CONSIDER_ALIGNED_COSTS
        for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++)
                c->costs.aligned[i] = LZX_NUM_ALIGNED_OFFSET_BITS << COST_SHIFT;
#endif

        lzx_compute_match_costs(c);
}

/* Update the current cost model to reflect the computed Huffman codes.  */
static void
lzx_set_costs_from_codes(struct lzx_compressor *c)
{
        unsigned i;
        const struct lzx_lens *lens = &c->codes[c->codes_index].lens;

        for (i = 0; i < c->num_main_syms; i++) {
                c->costs.main[i] = (lens->main[i] ? lens->main[i] :
                                    MAIN_CODEWORD_LIMIT) << COST_SHIFT;
        }

        for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++) {
                c->costs.len[i] = (lens->len[i] ? lens->len[i] :
                                   LENGTH_CODEWORD_LIMIT) << COST_SHIFT;
        }

#if CONSIDER_ALIGNED_COSTS
        for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
                c->costs.aligned[i] = (lens->aligned[i] ? lens->aligned[i] :
                                       ALIGNED_CODEWORD_LIMIT) << COST_SHIFT;
        }
#endif

        lzx_compute_match_costs(c);
}

/*
 * Choose a "near-optimal" literal/match sequence to use for the current block,
 * then flush the block.  Because the cost of each Huffman symbol is unknown
 * until the Huffman codes have been built and the Huffman codes themselves
 * depend on the symbol frequencies, this uses an iterative optimization
 * algorithm to approximate an optimal solution.  The first optimization pass
 * for the block uses default costs.  Additional passes use costs taken from the
 * Huffman codes computed in the previous pass.
 */
static inline struct lzx_lru_queue
lzx_optimize_and_flush_block(struct lzx_compressor * const restrict c,
                             struct lzx_output_bitstream * const restrict os,
                             const u8 * const restrict block_begin,
                             const u32 block_size,
                             const struct lzx_lru_queue initial_queue,
                             bool is_16_bit)
{
        unsigned num_passes_remaining = c->num_optim_passes;
        struct lzx_lru_queue new_queue;
        u32 seq_idx;

        lzx_set_default_costs(c, block_begin, block_size);

        for (;;) {
                new_queue = lzx_find_min_cost_path(c, block_begin, block_size,
                                                   initial_queue, is_16_bit);

                if (--num_passes_remaining == 0)
                        break;

                /* At least one optimization pass remains.  Update the costs. */
                lzx_reset_symbol_frequencies(c);
                lzx_tally_item_list(c, block_size, is_16_bit);
                lzx_build_huffman_codes(c);
                lzx_set_costs_from_codes(c);
        }

        /* Done optimizing.  Generate the sequence list and flush the block. */
        lzx_reset_symbol_frequencies(c);
        seq_idx = lzx_record_item_list(c, block_size, is_16_bit);
        lzx_flush_block(c, os, block_begin, block_size, seq_idx);
        return new_queue;
}

/*
 * This is the "near-optimal" LZX compressor.
 *
 * For each block, it performs a relatively thorough graph search to find an
 * inexpensive (in terms of compressed size) way to output the block.
 *
 * Note: there are actually many things this algorithm leaves on the table in
 * terms of compression ratio.  So although it may be "near-optimal", it is
 * certainly not "optimal".  The goal is not to produce the optimal compression
 * ratio, which for LZX is probably impossible within any practical amount of
 * time, but rather to produce a compression ratio significantly better than a
 * simpler "greedy" or "lazy" parse while still being relatively fast.
 */
static inline void
lzx_compress_near_optimal(struct lzx_compressor * restrict c,
                          const u8 * const restrict in_begin, size_t in_nbytes,
                          struct lzx_output_bitstream * restrict os,
                          bool is_16_bit)
{
        const u8 *       in_next = in_begin;
        const u8 * const in_end  = in_begin + in_nbytes;
        u32 max_len = LZX_MAX_MATCH_LEN;
        u32 nice_len = min(c->nice_match_length, max_len);
        u32 next_hashes[2] = {0, 0};
        struct lzx_lru_queue queue = LZX_QUEUE_INITIALIZER;

        /* Initialize the matchfinder. */
        CALL_BT_MF(is_16_bit, c, bt_matchfinder_init);

        do {
                /* Starting a new block */
                const u8 * const in_block_begin = in_next;
                const u8 * const in_max_block_end =
                        in_next + min(SOFT_MAX_BLOCK_SIZE, in_end - in_next);
                struct lz_match *cache_ptr = c->match_cache;
                const u8 *next_search_pos = in_next;
                const u8 *next_observation =
                        (in_max_block_end - in_next < MIN_BLOCK_SIZE) ?
                         in_max_block_end : in_next;
                const u8 *next_pause_point =
                        min(in_next + min(MIN_BLOCK_SIZE,
                                          in_max_block_end - in_next),
                            in_max_block_end - min(LZX_MAX_MATCH_LEN - 1,
                                                   in_max_block_end - in_next));

                lzx_init_block_split_stats(&c->split_stats);

                /*
                 * Run the input buffer through the matchfinder, caching the
                 * matches, until we decide to end the block.
                 *
                 * For a tighter matchfinding loop, we compute a "pause point",
                 * which is the next position at which we may need to check
                 * whether to end the block or to decrease max_len.  We then
                 * only do these extra checks upon reaching the pause point.
                 */
        resume_matchfinding:
                do {
                        if (in_next >= next_search_pos) {
                                /* Search for matches at this position. */
                                struct lz_match *lz_matchptr;
                                u32 best_len;

                                lz_matchptr = CALL_BT_MF(is_16_bit, c,
                                                         bt_matchfinder_get_matches,
                                                         in_begin,
                                                         in_next - in_begin,
                                                         max_len,
                                                         nice_len,
                                                         c->max_search_depth,
                                                         next_hashes,
                                                         &best_len,
                                                         cache_ptr + 1);
                                cache_ptr->length = lz_matchptr - (cache_ptr + 1);
                                cache_ptr = lz_matchptr;

                                /* Accumulate block split statistics. */
                                if (in_next >= next_observation) {
                                        if (best_len >= 4) {
                                                lzx_observe_match(&c->split_stats,
                                                                  best_len);
                                                next_observation = in_next + best_len;
                                        } else {
                                                lzx_observe_literal(&c->split_stats,
                                                                    *in_next);
                                                next_observation = in_next + 1;
                                        }
                                }

                                /*
                                 * If there was a very long match found, then
                                 * don't cache any matches for the bytes covered
                                 * by that match.  This avoids degenerate
                                 * behavior when compressing highly redundant
                                 * data, where the number of matches can be very
                                 * large.
                                 *
                                 * This heuristic doesn't actually hurt the
                                 * compression ratio very much.  If there's a
                                 * long match, then the data must be highly
                                 * compressible, so it doesn't matter as much
                                 * what we do.
                                 */
                                if (best_len >= nice_len)
                                        next_search_pos = in_next + best_len;
                        } else {
                                /* Don't search for matches at this position. */
                                CALL_BT_MF(is_16_bit, c,
                                           bt_matchfinder_skip_position,
                                           in_begin,
                                           in_next - in_begin,
                                           nice_len,
                                           c->max_search_depth,
                                           next_hashes);
                                cache_ptr->length = 0;
                                cache_ptr++;
                        }
                } while (++in_next < next_pause_point &&
                         likely(cache_ptr < &c->match_cache[CACHE_LENGTH]));

                /* Adjust max_len and nice_len if we're nearing the end of the
                 * input buffer. In addition, if we are so close to the end of
                 * the input buffer that there cannot be any more matches, then
                 * just advance through the last few positions and record no
                 * matches. */
                if (unlikely(max_len > in_end - in_next)) {
                        max_len = in_end - in_next;
                        nice_len = min(max_len, nice_len);
                        if (max_len < BT_MATCHFINDER_REQUIRED_NBYTES) {
                                while (in_next != in_end) {
                                        cache_ptr->length = 0;
                                        cache_ptr++;
                                        in_next++;
                                }
                        }
                }

                /* End the block if the match cache may overflow. */
                if (unlikely(cache_ptr >= &c->match_cache[CACHE_LENGTH]))
                        goto end_block;

                /* End the block if the soft maximum size has been reached. */
                if (in_next >= in_max_block_end)
                        goto end_block;

                /* End the block if the block splitting algorithm thinks this is
                 * a good place to do so. */
                if (c->split_stats.num_new_observations >=
                    NUM_OBSERVATIONS_PER_BLOCK_CHECK)
                {
                        if (in_max_block_end - in_next < MIN_BLOCK_SIZE)
                                next_observation = in_max_block_end;
                        else if (lzx_end_block_check(&c->split_stats))
                                goto end_block;
                }

                /* It's not time to end the block yet. Compute the next pause
                 * point and resume the matchfinding. */
                next_pause_point =
                        min(in_next + min(NUM_OBSERVATIONS_PER_BLOCK_CHECK * 2 -
                                            c->split_stats.num_new_observations,
                                          in_max_block_end - in_next),
                            in_max_block_end - min(LZX_MAX_MATCH_LEN - 1,
                                                   in_max_block_end - in_next));
                goto resume_matchfinding;

        end_block:
                /* We've decided on a block boundary and cached matches.  Now
                 * choose a match/literal sequence and flush the block. */
                queue = lzx_optimize_and_flush_block(c, os, in_block_begin,
                                                     in_next - in_block_begin,
                                                     queue, is_16_bit);
        } while (in_next != in_end);
}

static void
lzx_compress_near_optimal_16(struct lzx_compressor *c, const u8 *in,
                             size_t in_nbytes, struct lzx_output_bitstream *os)
{
        lzx_compress_near_optimal(c, in, in_nbytes, os, true);
}

static void
lzx_compress_near_optimal_32(struct lzx_compressor *c, const u8 *in,
                             size_t in_nbytes, struct lzx_output_bitstream *os)
{
        lzx_compress_near_optimal(c, in, in_nbytes, os, false);
}

/******************************************************************************/
/*                     Faster ("lazy") compression algorithm                  */
/*----------------------------------------------------------------------------*/

/*
 * Called when the compressor decides to use a literal.  This tallies the
 * Huffman symbol for the literal and increment the current literal run length.
 */
static inline void
lzx_choose_literal(struct lzx_compressor *c, unsigned literal, u32 *litrunlen_p)
{
        lzx_observe_literal(&c->split_stats, literal);
        c->freqs.main[literal]++;
        ++*litrunlen_p;
}

/*
 * Called when the compressor decides to use a match.  This tallies the Huffman
 * symbol(s) for a match, saves the match data and the length of the preceding
 * literal run, and updates the recent offsets queue.
 */
static inline void
lzx_choose_match(struct lzx_compressor *c, unsigned length, u32 offset_data,
                 u32 recent_offsets[LZX_NUM_RECENT_OFFSETS], bool is_16_bit,
                 u32 *litrunlen_p, struct lzx_sequence **next_seq_p)
{
        u32 litrunlen = *litrunlen_p;
        struct lzx_sequence *next_seq = *next_seq_p;
        unsigned offset_slot;
        unsigned v;

        lzx_observe_match(&c->split_stats, length);

        v = length - LZX_MIN_MATCH_LEN;

        /* Save the literal run length and adjusted length. */
        next_seq->litrunlen = litrunlen;
        next_seq->adjusted_length = v;

        /* Compute the length header, then tally the length symbol if needed. */
        if (v >= LZX_NUM_PRIMARY_LENS) {
                c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
                v = LZX_NUM_PRIMARY_LENS;
        }

        /* Compute the offset slot. */
        offset_slot = lzx_get_offset_slot(c, offset_data, is_16_bit);

        /* Compute the match header. */
        v += offset_slot * LZX_NUM_LEN_HEADERS;

        /* Save the adjusted offset and match header. */
        next_seq->adjusted_offset_and_match_hdr = (offset_data << 9) | v;

        /* Tally the main symbol. */
        c->freqs.main[LZX_NUM_CHARS + v]++;

        /* Update the recent offsets queue. */
        if (offset_data < LZX_NUM_RECENT_OFFSETS) {
                /* Repeat offset match. */
                swap(recent_offsets[0], recent_offsets[offset_data]);
        } else {
                /* Explicit offset match. */

                /* Tally the aligned offset symbol if needed. */
                if (offset_data >= 16)
                        c->freqs.aligned[offset_data & LZX_ALIGNED_OFFSET_BITMASK]++;

                recent_offsets[2] = recent_offsets[1];
                recent_offsets[1] = recent_offsets[0];
                recent_offsets[0] = offset_data - LZX_OFFSET_ADJUSTMENT;
        }

        /* Reset the literal run length and advance to the next sequence. */
        *next_seq_p = next_seq + 1;
        *litrunlen_p = 0;
}

/*
 * Called when the compressor ends a block.  This finshes the last lzx_sequence,
 * which is just a literal run which no following match.  This literal run might
 * be empty.
 */
static inline void
lzx_finish_sequence(struct lzx_sequence *last_seq, u32 litrunlen)
{
        last_seq->litrunlen = litrunlen;

        /* Special value to mark last sequence */
        last_seq->adjusted_offset_and_match_hdr = 0x80000000;
}

/*
 * Find the longest repeat offset match with the current position.  If a match
 * is found, return its length and set *best_rep_idx_ret to the index of its
 * offset in @recent_offsets.  Otherwise, return 0.
 *
 * Don't bother with length 2 matches; consider matches of length >= 3 only.
 */
static unsigned
lzx_find_longest_repeat_offset_match(const u8 * const in_next,
                                     const u32 recent_offsets[],
                                     const unsigned max_len,
                                     unsigned *best_rep_idx_ret)
{
        STATIC_ASSERT(LZX_NUM_RECENT_OFFSETS == 3); /* loop is unrolled */

        const u32 seq3 = load_u24_unaligned(in_next);
        const u8 *matchptr;
        unsigned best_rep_len;
        unsigned best_rep_idx;
        unsigned rep_len;

        /* Check for rep0 match (most recent offset) */
        matchptr = in_next - recent_offsets[0];
        if (load_u24_unaligned(matchptr) == seq3)
                best_rep_len = lz_extend(in_next, matchptr, 3, max_len);
        else
                best_rep_len = 0;
        best_rep_idx = 0;

        /* Check for rep1 match (second most recent offset) */
        matchptr = in_next - recent_offsets[1];
        if (load_u24_unaligned(matchptr) == seq3) {
                rep_len = lz_extend(in_next, matchptr, 3, max_len);
                if (rep_len > best_rep_len) {
                        best_rep_len = rep_len;
                        best_rep_idx = 1;
                }
        }

        /* Check for rep2 match (third most recent offset) */
        matchptr = in_next - recent_offsets[2];
        if (load_u24_unaligned(matchptr) == seq3) {
                rep_len = lz_extend(in_next, matchptr, 3, max_len);
                if (rep_len > best_rep_len) {
                        best_rep_len = rep_len;
                        best_rep_idx = 2;
                }
        }

        *best_rep_idx_ret = best_rep_idx;
        return best_rep_len;
}

/*
 * Fast heuristic scoring for lazy parsing: how "good" is this match?
 * This is mainly determined by the length: longer matches are better.
 * However, we also give a bonus to close (small offset) matches and to repeat
 * offset matches, since those require fewer bits to encode.
 */

static inline unsigned
lzx_explicit_offset_match_score(unsigned len, u32 adjusted_offset)
{
        unsigned score = len;

        if (adjusted_offset < 4096)
                score++;
        if (adjusted_offset < 256)
                score++;

        return score;
}

static inline unsigned
lzx_repeat_offset_match_score(unsigned rep_len, unsigned rep_idx)
{
        return rep_len + 3;
}

/*
 * This is the "lazy" LZX compressor.  The basic idea is that before it chooses
 * a match, it checks to see if there's a longer match at the next position.  If
 * yes, it chooses a literal and continues to the next position.  If no, it
 * chooses the match.
 *
 * Some additional heuristics are used as well.  Repeat offset matches are
 * considered favorably and sometimes are chosen immediately.  In addition, long
 * matches (at least "nice_len" bytes) are chosen immediately as well.  Finally,
 * when we decide whether a match is "better" than another, we take the offset
 * into consideration as well as the length.
 */
static inline void
lzx_compress_lazy(struct lzx_compressor * restrict c,
                  const u8 * const restrict in_begin, size_t in_nbytes,
                  struct lzx_output_bitstream * restrict os, bool is_16_bit)
{
        const u8 *       in_next = in_begin;
        const u8 * const in_end  = in_begin + in_nbytes;
        unsigned max_len = LZX_MAX_MATCH_LEN;
        unsigned nice_len = min(c->nice_match_length, max_len);
        STATIC_ASSERT(LZX_NUM_RECENT_OFFSETS == 3);
        u32 recent_offsets[3] = {1, 1, 1};
        u32 next_hashes[2] = {0, 0};

        /* Initialize the matchfinder. */
        CALL_HC_MF(is_16_bit, c, hc_matchfinder_init);

        do {
                /* Starting a new block */

                const u8 * const in_block_begin = in_next;
                const u8 * const in_max_block_end =
                        in_next + min(SOFT_MAX_BLOCK_SIZE, in_end - in_next);
                struct lzx_sequence *next_seq = c->chosen_sequences;
                u32 litrunlen = 0;
                unsigned cur_len;
                u32 cur_offset;
                u32 cur_offset_data;
                unsigned cur_score;
                unsigned next_len;
                u32 next_offset;
                u32 next_offset_data;
                unsigned next_score;
                unsigned best_rep_len;
                unsigned best_rep_idx;
                unsigned rep_score;
                unsigned skip_len;

                lzx_reset_symbol_frequencies(c);
                lzx_init_block_split_stats(&c->split_stats);

                do {
                        /* Adjust max_len and nice_len if we're nearing the end
                         * of the input buffer. */
                        if (unlikely(max_len > in_end - in_next)) {
                                max_len = in_end - in_next;
                                nice_len = min(max_len, nice_len);
                        }

                        /* Find the longest match (subject to the
                         * max_search_depth cutoff parameter) with the current
                         * position.  Don't bother with length 2 matches; only
                         * look for matches of length >= 3. */
                        cur_len = CALL_HC_MF(is_16_bit, c,
                                             hc_matchfinder_longest_match,
                                             in_begin,
                                             in_next - in_begin,
                                             2,
                                             max_len,
                                             nice_len,
                                             c->max_search_depth,
                                             next_hashes,
                                             &cur_offset);

                        /* If there was no match found, or the only match found
                         * was a distant short match, then choose a literal. */
                        if (cur_len < 3 ||
                            (cur_len == 3 &&
                             cur_offset >= 8192 - LZX_OFFSET_ADJUSTMENT &&
                             cur_offset != recent_offsets[0] &&
                             cur_offset != recent_offsets[1] &&
                             cur_offset != recent_offsets[2]))
                        {
                                lzx_choose_literal(c, *in_next, &litrunlen);
                                in_next++;
                                continue;
                        }

                        /* Heuristic: if this match has the most recent offset,
                         * then go ahead and choose it as a rep0 match. */
                        if (cur_offset == recent_offsets[0]) {
                                in_next++;
                                cur_offset_data = 0;
                                skip_len = cur_len - 1;
                                goto choose_cur_match;
                        }

                        /* Compute the longest match's score as an explicit
                         * offset match. */
                        cur_offset_data = cur_offset + LZX_OFFSET_ADJUSTMENT;
                        cur_score = lzx_explicit_offset_match_score(cur_len, cur_offset_data);

                        /* Find the longest repeat offset match at this
                         * position.  If we find one and it's "better" than the
                         * explicit offset match we found, then go ahead and
                         * choose the repeat offset match immediately. */
                        best_rep_len = lzx_find_longest_repeat_offset_match(in_next,
                                                                            recent_offsets,
                                                                            max_len,
                                                                            &best_rep_idx);
                        in_next++;

                        if (best_rep_len != 0 &&
                            (rep_score = lzx_repeat_offset_match_score(best_rep_len,
                                                                       best_rep_idx)) >= cur_score)
                        {
                                cur_len = best_rep_len;
                                cur_offset_data = best_rep_idx;
                                skip_len = best_rep_len - 1;
                                goto choose_cur_match;
                        }

                have_cur_match:
                        /*
                         * We have a match at the current position.  If the
                         * match is very long, then choose it immediately.
                         * Otherwise, see if there's a better match at the next
                         * position.
                         */

                        if (cur_len >= nice_len) {
                                skip_len = cur_len - 1;
                                goto choose_cur_match;
                        }

                        if (unlikely(max_len > in_end - in_next)) {
                                max_len = in_end - in_next;
                                nice_len = min(max_len, nice_len);
                        }

                        next_len = CALL_HC_MF(is_16_bit, c,
                                              hc_matchfinder_longest_match,
                                              in_begin,
                                              in_next - in_begin,
                                              cur_len - 2,
                                              max_len,
                                              nice_len,
                                              c->max_search_depth / 2,
                                              next_hashes,
                                              &next_offset);

                        if (next_len <= cur_len - 2) {
                                /* No potentially better match was found. */
                                in_next++;
                                skip_len = cur_len - 2;
                                goto choose_cur_match;
                        }

                        next_offset_data = next_offset + LZX_OFFSET_ADJUSTMENT;
                        next_score = lzx_explicit_offset_match_score(next_len, next_offset_data);

                        best_rep_len = lzx_find_longest_repeat_offset_match(in_next,
                                                                            recent_offsets,
                                                                            max_len,
                                                                            &best_rep_idx);
                        in_next++;

                        if (best_rep_len != 0 &&
                            (rep_score = lzx_repeat_offset_match_score(best_rep_len,
                                                                       best_rep_idx)) >= next_score)
                        {

                                if (rep_score > cur_score) {
                                        /* The next match is better, and it's a
                                         * repeat offset match. */
                                        lzx_choose_literal(c, *(in_next - 2),
                                                           &litrunlen);
                                        cur_len = best_rep_len;
                                        cur_offset_data = best_rep_idx;
                                        skip_len = cur_len - 1;
                                        goto choose_cur_match;
                                }
                        } else {
                                if (next_score > cur_score) {
                                        /* The next match is better, and it's an
                                         * explicit offset match. */
                                        lzx_choose_literal(c, *(in_next - 2),
                                                           &litrunlen);
                                        cur_len = next_len;
                                        cur_offset_data = next_offset_data;
                                        cur_score = next_score;
                                        goto have_cur_match;
                                }
                        }

                        /* The original match was better; choose it. */
                        skip_len = cur_len - 2;

                choose_cur_match:
                        /* Choose a match and have the matchfinder skip over its
                         * remaining bytes. */
                        lzx_choose_match(c, cur_len, cur_offset_data,
                                         recent_offsets, is_16_bit,
                                         &litrunlen, &next_seq);
                        in_next = CALL_HC_MF(is_16_bit, c,
                                             hc_matchfinder_skip_positions,
                                             in_begin,
                                             in_next - in_begin,
                                             in_end - in_begin,
                                             skip_len,
                                             next_hashes);

                        /* Keep going until it's time to end the block. */
                } while (in_next < in_max_block_end &&
                         !lzx_should_end_block(&c->split_stats, in_block_begin,
                                               in_next, in_end));

                /* Flush the block. */
                lzx_finish_sequence(next_seq, litrunlen);
                lzx_flush_block(c, os, in_block_begin, in_next - in_block_begin, 0);

                /* Keep going until we've reached the end of the input buffer. */
        } while (in_next != in_end);
}

static void
lzx_compress_lazy_16(struct lzx_compressor *c, const u8 *in, size_t in_nbytes,
                     struct lzx_output_bitstream *os)
{
        lzx_compress_lazy(c, in, in_nbytes, os, true);
}

static void
lzx_compress_lazy_32(struct lzx_compressor *c, const u8 *in, size_t in_nbytes,
                     struct lzx_output_bitstream *os)
{
        lzx_compress_lazy(c, in, in_nbytes, os, false);
}

/******************************************************************************/
/*                          Compressor operations                             */
/*----------------------------------------------------------------------------*/

/*
 * Generate tables for mapping match offsets (actually, "adjusted" match
 * offsets) to offset slots.
 */
static void
lzx_init_offset_slot_tabs(struct lzx_compressor *c)
{
        u32 adjusted_offset = 0;
        unsigned slot = 0;

        /* slots [0, 29] */
        for (; adjusted_offset < ARRAY_LEN(c->offset_slot_tab_1);
             adjusted_offset++)
        {
                if (adjusted_offset >= lzx_offset_slot_base[slot + 1])
                        slot++;
                c->offset_slot_tab_1[adjusted_offset] = slot;
        }

        /* slots [30, 49] */
        for (; adjusted_offset < LZX_MAX_WINDOW_SIZE;
             adjusted_offset += (u32)1 << 14)
        {
                if (adjusted_offset >= lzx_offset_slot_base[slot + 1])
                        slot++;
                c->offset_slot_tab_2[adjusted_offset >> 14] = slot;
        }
}

static size_t
lzx_get_compressor_size(size_t max_bufsize, unsigned compression_level)
{
        if (compression_level <= MAX_FAST_LEVEL) {
                if (lzx_is_16_bit(max_bufsize))
                        return offsetof(struct lzx_compressor, hc_mf_16) +
                               hc_matchfinder_size_16(max_bufsize);
                else
                        return offsetof(struct lzx_compressor, hc_mf_32) +
                               hc_matchfinder_size_32(max_bufsize);
        } else {
                if (lzx_is_16_bit(max_bufsize))
                        return offsetof(struct lzx_compressor, bt_mf_16) +
                               bt_matchfinder_size_16(max_bufsize);
                else
                        return offsetof(struct lzx_compressor, bt_mf_32) +
                               bt_matchfinder_size_32(max_bufsize);
        }
}

/* Compute the amount of memory needed to allocate an LZX compressor. */
static u64
lzx_get_needed_memory(size_t max_bufsize, unsigned compression_level,
                      bool destructive)
{
        u64 size = 0;

        if (max_bufsize > LZX_MAX_WINDOW_SIZE)
                return 0;

        size += lzx_get_compressor_size(max_bufsize, compression_level);
        if (!destructive)
                size += max_bufsize; /* account for in_buffer */
        return size;
}

/* Allocate an LZX compressor. */
static int
lzx_create_compressor(size_t max_bufsize, unsigned compression_level,
                      bool destructive, void **c_ret)
{
        unsigned window_order;
        struct lzx_compressor *c;

        /* Validate the maximum buffer size and get the window order from it. */
        window_order = lzx_get_window_order(max_bufsize);
        if (window_order == 0)
                return WIMLIB_ERR_INVALID_PARAM;

        /* Allocate the compressor. */
        c = MALLOC(lzx_get_compressor_size(max_bufsize, compression_level));
        if (!c)
                goto oom0;

        c->window_order = window_order;
        c->num_main_syms = lzx_get_num_main_syms(window_order);
        c->destructive = destructive;

        /* Allocate the buffer for preprocessed data if needed. */
        if (!c->destructive) {
                c->in_buffer = MALLOC(max_bufsize);
                if (!c->in_buffer)
                        goto oom1;
        }

        if (compression_level <= MAX_FAST_LEVEL) {

                /* Fast compression: Use lazy parsing. */
                if (lzx_is_16_bit(max_bufsize))
                        c->impl = lzx_compress_lazy_16;
                else
                        c->impl = lzx_compress_lazy_32;

                /* Scale max_search_depth and nice_match_length with the
                 * compression level. */
                c->max_search_depth = (60 * compression_level) / 20;
                c->nice_match_length = (80 * compression_level) / 20;

                /* lzx_compress_lazy() needs max_search_depth >= 2 because it
                 * halves the max_search_depth when attempting a lazy match, and
                 * max_search_depth must be at least 1. */
                c->max_search_depth = max(c->max_search_depth, 2);
        } else {

                /* Normal / high compression: Use near-optimal parsing. */
                if (lzx_is_16_bit(max_bufsize))
                        c->impl = lzx_compress_near_optimal_16;
                else
                        c->impl = lzx_compress_near_optimal_32;

                /* Scale max_search_depth and nice_match_length with the
                 * compression level. */
                c->max_search_depth = (24 * compression_level) / 50;
                c->nice_match_length = (48 * compression_level) / 50;

                /* Also scale num_optim_passes with the compression level.  But
                 * the more passes there are, the less they help --- so don't
                 * add them linearly.  */
                c->num_optim_passes = 1;
                c->num_optim_passes += (compression_level >= 45);
                c->num_optim_passes += (compression_level >= 70);
                c->num_optim_passes += (compression_level >= 100);
                c->num_optim_passes += (compression_level >= 150);
                c->num_optim_passes += (compression_level >= 200);
                c->num_optim_passes += (compression_level >= 300);

                /* max_search_depth must be at least 1. */
                c->max_search_depth = max(c->max_search_depth, 1);
        }

        /* Prepare the offset => offset slot mapping. */
        lzx_init_offset_slot_tabs(c);

        *c_ret = c;
        return 0;

oom1:
        FREE(c);
oom0:
        return WIMLIB_ERR_NOMEM;
}

/* Compress a buffer of data. */
static size_t
lzx_compress(const void *restrict in, size_t in_nbytes,
             void *restrict out, size_t out_nbytes_avail, void *restrict _c)
{
        struct lzx_compressor *c = _c;
        struct lzx_output_bitstream os;
        size_t result;

        /* Don't bother trying to compress very small inputs. */
        if (in_nbytes < 64)
                return 0;

        /* Copy the input data into the internal buffer if needed. */
        if (!c->destructive) {
                memcpy(c->in_buffer, in, in_nbytes);
                in = c->in_buffer;
        }

        /* Preprocess the input data. */
        lzx_preprocess((void *)in, in_nbytes);

        /* Initially, the previous Huffman codeword lengths are all zeroes. */
        c->codes_index = 0;
        memset(&c->codes[1].lens, 0, sizeof(struct lzx_lens));

        /* Initialize the output bitstream. */
        lzx_init_output(&os, out, out_nbytes_avail);

        /* Call the compression level-specific compress() function. */
        (*c->impl)(c, in, in_nbytes, &os);

        /* Flush the output bitstream. */
        result = lzx_flush_output(&os);

        /* If the data did not compress to less than its original size and we
         * preprocessed the original buffer, then postprocess it to restore it
         * to its original state. */
        if (result == 0 && c->destructive)
                lzx_postprocess((void *)in, in_nbytes);

        /* Return the number of compressed bytes, or 0 if the input did not
         * compress to less than its original size. */
        return result;
}

/* Free an LZX compressor. */
static void
lzx_free_compressor(void *_c)
{
        struct lzx_compressor *c = _c;

        if (!c->destructive)
                FREE(c->in_buffer);
        FREE(c);
}

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