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[wimlib] / src / lzx-compress.c

/*
 * lzx-compress.c
 *
 * LZX compression routines, originally based on code written by Matthew T.
 * Russotto (liblzxcomp), but heavily modified.
 */

/*
 * Copyright (C) 2002 Matthew T. Russotto
 * Copyright (C) 2012, 2013 Eric Biggers
 *
 * This file is part of wimlib, a library for working with WIM files.
 *
 * wimlib is free software; you can redistribute it and/or modify it under the
 * terms of the GNU General Public License as published by the Free
 * Software Foundation; either version 3 of the License, or (at your option)
 * any later version.
 *
 * wimlib is distributed in the hope that it will be useful, but WITHOUT ANY
 * WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
 * A PARTICULAR PURPOSE. See the GNU General Public License for more
 * details.
 *
 * You should have received a copy of the GNU General Public License
 * along with wimlib; if not, see http://www.gnu.org/licenses/.
 */


/*
 * This file provides lzx_compress(), a function to compress an in-memory buffer
 * of data using LZX compression, as used in the WIM file format.
 *
 * Please see the comments in lzx-decompress.c for more information about this
 * compression format.
 *
 * One thing to keep in mind is that there is no sliding window, since the
 * window is always the entirety of a WIM chunk, which is at most WIM_CHUNK_SIZE
 * ( = 32768) bytes.
 *
 * The basic compression algorithm used here should be familiar if you are
 * familiar with Huffman trees and with other LZ77 and Huffman-based formats
 * such as DEFLATE.  Otherwise it can be quite tricky to understand.  Basically
 * it is the following:
 *
 * - Preprocess the input data (LZX-specific)
 * - Go through the input data and determine matches.  This part is based on
 *       code from zlib, and a hash table of 3-character strings is used to
 *       accelerate the process of finding matches.
 * - Build the Huffman trees based on the frequencies of symbols determined
 *       while recording matches.
 * - Output the block header, including the Huffman trees; then output the
 *       compressed stream of matches and literal characters.
 *
 * It is possible for a WIM chunk to include multiple LZX blocks, since for some
 * input data this will produce a better compression ratio (especially since
 * each block can include new Huffman codes).  However, producing multiple LZX
 * blocks from one input chunk is not yet implemented.
 */

#include "lzx.h"
#include "compress.h"
#include <stdlib.h>
#include <string.h>


/* Structure to contain the Huffman codes for the main, length, and aligned
 * offset trees. */
struct lzx_codes {
        u16 main_codewords[LZX_MAINTREE_NUM_SYMBOLS];
        u8  main_lens[LZX_MAINTREE_NUM_SYMBOLS];

        u16 len_codewords[LZX_LENTREE_NUM_SYMBOLS];
        u8  len_lens[LZX_LENTREE_NUM_SYMBOLS];

        u16 aligned_codewords[LZX_ALIGNEDTREE_NUM_SYMBOLS];
        u8  aligned_lens[LZX_ALIGNEDTREE_NUM_SYMBOLS];
};

struct lzx_freq_tables {
        freq_t main_freq_table[LZX_MAINTREE_NUM_SYMBOLS];
        freq_t len_freq_table[LZX_LENTREE_NUM_SYMBOLS];
        freq_t aligned_freq_table[LZX_ALIGNEDTREE_NUM_SYMBOLS];
};

/* Returns the LZX position slot that corresponds to a given formatted offset.
 *
 * Logically, this returns the smallest i such that
 * formatted_offset >= lzx_position_base[i].
 *
 * The actual implementation below takes advantage of the regularity of the
 * numbers in the lzx_position_base array to calculate the slot directly from
 * the formatted offset without actually looking at the array.
 */
static inline unsigned lzx_get_position_slot(unsigned formatted_offset)
{
#if 0
        /*
         * Slots 36-49 (formatted_offset >= 262144) can be found by
         * (formatted_offset/131072) + 34 == (formatted_offset >> 17) + 34;
         * however, this check for formatted_offset >= 262144 is commented out
         * because WIM chunks cannot be that large.
         */
        if (formatted_offset >= 262144) {
                return (formatted_offset >> 17) + 34;
        } else
#endif
        {
                /* Note: this part here only works if:
                 *
                 *    2 <= formatted_offset < 655360
                 *
                 * It is < 655360 because the frequency of the position bases
                 * increases starting at the 655360 entry, and it is >= 2
                 * because the below calculation fails if the most significant
                 * bit is lower than the 2's place. */
                wimlib_assert(formatted_offset >= 2 && formatted_offset < 655360);
                unsigned mssb_idx = bsr32(formatted_offset);
                return (mssb_idx << 1) |
                        ((formatted_offset >> (mssb_idx - 1)) & 1);
        }
}

static u32 lzx_record_literal(u8 literal, void *__main_freq_tab)
{
        freq_t *main_freq_tab = __main_freq_tab;
        main_freq_tab[literal]++;
        return literal;
}

/* Constructs a match from an offset and a length, and updates the LRU queue and
 * the frequency of symbols in the main, length, and aligned offset alphabets.
 * The return value is a 32-bit number that provides the match in an
 * intermediate representation documented below. */
static u32 lzx_record_match(unsigned match_offset, unsigned match_len,
                            void *__freq_tabs, void *__queue)
{
        struct lzx_freq_tables *freq_tabs = __freq_tabs;
        struct lru_queue *queue = __queue;
        unsigned position_slot;
        unsigned position_footer = 0;
        u32 match;
        u32 len_header;
        u32 len_pos_header;
        unsigned len_footer;
        unsigned adjusted_match_len;

        wimlib_assert(match_len >= LZX_MIN_MATCH && match_len <= LZX_MAX_MATCH);
        wimlib_assert(match_offset != 0);

        /* If possible, encode this offset as a repeated offset. */
        if (match_offset == queue->R0) {
                position_slot = 0;
        } else if (match_offset == queue->R1) {
                swap(queue->R0, queue->R1);
                position_slot = 1;
        } else if (match_offset == queue->R2) {
                swap(queue->R0, queue->R2);
                position_slot = 2;
        } else {
                /* Not a repeated offset. */

                /* offsets of 0, 1, and 2 are reserved for the repeated offset
                 * codes, so non-repeated offsets must be encoded as 3+.  The
                 * minimum offset is 1, so encode the offsets offset by 2. */
                unsigned formatted_offset = match_offset + LZX_MIN_MATCH;

                queue->R2 = queue->R1;
                queue->R1 = queue->R0;
                queue->R0 = match_offset;

                /* The (now-formatted) offset will actually be encoded as a
                 * small position slot number that maps to a certain hard-coded
                 * offset (position base), followed by a number of extra bits---
                 * the position footer--- that are added to the position base to
                 * get the original formatted offset. */

                position_slot = lzx_get_position_slot(formatted_offset);
                position_footer = formatted_offset &
                                  ((1 << lzx_get_num_extra_bits(position_slot)) - 1);
        }

        adjusted_match_len = match_len - LZX_MIN_MATCH;

        /* Pack the position slot, position footer, and match length into an
         * intermediate representation.
         *
         * bits    description
         * ----    -----------------------------------------------------------
         *
         * 31      1 if a match, 0 if a literal.
         *
         * 30-25   position slot.  This can be at most 50, so it will fit in 6
         *         bits.
         *
         * 8-24    position footer.  This is the offset of the real formatted
         *         offset from the position base.  This can be at most 17 bits
         *         (since lzx_extra_bits[LZX_NUM_POSITION_SLOTS - 1] is 17).
         *
         * 0-7     length of match, offset by 2.  This can be at most
         *         (LZX_MAX_MATCH - 2) == 255, so it will fit in 8 bits.  */
        match = 0x80000000 |
                (position_slot << 25) |
                (position_footer << 8) |
                (adjusted_match_len);

        /* The match length must be at least 2, so let the adjusted match length
         * be the match length minus 2.
         *
         * If it is less than 7, the adjusted match length is encoded as a 3-bit
         * number offset by 2.  Otherwise, the 3-bit length header is all 1's
         * and the actual adjusted length is given as a symbol encoded with the
         * length tree, offset by 7.
         */
        if (adjusted_match_len < LZX_NUM_PRIMARY_LENS) {
                len_header = adjusted_match_len;
        } else {
                len_header = LZX_NUM_PRIMARY_LENS;
                len_footer = adjusted_match_len - LZX_NUM_PRIMARY_LENS;
                freq_tabs->len_freq_table[len_footer]++;
        }
        len_pos_header = (position_slot << 3) | len_header;

        wimlib_assert(len_pos_header < LZX_MAINTREE_NUM_SYMBOLS - LZX_NUM_CHARS);

        freq_tabs->main_freq_table[len_pos_header + LZX_NUM_CHARS]++;

        /* Equivalent to:
         * if (lzx_extra_bits[position_slot] >= 3) */
        if (position_slot >= 8)
                freq_tabs->aligned_freq_table[position_footer & 7]++;

        return match;
}

/*
 * Writes a compressed literal match to the output.
 *
 * @out:         The output bitstream.
 * @block_type:  The type of the block (LZX_BLOCKTYPE_ALIGNED or LZX_BLOCKTYPE_VERBATIM)
 * @match:       The match, encoded as a 32-bit number.
 * @codes:      Pointer to a structure that contains the codewords for the
 *                      main, length, and aligned offset Huffman codes.
 */
static int lzx_write_match(struct output_bitstream *out, int block_type,
                           u32 match, const struct lzx_codes *codes)
{
        /* low 8 bits are the match length minus 2 */
        unsigned match_len_minus_2 = match & 0xff;
        /* Next 17 bits are the position footer */
        unsigned position_footer = (match >> 8) & 0x1ffff;      /* 17 bits */
        /* Next 6 bits are the position slot. */
        unsigned position_slot = (match >> 25) & 0x3f;  /* 6 bits */
        unsigned len_header;
        unsigned len_footer;
        unsigned len_pos_header;
        unsigned main_symbol;
        unsigned num_extra_bits;
        unsigned verbatim_bits;
        unsigned aligned_bits;
        int ret;

        /* If the match length is less than MIN_MATCH (= 2) +
         * NUM_PRIMARY_LENS (= 7), the length header contains
         * the match length minus MIN_MATCH, and there is no
         * length footer.
         *
         * Otherwise, the length header contains
         * NUM_PRIMARY_LENS, and the length footer contains
         * the match length minus NUM_PRIMARY_LENS minus
         * MIN_MATCH. */
        if (match_len_minus_2 < LZX_NUM_PRIMARY_LENS) {
                len_header = match_len_minus_2;
                /* No length footer-- mark it with a special
                 * value. */
                len_footer = (unsigned)(-1);
        } else {
                len_header = LZX_NUM_PRIMARY_LENS;
                len_footer = match_len_minus_2 - LZX_NUM_PRIMARY_LENS;
        }

        /* Combine the position slot with the length header into
         * a single symbol that will be encoded with the main
         * tree. */
        len_pos_header = (position_slot << 3) | len_header;

        /* The actual main symbol is offset by LZX_NUM_CHARS because
         * values under LZX_NUM_CHARS are used to indicate a literal
         * byte rather than a match. */
        main_symbol = len_pos_header + LZX_NUM_CHARS;

        /* Output main symbol. */
        ret = bitstream_put_bits(out, codes->main_codewords[main_symbol],
                                 codes->main_lens[main_symbol]);
        if (ret != 0)
                return ret;

        /* If there is a length footer, output it using the
         * length Huffman code. */
        if (len_footer != (unsigned)(-1)) {
                ret = bitstream_put_bits(out, codes->len_codewords[len_footer],
                                         codes->len_lens[len_footer]);
                if (ret != 0)
                        return ret;
        }

        wimlib_assert(position_slot < LZX_NUM_POSITION_SLOTS);

        num_extra_bits = lzx_get_num_extra_bits(position_slot);

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

                verbatim_bits = position_footer >> 3;
                ret = bitstream_put_bits(out, verbatim_bits,
                                         num_extra_bits - 3);
                if (ret != 0)
                        return ret;

                aligned_bits = (position_footer & 7);
                ret = bitstream_put_bits(out,
                                         codes->aligned_codewords[aligned_bits],
                                         codes->aligned_lens[aligned_bits]);
                if (ret != 0)
                        return ret;
        } else {
                /* verbatim bits is the same as the position
                 * footer, in this case. */
                ret = bitstream_put_bits(out, position_footer, num_extra_bits);
                if (ret != 0)
                        return ret;
        }
        return 0;
}

/*
 * Writes all compressed literals in a block, both matches and literal bytes, to
 * the output bitstream.
 *
 * @out:         The output bitstream.
 * @block_type:  The type of the block (LZX_BLOCKTYPE_ALIGNED or LZX_BLOCKTYPE_VERBATIM)
 * @match_tab[]:   The array of matches that will be output.  It has length
 *                      of @num_compressed_literals.
 * @num_compressed_literals:  Number of compressed literals to be output.
 * @codes:      Pointer to a structure that contains the codewords for the
 *                      main, length, and aligned offset Huffman codes.
 */
static int lzx_write_compressed_literals(struct output_bitstream *ostream,
                                         int block_type,
                                         const u32 match_tab[],
                                         unsigned  num_compressed_literals,
                                         const struct lzx_codes *codes)
{
        unsigned i;
        u32 match;
        int ret;

        for (i = 0; i < num_compressed_literals; i++) {
                match = match_tab[i];

                /* High bit of the match indicates whether the match is an
                 * actual match (1) or a literal uncompressed byte (0) */
                if (match & 0x80000000) {
                        /* match */
                        ret = lzx_write_match(ostream, block_type, match,
                                              codes);
                        if (ret != 0)
                                return ret;
                } else {
                        /* literal byte */
                        wimlib_assert(match < LZX_NUM_CHARS);
                        ret = bitstream_put_bits(ostream,
                                                 codes->main_codewords[match],
                                                 codes->main_lens[match]);
                        if (ret != 0)
                                return ret;
                }
        }
        return 0;
}

/*
 * Writes a compressed Huffman tree to the output, preceded by the pretree for
 * it.
 *
 * The Huffman tree is represented in the output as a series of path lengths
 * from which the canonical Huffman code can be reconstructed.  The path lengths
 * themselves are compressed using a separate Huffman code, the pretree, which
 * consists of LZX_PRETREE_NUM_SYMBOLS (= 20) symbols that cover all possible code
 * lengths, plus extra codes for repeated lengths.  The path lengths of the
 * pretree precede the path lengths of the larger code and are uncompressed,
 * consisting of 20 entries of 4 bits each.
 *
 * @out:        The bitstream for the compressed output.
 * @lens:       The code lengths for the Huffman tree, indexed by symbol.
 * @num_symbols:        The number of symbols in the code.
 */
static int lzx_write_compressed_tree(struct output_bitstream *out,
                                     const u8 lens[], unsigned num_symbols)
{
        /* Frequencies of the length symbols, including the RLE symbols (NOT the
         * actual lengths themselves). */
        freq_t pretree_freqs[LZX_PRETREE_NUM_SYMBOLS];
        u8 pretree_lens[LZX_PRETREE_NUM_SYMBOLS];
        u16 pretree_codewords[LZX_PRETREE_NUM_SYMBOLS];
        u8 output_syms[num_symbols * 2];
        unsigned output_syms_idx;
        unsigned cur_run_len;
        unsigned i;
        unsigned len_in_run;
        unsigned additional_bits;
        char delta;
        u8 pretree_sym;

        ZERO_ARRAY(pretree_freqs);

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

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

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

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

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

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

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

                        /* The magic length 17 indicates a run of 4 + n zeroes,
                         * where n is an uncompressed literal 4-bit integer that
                         * follows the magic length. */
                        while (cur_run_len >= 4) {
                                additional_bits = min(cur_run_len - 4, 0xf);
                                pretree_freqs[17]++;
                                output_syms[output_syms_idx++] = 17;
                                output_syms[output_syms_idx++] = additional_bits;
                                cur_run_len -= 4 + additional_bits;
                        }

                } else {

                        /* A run of nonzero lengths. */

                        /* The magic length 19 indicates a run of 4 + n
                         * nonzeroes, where n is a literal bit that follows the
                         * magic length, and where the value of the lengths in
                         * the run is given by an extra length symbol, encoded
                         * with the pretree, that follows the literal bit.
                         *
                         * The extra length symbol is encoded as a difference
                         * from the length of the codeword for the first symbol
                         * in the run in the previous tree.
                         * */
                        while (cur_run_len >= 4) {
                                additional_bits = (cur_run_len > 4);
                                delta = -(char)len_in_run;
                                if (delta < 0)
                                        delta += 17;
                                pretree_freqs[19]++;
                                pretree_freqs[(unsigned char)delta]++;
                                output_syms[output_syms_idx++] = 19;
                                output_syms[output_syms_idx++] = additional_bits;
                                output_syms[output_syms_idx++] = delta;
                                cur_run_len -= 4 + additional_bits;
                        }
                }

                /* Any remaining lengths in the run are outputted without RLE,
                 * as a difference from the length of that codeword in the
                 * previous tree. */
                while (cur_run_len--) {
                        delta = -(char)len_in_run;
                        if (delta < 0)
                                delta += 17;

                        pretree_freqs[(unsigned char)delta]++;
                        output_syms[output_syms_idx++] = delta;
                }

                cur_run_len = 1;
        }

        wimlib_assert(output_syms_idx < ARRAY_LEN(output_syms));

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

        make_canonical_huffman_code(LZX_PRETREE_NUM_SYMBOLS,
                                    LZX_MAX_CODEWORD_LEN,
                                    pretree_freqs, pretree_lens,
                                    pretree_codewords);

        /* Write the lengths of the pretree codes to the output. */
        for (i = 0; i < LZX_PRETREE_NUM_SYMBOLS; i++)
                bitstream_put_bits(out, pretree_lens[i],
                                   LZX_PRETREE_ELEMENT_SIZE);

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

        i = 0;
        while (i < output_syms_idx) {
                pretree_sym = output_syms[i++];

                bitstream_put_bits(out, pretree_codewords[pretree_sym],
                                   pretree_lens[pretree_sym]);
                switch (pretree_sym) {
                case 17:
                        bitstream_put_bits(out, output_syms[i++], 4);
                        break;
                case 18:
                        bitstream_put_bits(out, output_syms[i++], 5);
                        break;
                case 19:
                        bitstream_put_bits(out, output_syms[i++], 1);
                        bitstream_put_bits(out,
                                           pretree_codewords[output_syms[i]],
                                           pretree_lens[output_syms[i]]);
                        i++;
                        break;
                default:
                        break;
                }
        }
        return 0;
}

/* Builds the canonical Huffman code for the main tree, the length tree, and the
 * aligned offset tree. */
static void lzx_make_huffman_codes(const struct lzx_freq_tables *freq_tabs,
                                struct lzx_codes *codes)
{
        make_canonical_huffman_code(LZX_MAINTREE_NUM_SYMBOLS,
                                        LZX_MAX_CODEWORD_LEN,
                                        freq_tabs->main_freq_table,
                                        codes->main_lens,
                                        codes->main_codewords);

        make_canonical_huffman_code(LZX_LENTREE_NUM_SYMBOLS,
                                        LZX_MAX_CODEWORD_LEN,
                                        freq_tabs->len_freq_table,
                                        codes->len_lens,
                                        codes->len_codewords);

        make_canonical_huffman_code(LZX_ALIGNEDTREE_NUM_SYMBOLS, 8,
                                        freq_tabs->aligned_freq_table,
                                        codes->aligned_lens,
                                        codes->aligned_codewords);
}

static void do_call_insn_translation(u32 *call_insn_target, int input_pos,
                                     int32_t file_size)
{
        int32_t abs_offset;
        int32_t rel_offset;

        rel_offset = le32_to_cpu(*call_insn_target);
        if (rel_offset >= -input_pos && rel_offset < file_size) {
                if (rel_offset < file_size - input_pos) {
                        /* "good translation" */
                        abs_offset = rel_offset + input_pos;
                } else {
                        /* "compensating translation" */
                        abs_offset = rel_offset - file_size;
                }
                *call_insn_target = cpu_to_le32(abs_offset);
        }
}

/* This is the reverse of undo_call_insn_preprocessing() in lzx-decompress.c.
 * See the comment above that function for more information. */
static void do_call_insn_preprocessing(u8 uncompressed_data[],
                                       int uncompressed_data_len)
{
        for (int i = 0; i < uncompressed_data_len - 10; i++) {
                if (uncompressed_data[i] == 0xe8) {
                        do_call_insn_translation((u32*)&uncompressed_data[i + 1],
                                                 i,
                                                 LZX_WIM_MAGIC_FILESIZE);
                        i += 4;
                }
        }
}


static const struct lz_params lzx_lz_params = {

         /* LZX_MIN_MATCH == 2, but 2-character matches are rarely useful; the
          * minimum match for compression is set to 3 instead. */
        .min_match      = 3,

        .max_match      = LZX_MAX_MATCH,
        .good_match     = LZX_MAX_MATCH,
        .nice_match     = LZX_MAX_MATCH,
        .max_chain_len  = LZX_MAX_MATCH,
        .max_lazy_match = LZX_MAX_MATCH,
        .too_far        = 4096,
};

/*
 * Performs LZX compression on a block of data.
 *
 * @__uncompressed_data:  Pointer to the data to be compressed.
 * @uncompressed_len:     Length, in bytes, of the data to be compressed.
 * @compressed_data:      Pointer to a location at least (@uncompressed_len - 1)
 *                              bytes long into which the compressed data may be
 *                              written.
 * @compressed_len_ret:   A pointer to an unsigned int into which the length of
 *                              the compressed data may be returned.
 *
 * Returns zero if compression was successfully performed.  In that case
 * @compressed_data and @compressed_len_ret will contain the compressed data and
 * its length.  A return value of nonzero means that compressing the data did
 * not reduce its size, and @compressed_data will not contain the full
 * compressed data.
 */
int lzx_compress(const void *__uncompressed_data, unsigned uncompressed_len,
                 void *compressed_data, unsigned *compressed_len_ret)
{
        struct output_bitstream ostream;
        u8 uncompressed_data[uncompressed_len + 8];
        struct lzx_freq_tables freq_tabs;
        struct lzx_codes codes;
        u32 match_tab[uncompressed_len];
        struct lru_queue queue;
        unsigned num_matches;
        unsigned compressed_len;
        unsigned i;
        int ret;
        int block_type = LZX_BLOCKTYPE_ALIGNED;

        if (uncompressed_len < 100)
                return 1;

        memset(&freq_tabs, 0, sizeof(freq_tabs));
        queue.R0 = 1;
        queue.R1 = 1;
        queue.R2 = 1;

        /* The input data must be preprocessed. To avoid changing the original
         * input, copy it to a temporary buffer. */
        memcpy(uncompressed_data, __uncompressed_data, uncompressed_len);

        /* Before doing any actual compression, do the call instruction (0xe8
         * byte) translation on the uncompressed data. */
        do_call_insn_preprocessing(uncompressed_data, uncompressed_len);

        /* Determine the sequence of matches and literals that will be output,
         * and in the process, keep counts of the number of times each symbol
         * will be output, so that the Huffman trees can be made. */

        num_matches = lz_analyze_block(uncompressed_data, uncompressed_len,
                                       match_tab, lzx_record_match,
                                       lzx_record_literal, &freq_tabs,
                                       &queue, freq_tabs.main_freq_table,
                                       &lzx_lz_params);

        lzx_make_huffman_codes(&freq_tabs, &codes);

        /* Initialize the output bitstream. */
        init_output_bitstream(&ostream, compressed_data, uncompressed_len - 1);

        /* The first three bits tell us what kind of block it is, and are one
         * of the LZX_BLOCKTYPE_* values.  */
        bitstream_put_bits(&ostream, block_type, 3);

        /* The next bit indicates whether the block size is the default (32768),
         * indicated by a 1 bit, or whether the block size is given by the next
         * 16 bits, indicated by a 0 bit. */
        if (uncompressed_len == 32768) {
                bitstream_put_bits(&ostream, 1, 1);
        } else {
                bitstream_put_bits(&ostream, 0, 1);
                bitstream_put_bits(&ostream, uncompressed_len, 16);
        }

        /* Write out the aligned offset tree. Note that M$ lies and says that
         * the aligned offset tree comes after the length tree, but that is
         * wrong; it actually is before the main tree.  */
        if (block_type == LZX_BLOCKTYPE_ALIGNED)
                for (i = 0; i < LZX_ALIGNEDTREE_NUM_SYMBOLS; i++)
                        bitstream_put_bits(&ostream, codes.aligned_lens[i],
                                           LZX_ALIGNEDTREE_ELEMENT_SIZE);

        /* Write the pre-tree and lengths for the first LZX_NUM_CHARS symbols in the
         * main tree. */
        ret = lzx_write_compressed_tree(&ostream, codes.main_lens,
                                        LZX_NUM_CHARS);
        if (ret != 0)
                return ret;

        /* Write the pre-tree and symbols for the rest of the main tree. */
        ret = lzx_write_compressed_tree(&ostream, codes.main_lens +
                                        LZX_NUM_CHARS,
                                        LZX_MAINTREE_NUM_SYMBOLS -
                                                LZX_NUM_CHARS);
        if (ret != 0)
                return ret;

        /* Write the pre-tree and symbols for the length tree. */
        ret = lzx_write_compressed_tree(&ostream, codes.len_lens,
                                        LZX_LENTREE_NUM_SYMBOLS);
        if (ret != 0)
                return ret;

        /* Write the compressed literals. */
        ret = lzx_write_compressed_literals(&ostream, block_type,
                                            match_tab, num_matches, &codes);
        if (ret != 0)
                return ret;

        ret = flush_output_bitstream(&ostream);
        if (ret != 0)
                return ret;

        compressed_len = ostream.bit_output - (u8*)compressed_data;

        *compressed_len_ret = compressed_len;

#ifdef ENABLE_VERIFY_COMPRESSION
        /* Verify that we really get the same thing back when decompressing. */
        u8 buf[uncompressed_len];
        ret = lzx_decompress(compressed_data, compressed_len, buf,
                             uncompressed_len);
        if (ret != 0) {
                ERROR("lzx_compress(): Failed to decompress data we compressed");
                abort();
        }

        for (i = 0; i < uncompressed_len; i++) {
                if (buf[i] != *((u8*)__uncompressed_data + i)) {
                        ERROR("lzx_compress(): Data we compressed didn't "
                              "decompress to the original data (difference at "
                              "byte %u of %u)", i + 1, uncompressed_len);
                        abort();
                }
        }
#endif
        return 0;
}