[wimlib] / src / lzx-decompress.c

/*
 * lzx-decompress.c
 *
 * LZX decompression routines, originally based on code taken from cabextract
 * v0.5, which was, itself, a modified version of the lzx decompression code
 * from unlzx.
 */

/*
 * 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/.
 */

/*
 * LZX is a LZ77 and Huffman-code based compression format that has many
 * similarities to the DEFLATE format used in zlib.  The compression ratio is as
 * good or better than DEFLATE.
 *
 * Some notes on the LZX compression format as used in Windows Imaging (WIM)
 * files:
 *
 * A compressed WIM resource consists of a table of chunk offsets followed by
 * the compressed chunks themselves.  All compressed chunks except possibly the
 * last decompress to a fixed number of bytes, by default 32768.  This is quite
 * similar to the cabinet (.cab) file format, but they are not the same.
 * According to the cabinet format documentation, the LZX block size is
 * independent from the CFDATA blocks, and a LZX block may span several CFDATA
 * blocks.  However, in WIMs, LZX blocks do not appear to ever span multiple WIM
 * chunks.  Note that this means any WIM chunk may be decompressed or compressed
 * independently from any other chunk, which allows random access.
 *
 * A LZX compressed WIM chunk contains one or more LZX blocks of the aligned,
 * verbatim, or uncompressed block types.  For aligned and verbatim blocks, the
 * size of the block in uncompressed bytes is specified by a bit following the 3
 * bits that specify the block type, possibly followed by an additional 16 bits.
 * '1' means to use the default block size (equal to 32768, the default size of
 * a WIM chunk), while '0' means that the block size is provided by the next 16
 * bits.
 *
 * The cabinet format, as documented, allows for the possibility that a
 * compressed CFDATA chunk is up to 6144 bytes larger than the data it
 * uncompresses to.  However, in the WIM format it appears that every chunk that
 * would be 32768 bytes or more when compressed is actually stored fully
 * uncompressed.
 *
 * The 'e8' preprocessing step that changes x86 call instructions to use
 * absolute offsets instead of relative offsets relies on a filesize parameter.
 * There is no such parameter for this in the WIM files (even though the size of
 * the file resource could be used for this purpose), and instead a magic file
 * size of 12000000 is used.  The 'e8' preprocessing is always done, and there
 * is no bit to indicate whether it is done or not.
 */

/*
 * Some more notes about errors in Microsoft's LZX documentation:
 *
 * Microsoft's LZX document and their implementation of the com.ms.util.cab Java
 * package do not concur.
 *
 * In the LZX document, there is a table showing the correlation between window
 * size and the number of position slots. It states that the 1MB window = 40
 * slots and the 2MB window = 42 slots. In the implementation, 1MB = 42 slots,
 * 2MB = 50 slots. The actual calculation is 'find the first slot whose position
 * base is equal to or more than the required window size'. This would explain
 * why other tables in the document refer to 50 slots rather than 42.
 *
 * The constant NUM_PRIMARY_LENS used in the decompression pseudocode is not
 * defined in the specification.
 *
 * The LZX document states that aligned offset blocks have their aligned offset
 * Huffman tree AFTER the main and length trees. The implementation suggests
 * that the aligned offset tree is BEFORE the main and length trees.
 *
 * The LZX document decoding algorithm states that, in an aligned offset block,
 * if an extra_bits value is 1, 2 or 3, then that number of bits should be read
 * and the result added to the match offset. This is correct for 1 and 2, but
 * not 3, where just a Huffman symbol (using the aligned tree) should be read.
 *
 * Regarding the E8 preprocessing, the LZX document states 'No translation may
 * be performed on the last 6 bytes of the input block'. This is correct.
 * However, the pseudocode provided checks for the *E8 leader* up to the last 6
 * bytes. If the leader appears between -10 and -7 bytes from the end, this
 * would cause the next four bytes to be modified, at least one of which would
 * be in the last 6 bytes, which is not allowed according to the spec.
 *
 * The specification states that the Huffman trees must always contain at least
 * one element. However, many CAB files contain blocks where the length tree is
 * completely empty (because there are no matches), and this is expected to
 * succeed.
 */

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

#include "wimlib.h"
#include "wimlib/decompress.h"
#include "wimlib/lzx.h"
#include "wimlib/util.h"

#include <string.h>

/* Huffman decoding tables and maps from symbols to code lengths. */
struct lzx_tables {

        u16 maintree_decode_table[(1 << LZX_MAINCODE_TABLEBITS) +
                                        (LZX_MAINCODE_MAX_NUM_SYMBOLS * 2)]
                                        _aligned_attribute(DECODE_TABLE_ALIGNMENT);
        u8 maintree_lens[LZX_MAINCODE_MAX_NUM_SYMBOLS];


        u16 lentree_decode_table[(1 << LZX_LENCODE_TABLEBITS) +
                                        (LZX_LENCODE_NUM_SYMBOLS * 2)]
                                        _aligned_attribute(DECODE_TABLE_ALIGNMENT);
        u8 lentree_lens[LZX_LENCODE_NUM_SYMBOLS];


        u16 alignedtree_decode_table[(1 << LZX_ALIGNEDCODE_TABLEBITS) +
                                        (LZX_ALIGNEDCODE_NUM_SYMBOLS * 2)]
                                        _aligned_attribute(DECODE_TABLE_ALIGNMENT);
        u8 alignedtree_lens[LZX_ALIGNEDCODE_NUM_SYMBOLS];
} _aligned_attribute(DECODE_TABLE_ALIGNMENT);


/*
 * Reads a Huffman-encoded symbol using the pre-tree.
 */
static inline int
read_huffsym_using_pretree(struct input_bitstream *istream,
                           const u16 pretree_decode_table[],
                           const u8 pretree_lens[], unsigned *n)
{
        return read_huffsym(istream, pretree_decode_table, pretree_lens,
                            LZX_PRECODE_NUM_SYMBOLS, LZX_PRECODE_TABLEBITS, n,
                            LZX_MAX_PRE_CODEWORD_LEN);
}

/* Reads a Huffman-encoded symbol using the main tree. */
static inline int
read_huffsym_using_maintree(struct input_bitstream *istream,
                            const struct lzx_tables *tables,
                            unsigned *n,
                            unsigned num_main_syms)
{
        return read_huffsym(istream, tables->maintree_decode_table,
                            tables->maintree_lens, num_main_syms,
                            LZX_MAINCODE_TABLEBITS, n, LZX_MAX_MAIN_CODEWORD_LEN);
}

/* Reads a Huffman-encoded symbol using the length tree. */
static inline int
read_huffsym_using_lentree(struct input_bitstream *istream,
                           const struct lzx_tables *tables,
                           unsigned *n)
{
        return read_huffsym(istream, tables->lentree_decode_table,
                            tables->lentree_lens, LZX_LENCODE_NUM_SYMBOLS,
                            LZX_LENCODE_TABLEBITS, n, LZX_MAX_LEN_CODEWORD_LEN);
}

/* Reads a Huffman-encoded symbol using the aligned offset tree. */
static inline int
read_huffsym_using_alignedtree(struct input_bitstream *istream,
                               const struct lzx_tables *tables,
                               unsigned *n)
{
        return read_huffsym(istream, tables->alignedtree_decode_table,
                            tables->alignedtree_lens,
                            LZX_ALIGNEDCODE_NUM_SYMBOLS,
                            LZX_ALIGNEDCODE_TABLEBITS, n,
                            LZX_MAX_ALIGNED_CODEWORD_LEN);
}

/*
 * Reads the pretree from the input, then uses the pretree to decode @num_lens
 * code length values from the input.
 *
 * @istream:    The bit stream for the input.  It is positioned on the beginning
 *                      of the pretree for the code length values.
 * @lens:       An array that contains the length values from the previous time
 *                      the code lengths for this Huffman tree were read, or all
 *                      0's if this is the first time.
 * @num_lens:   Number of length values to decode and return.
 *
 */
static int
lzx_read_code_lens(struct input_bitstream *istream, u8 lens[],
                   unsigned num_lens)
{
        /* Declare the decoding table and length table for the pretree. */
        u16 pretree_decode_table[(1 << LZX_PRECODE_TABLEBITS) +
                                        (LZX_PRECODE_NUM_SYMBOLS * 2)]
                                        _aligned_attribute(DECODE_TABLE_ALIGNMENT);
        u8 pretree_lens[LZX_PRECODE_NUM_SYMBOLS];
        unsigned i;
        u32 len;
        int ret;

        /* Read the code lengths of the pretree codes.  There are 20 lengths of
         * 4 bits each. */
        for (i = 0; i < LZX_PRECODE_NUM_SYMBOLS; i++) {
                ret = bitstream_read_bits(istream, LZX_PRECODE_ELEMENT_SIZE,
                                          &len);
                if (ret)
                        return ret;
                pretree_lens[i] = len;
        }

        /* Make the decoding table for the pretree. */
        ret = make_huffman_decode_table(pretree_decode_table,
                                        LZX_PRECODE_NUM_SYMBOLS,
                                        LZX_PRECODE_TABLEBITS,
                                        pretree_lens,
                                        LZX_MAX_PRE_CODEWORD_LEN);
        if (ret)
                return ret;

        /* Pointer past the last length value that needs to be filled in. */
        u8 *lens_end = lens + num_lens;

        while (1) {

                /* Decode a symbol from the input.  If the symbol is between 0
                 * and 16, it is the difference from the old length.  If it is
                 * between 17 and 19, it is a special code that indicates that
                 * some number of the next lengths are all 0, or some number of
                 * the next lengths are all equal to the next symbol in the
                 * input. */
                unsigned tree_code;
                u32 num_zeroes;
                unsigned code;
                u32 num_same;
                signed char value;

                ret = read_huffsym_using_pretree(istream, pretree_decode_table,
                                                 pretree_lens, &tree_code);
                if (ret)
                        return ret;
                switch (tree_code) {
                case 17: /* Run of 0's */
                        ret = bitstream_read_bits(istream, 4, &num_zeroes);
                        if (ret)
                                return ret;
                        num_zeroes += 4;
                        while (num_zeroes--) {
                                *lens = 0;
                                if (++lens == lens_end)
                                        return 0;
                        }
                        break;
                case 18: /* Longer run of 0's */
                        ret = bitstream_read_bits(istream, 5, &num_zeroes);
                        if (ret)
                                return ret;
                        num_zeroes += 20;
                        while (num_zeroes--) {
                                *lens = 0;
                                if (++lens == lens_end)
                                        return 0;
                        }
                        break;
                case 19: /* Run of identical lengths */
                        ret = bitstream_read_bits(istream, 1, &num_same);
                        if (ret)
                                return ret;
                        num_same += 4;
                        ret = read_huffsym_using_pretree(istream,
                                                         pretree_decode_table,
                                                         pretree_lens,
                                                         &code);
                        if (ret)
                                return ret;
                        value = (signed char)*lens - (signed char)code;
                        if (value < 0)
                                value += 17;
                        while (num_same--) {
                                *lens = value;
                                if (++lens == lens_end)
                                        return 0;
                        }
                        break;
                default: /* Difference from old length. */
                        value = (signed char)*lens - (signed char)tree_code;
                        if (value < 0)
                                value += 17;
                        *lens = value;
                        if (++lens == lens_end)
                                return 0;
                        break;
                }
        }
}

/*
 * Reads the header for an LZX-compressed block.
 *
 * @istream:            The input bitstream.
 * @block_size_ret:     A pointer to an int into which the size of the block,
 *                              in bytes, will be returned.
 * @block_type_ret:     A pointer to an int into which the type of the block
 *                              (LZX_BLOCKTYPE_*) will be returned.
 * @tables:             A pointer to a lzx_tables structure in which the
 *                              main tree, the length tree, and possibly the
 *                              aligned offset tree will be constructed.
 * @queue:      A pointer to the least-recently-used queue into which
 *                      R0, R1, and R2 will be written (only for uncompressed
 *                      blocks, which contain this information in the header)
 */
static int
lzx_read_block_header(struct input_bitstream *istream,
                      unsigned num_main_syms,
                      unsigned max_window_size,
                      unsigned *block_size_ret,
                      unsigned *block_type_ret,
                      struct lzx_tables *tables,
                      struct lzx_lru_queue *queue)
{
        int ret;
        unsigned block_type;
        unsigned block_size;

        ret = bitstream_ensure_bits(istream, 4);
        if (ret)
                return ret;

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

        /* Read the block size.  This mirrors the behavior
         * lzx_write_compressed_block() in lzx-compress.c; see that for more
         * details.  */
        if (bitstream_read_bits_nocheck(istream, 1)) {
                block_size = LZX_DEFAULT_BLOCK_SIZE;
        } else {
                u32 tmp;
                block_size = 0;

                ret = bitstream_read_bits(istream, 8, &tmp);
                if (ret)
                        return ret;
                block_size |= tmp;

                ret = bitstream_read_bits(istream, 8, &tmp);
                if (ret)
                        return ret;
                block_size <<= 8;
                block_size |= tmp;

                if (max_window_size >= 65536) {
                        ret = bitstream_read_bits(istream, 8, &tmp);
                        if (ret)
                                return ret;
                        block_size <<= 8;
                        block_size |= tmp;
                }
        }

        switch (block_type) {
        case LZX_BLOCKTYPE_ALIGNED:
                /* Read the path lengths for the elements of the aligned tree,
                 * then build it. */

                for (unsigned i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
                        u32 len;

                        ret = bitstream_read_bits(istream,
                                                  LZX_ALIGNEDCODE_ELEMENT_SIZE,
                                                  &len);
                        if (ret)
                                return ret;
                        tables->alignedtree_lens[i] = len;
                }

                LZX_DEBUG("Building the aligned tree.");
                ret = make_huffman_decode_table(tables->alignedtree_decode_table,
                                                LZX_ALIGNEDCODE_NUM_SYMBOLS,
                                                LZX_ALIGNEDCODE_TABLEBITS,
                                                tables->alignedtree_lens,
                                                LZX_MAX_ALIGNED_CODEWORD_LEN);
                if (ret) {
                        LZX_DEBUG("Failed to make the decode table for the "
                                  "aligned offset tree");
                        return ret;
                }

                /* Fall though, since the rest of the header for aligned offset
                 * blocks is the same as that for verbatim blocks */

        case LZX_BLOCKTYPE_VERBATIM:
                if (block_type == LZX_BLOCKTYPE_VERBATIM)
                        LZX_DEBUG("Found verbatim block.");

                LZX_DEBUG("Reading path lengths for main tree.");
                /* Read the path lengths for the first 256 elements of the main
                 * tree. */
                ret = lzx_read_code_lens(istream, tables->maintree_lens,
                                         LZX_NUM_CHARS);
                if (ret) {
                        LZX_DEBUG("Failed to read the code lengths for the "
                                  "first 256 elements of the main tree");
                        return ret;
                }

                /* Read the path lengths for the remaining elements of the main
                 * tree. */
                LZX_DEBUG("Reading path lengths for remaining elements of "
                          "main tree (%d elements).",
                          num_main_syms - LZX_NUM_CHARS);
                ret = lzx_read_code_lens(istream,
                                         tables->maintree_lens + LZX_NUM_CHARS,
                                         num_main_syms - LZX_NUM_CHARS);
                if (ret) {
                        LZX_DEBUG("Failed to read the path lengths for the "
                                  "remaining elements of the main tree");
                        return ret;
                }

                LZX_DEBUG("Building the Huffman decoding "
                          "table for the main tree.");

                ret = make_huffman_decode_table(tables->maintree_decode_table,
                                                num_main_syms,
                                                LZX_MAINCODE_TABLEBITS,
                                                tables->maintree_lens,
                                                LZX_MAX_MAIN_CODEWORD_LEN);
                if (ret) {
                        LZX_DEBUG("Failed to make the decode "
                                  "table for the main tree");
                        return ret;
                }

                LZX_DEBUG("Reading path lengths for the length tree.");
                ret = lzx_read_code_lens(istream, tables->lentree_lens,
                                         LZX_LENCODE_NUM_SYMBOLS);
                if (ret) {
                        LZX_DEBUG("Failed to read the path "
                                  "lengths for the length tree");
                        return ret;
                }

                LZX_DEBUG("Building the length tree.");
                ret = make_huffman_decode_table(tables->lentree_decode_table,
                                                LZX_LENCODE_NUM_SYMBOLS,
                                                LZX_LENCODE_TABLEBITS,
                                                tables->lentree_lens,
                                                LZX_MAX_LEN_CODEWORD_LEN);
                if (ret) {
                        LZX_DEBUG("Failed to build the length Huffman tree");
                        return ret;
                }
                /* The bitstream of compressed literals and matches for this
                 * block directly follows and will be read in
                 * lzx_decompress_block(). */
                break;
        case LZX_BLOCKTYPE_UNCOMPRESSED:
                LZX_DEBUG("Found uncompressed block.");
                /* Before reading the three LRU match offsets from the
                 * uncompressed block header, the stream needs to be aligned on
                 * a 16-bit boundary.  But, unexpectedly, if the stream is
                 * *already* aligned, the correct thing to do is to throw away
                 * the next 16 bits. */
                if (istream->bitsleft == 0) {
                        if (istream->data_bytes_left < 14) {
                                LZX_DEBUG("Insufficient length in "
                                          "uncompressed block");
                                return -1;
                        }
                        istream->data += 2;
                        istream->data_bytes_left -= 2;
                } else {
                        if (istream->data_bytes_left < 12) {
                                LZX_DEBUG("Insufficient length in "
                                          "uncompressed block");
                                return -1;
                        }
                        istream->bitsleft = 0;
                        istream->bitbuf = 0;
                }
                queue->R[0] = le32_to_cpu(*(le32*)(istream->data + 0));
                queue->R[1] = le32_to_cpu(*(le32*)(istream->data + 4));
                queue->R[2] = le32_to_cpu(*(le32*)(istream->data + 8));
                istream->data += 12;
                istream->data_bytes_left -= 12;
                /* The uncompressed data of this block directly follows and will
                 * be read in lzx_decompress(). */
                break;
        default:
                LZX_DEBUG("Found invalid block");
                return -1;
        }
        *block_type_ret = block_type;
        *block_size_ret = block_size;
        return 0;
}

/*
 * Decodes a compressed match from a block of LZX-compressed data.  A match
 * refers to some match_offset to a point earlier in the window as well as some
 * match_len, for which the data is to be copied to the current position in the
 * window.
 *
 * @main_element:       The start of the match data, as decoded using the main
 *                      tree.
 *
 * @block_type:         The type of the block (LZX_BLOCKTYPE_ALIGNED or
 *                      LZX_BLOCKTYPE_VERBATIM)
 *
 * @bytes_remaining:    The amount of uncompressed data remaining to be
 *                      uncompressed in this block.  It is an error if the match
 *                      is longer than this number.
 *
 * @window:             A pointer to the window into which the uncompressed
 *                      data is being written.
 *
 * @window_pos:         The current byte offset in the window.
 *
 * @tables:             The Huffman decoding tables for this LZX block (main
 *                      code, length code, and for LZX_BLOCKTYPE_ALIGNED blocks,
 *                      also the aligned offset code).
 *
 * @queue:              The least-recently used queue for match offsets.
 *
 * @istream:            The input bitstream.
 *
 * Returns the length of the match, or a negative number on error.  The possible
 * error cases are:
 *      - Match would exceed the amount of data remaining to be uncompressed.
 *      - Match refers to data before the window.
 *      - The input bitstream ended unexpectedly.
 */
static int
lzx_decode_match(unsigned main_element, int block_type,
                 unsigned bytes_remaining, u8 *window,
                 unsigned window_pos,
                 const struct lzx_tables *tables,
                 struct lzx_lru_queue *queue,
                 struct input_bitstream *istream)
{
        unsigned length_header;
        unsigned position_slot;
        unsigned match_len;
        unsigned match_offset;
        unsigned additional_len;
        unsigned num_extra_bits;
        u32 verbatim_bits;
        u32 aligned_bits;
        unsigned i;
        int ret;
        u8 *match_dest;
        u8 *match_src;

        /* The main element is offset by 256 because values under 256 indicate a
         * literal value. */
        main_element -= LZX_NUM_CHARS;

        /* The length header consists of the lower 3 bits of the main element.
         * The position slot is the rest of it. */
        length_header = main_element & LZX_NUM_PRIMARY_LENS;
        position_slot = main_element >> 3;

        /* If the length_header is less than LZX_NUM_PRIMARY_LENS (= 7), it
         * gives the match length as the offset from LZX_MIN_MATCH_LEN.
         * Otherwise, the length is given by an additional symbol encoded using
         * the length tree, offset by 9 (LZX_MIN_MATCH_LEN +
         * LZX_NUM_PRIMARY_LENS) */
        match_len = LZX_MIN_MATCH_LEN + length_header;
        if (length_header == LZX_NUM_PRIMARY_LENS) {
                ret = read_huffsym_using_lentree(istream, tables,
                                                 &additional_len);
                if (ret)
                        return ret;
                match_len += additional_len;
        }


        /* If the position_slot is 0, 1, or 2, the match offset is retrieved
         * from the LRU queue.  Otherwise, the match offset is not in the LRU
         * queue. */
        switch (position_slot) {
        case 0:
                match_offset = queue->R[0];
                break;
        case 1:
                match_offset = queue->R[1];
                swap(queue->R[0], queue->R[1]);
                break;
        case 2:
                /* The queue doesn't work quite the same as a real LRU queue,
                 * since using the R2 offset doesn't bump the R1 offset down to
                 * R2. */
                match_offset = queue->R[2];
                swap(queue->R[0], queue->R[2]);
                break;
        default:
                /* Otherwise, the offset was not encoded as one the offsets in
                 * the queue.  Depending on the position slot, there is a
                 * certain number of extra bits that need to be read to fully
                 * decode the match offset. */

                /* Look up the number of extra bits that need to be read. */
                num_extra_bits = lzx_get_num_extra_bits(position_slot);

                /* For aligned blocks, if there are at least 3 extra bits, the
                 * actual number of extra bits is 3 less, and they encode a
                 * number of 8-byte words that are added to the offset; there
                 * is then an additional symbol read using the aligned tree that
                 * specifies the actual byte alignment. */
                if (block_type == LZX_BLOCKTYPE_ALIGNED && num_extra_bits >= 3) {

                        /* There is an error in the LZX "specification" at this
                         * point; it indicates that a Huffman symbol is to be
                         * read only if num_extra_bits is greater than 3, but
                         * actually it is if num_extra_bits is greater than or
                         * equal to 3.  (Note that in the case with
                         * num_extra_bits == 3, the assignment to verbatim_bits
                         * will just set it to 0. ) */
                        ret = bitstream_read_bits(istream, num_extra_bits - 3,
                                                  &verbatim_bits);
                        if (ret)
                                return ret;

                        verbatim_bits <<= 3;

                        ret = read_huffsym_using_alignedtree(istream, tables,
                                                             &aligned_bits);
                        if (ret)
                                return ret;
                } else {
                        /* For non-aligned blocks, or for aligned blocks with
                         * less than 3 extra bits, the extra bits are added
                         * directly to the match offset, and the correction for
                         * the alignment is taken to be 0. */
                        ret = bitstream_read_bits(istream, num_extra_bits,
                                                  &verbatim_bits);
                        if (ret)
                                return ret;

                        aligned_bits = 0;
                }

                /* Calculate the match offset. */
                match_offset = lzx_position_base[position_slot] +
                               verbatim_bits + aligned_bits - LZX_OFFSET_OFFSET;

                /* Update the LRU queue. */
                queue->R[2] = queue->R[1];
                queue->R[1] = queue->R[0];
                queue->R[0] = match_offset;
                break;
        }

        /* Verify that the match is in the bounds of the part of the window
         * currently in use, then copy the source of the match to the current
         * position. */

        if (match_len > bytes_remaining) {
                LZX_DEBUG("Match of length %u bytes overflows "
                          "uncompressed block size", match_len);
                return -1;
        }

        if (match_offset > window_pos) {
                LZX_DEBUG("Match of length %u bytes references "
                          "data before window (match_offset = %u, "
                          "window_pos = %u)",
                          match_len, match_offset, window_pos);
                return -1;
        }

        match_dest = window + window_pos;
        match_src = match_dest - match_offset;

#if 0
        printf("Match: src %u, dst %u, len %u\n", match_src - window,
                                                match_dest - window,
                                                match_len);
        putchar('|');
        for (i = 0; i < match_len; i++) {
                match_dest[i] = match_src[i];
                putchar(match_src[i]);
        }
        putchar('|');
        putchar('\n');
#else
        for (i = 0; i < match_len; i++)
                match_dest[i] = match_src[i];
#endif

        return match_len;
}

static void
undo_call_insn_translation(u32 *call_insn_target, int input_pos,
                           s32 file_size)
{
        s32 abs_offset;
        s32 rel_offset;

        abs_offset = le32_to_cpu(*call_insn_target);
        if (abs_offset >= -input_pos && abs_offset < file_size) {
                if (abs_offset >= 0) {
                        /* "good translation" */
                        rel_offset = abs_offset - input_pos;
                } else {
                        /* "compensating translation" */
                        rel_offset = abs_offset + file_size;
                }
                *call_insn_target = cpu_to_le32(rel_offset);
        }
}

/* Undo the 'E8' preprocessing, where the targets of x86 CALL instructions were
 * changed from relative offsets to absolute offsets.
 *
 * Note that this call instruction preprocessing can and will be used on any
 * data even if it is not actually x86 machine code.  In fact, this type of
 * preprocessing appears to always be used in LZX-compressed resources in WIM
 * files; there is no bit to indicate whether it is used or not, unlike in the
 * LZX compressed format as used in cabinet files, where a bit is reserved for
 * that purpose.
 *
 * Call instruction preprocessing is disabled in the last 6 bytes of the
 * uncompressed data, which really means the 5-byte call instruction cannot
 * start in the last 10 bytes of the uncompressed data.  This is one of the
 * errors in the LZX documentation.
 *
 * Call instruction preprocessing does not appear to be disabled after the
 * 32768th chunk of a WIM stream, which is apparently is yet another difference
 * from the LZX compression used in cabinet files.
 *
 * Call instruction processing is supposed to take the file size as a parameter,
 * as it is used in calculating the translated jump targets.  But in WIM files,
 * this file size is always the same (LZX_WIM_MAGIC_FILESIZE == 12000000).*/
static void
undo_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) {
                        undo_call_insn_translation((u32*)&uncompressed_data[i + 1],
                                                   i,
                                                   LZX_WIM_MAGIC_FILESIZE);
                        i += 4;
                }
        }
}

/*
 * Decompresses a LZX-compressed block of data from which the header has already
 * been read.
 *
 * @block_type: The type of the block (LZX_BLOCKTYPE_VERBATIM or
 *              LZX_BLOCKTYPE_ALIGNED)
 * @block_size: The size of the block, in bytes.
 * @num_main_syms:      Number of symbols in the main alphabet.
 * @window:     Pointer to the decompression window.
 * @window_pos: The current position in the window.  Will be 0 for the first
 *                      block.
 * @tables:     The Huffman decoding tables for the block (main, length, and
 *                      aligned offset, the latter only for LZX_BLOCKTYPE_ALIGNED)
 * @queue:      The least-recently-used queue for match offsets.
 * @istream:    The input bitstream for the compressed literals.
 */
static int
lzx_decompress_block(int block_type, unsigned block_size,
                     unsigned num_main_syms,
                     u8 *window,
                     unsigned window_pos,
                     const struct lzx_tables *tables,
                     struct lzx_lru_queue *queue,
                     struct input_bitstream *istream)
{
        unsigned main_element;
        unsigned end;
        int ret;
        int match_len;

        end = window_pos + block_size;
        while (window_pos < end) {
                ret = read_huffsym_using_maintree(istream, tables,
                                                  &main_element,
                                                  num_main_syms);
                if (ret)
                        return ret;

                if (main_element < LZX_NUM_CHARS) {
                        /* literal: 0 to LZX_NUM_CHARS - 1 */
                        window[window_pos++] = main_element;
                } else {
                        /* match: LZX_NUM_CHARS to num_main_syms - 1 */
                        match_len = lzx_decode_match(main_element,
                                                     block_type,
                                                     end - window_pos,
                                                     window,
                                                     window_pos,
                                                     tables,
                                                     queue,
                                                     istream);
                        if (match_len < 0)
                                return match_len;
                        window_pos += match_len;
                }
        }
        return 0;
}

/* API function documented in wimlib.h  */
WIMLIBAPI int
wimlib_lzx_decompress2(const void *compressed_data, unsigned compressed_len,
                       void *uncompressed_data, unsigned uncompressed_len,
                       u32 max_window_size)
{
        struct lzx_tables tables;
        struct input_bitstream istream;
        struct lzx_lru_queue queue;
        unsigned window_pos;
        unsigned block_size;
        unsigned block_type;
        unsigned num_main_syms;
        int ret;
        bool e8_preprocessing_done;

        LZX_DEBUG("compressed_data = %p, compressed_len = %u, "
                  "uncompressed_data = %p, uncompressed_len = %u, "
                  "max_window_size=%u).",
                  compressed_data, compressed_len,
                  uncompressed_data, uncompressed_len, max_window_size);

        if (!lzx_window_size_valid(max_window_size)) {
                LZX_DEBUG("Window size of %u is invalid!",
                          max_window_size);
                return -1;
        }

        num_main_syms = lzx_get_num_main_syms(max_window_size);

        if (uncompressed_len > max_window_size) {
                LZX_DEBUG("Uncompressed chunk size of %u exceeds "
                          "window size of %u!",
                          uncompressed_len, max_window_size);
                return -1;
        }

        memset(tables.maintree_lens, 0, sizeof(tables.maintree_lens));
        memset(tables.lentree_lens, 0, sizeof(tables.lentree_lens));
        lzx_lru_queue_init(&queue);
        init_input_bitstream(&istream, compressed_data, compressed_len);

        e8_preprocessing_done = false; /* Set to true if there may be 0xe8 bytes
                                          in the uncompressed data. */

        /* The compressed data will consist of one or more blocks.  The
         * following loop decompresses one block, and it runs until there all
         * the compressed data has been decompressed, so there are no more
         * blocks.  */

        for (window_pos = 0;
             window_pos < uncompressed_len;
             window_pos += block_size)
        {
                LZX_DEBUG("Reading block header.");
                ret = lzx_read_block_header(&istream, num_main_syms,
                                            max_window_size, &block_size,
                                            &block_type, &tables, &queue);
                if (ret)
                        return ret;

                LZX_DEBUG("block_size = %u, window_pos = %u",
                          block_size, window_pos);

                if (block_size > uncompressed_len - window_pos) {
                        LZX_DEBUG("Expected a block size of at "
                                  "most %u bytes (found %u bytes)",
                                  uncompressed_len - window_pos, block_size);
                        return -1;
                }

                switch (block_type) {
                case LZX_BLOCKTYPE_VERBATIM:
                case LZX_BLOCKTYPE_ALIGNED:
                        if (block_type == LZX_BLOCKTYPE_VERBATIM)
                                LZX_DEBUG("LZX_BLOCKTYPE_VERBATIM");
                        else
                                LZX_DEBUG("LZX_BLOCKTYPE_ALIGNED");
                        ret = lzx_decompress_block(block_type,
                                                   block_size,
                                                   num_main_syms,
                                                   uncompressed_data,
                                                   window_pos,
                                                   &tables,
                                                   &queue,
                                                   &istream);
                        if (ret)
                                return ret;

                        if (tables.maintree_lens[0xe8] != 0)
                                e8_preprocessing_done = true;
                        break;
                case LZX_BLOCKTYPE_UNCOMPRESSED:
                        LZX_DEBUG("LZX_BLOCKTYPE_UNCOMPRESSED");
                        if (istream.data_bytes_left < block_size) {
                                LZX_DEBUG("Unexpected end of input when "
                                          "reading %u bytes from LZX bitstream "
                                          "(only have %u bytes left)",
                                          block_size, istream.data_bytes_left);
                                return -1;
                        }
                        memcpy(&((u8*)uncompressed_data)[window_pos], istream.data,
                               block_size);
                        istream.data += block_size;
                        istream.data_bytes_left -= block_size;
                        /* Re-align bitstream if an odd number of bytes were
                         * read.  */
                        if (istream.data_bytes_left && (block_size & 1)) {
                                istream.data_bytes_left--;
                                istream.data++;
                        }
                        e8_preprocessing_done = true;
                        break;
                }
        }
        if (e8_preprocessing_done)
                undo_call_insn_preprocessing(uncompressed_data, uncompressed_len);
        return 0;
}

/* API function documented in wimlib.h  */
WIMLIBAPI int
wimlib_lzx_decompress(const void *compressed_data, unsigned compressed_len,
                      void *uncompressed_data, unsigned uncompressed_len)
{
        return wimlib_lzx_decompress2(compressed_data, compressed_len,
                                      uncompressed_data, uncompressed_len,
                                      32768);
}