--- /dev/null
+/*
+ * xpress_decompress.c
+ *
+ * A decompressor for the XPRESS compression format (Huffman variant).
+ */
+
+/*
+ *
+ * Copyright (C) 2012, 2013 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/.
+ */
+
+
+/*
+ * The XPRESS compression format is an LZ77 and Huffman-code based algorithm.
+ * That means it is fairly similar to LZX compression, but XPRESS is simpler, so
+ * it is a little faster to compress and decompress.
+ *
+ * The XPRESS compression format is mostly documented in a file called "[MS-XCA]
+ * Xpress Compression Algorithm". In the MSDN library, it can currently be
+ * found under Open Specifications => Protocols => Windows Protocols => Windows
+ * Server Protocols => [MS-XCA] Xpress Compression Algorithm". The format in
+ * WIMs is specifically the algorithm labeled as the "LZ77+Huffman Algorithm"
+ * (there apparently are some other versions of XPRESS as well).
+ *
+ * If you are already familiar with the LZ77 algorithm and Huffman coding, the
+ * XPRESS format is fairly simple. The compressed data begins with 256 bytes
+ * that contain 512 4-bit integers that are the lengths of the symbols in the
+ * Huffman code used for match/literal headers. In contrast with more
+ * complicated formats such as DEFLATE and LZX, this is the only Huffman code
+ * that is used for the entirety of the XPRESS compressed data, and the codeword
+ * lengths are not encoded with a pretree.
+ *
+ * The rest of the compressed data is Huffman-encoded symbols. Values 0 through
+ * 255 represent the corresponding literal bytes. Values 256 through 511
+ * represent matches and may require extra bits or bytes to be read to get the
+ * match offset and match length.
+ *
+ * The trickiest part is probably the way in which literal bytes for match
+ * lengths are interleaved in the bitstream.
+ *
+ * Also, a caveat--- according to Microsoft's documentation for XPRESS,
+ *
+ * "Some implementation of the decompression algorithm expect an extra
+ * symbol to mark the end of the data. Specifically, some implementations
+ * fail during decompression if the Huffman symbol 256 is not found after
+ * the actual data."
+ *
+ * This is the case for the implementation in WIMGAPI. However, wimlib's
+ * decompressor in this file currently does not care if this extra symbol is
+ * there or not.
+ */
+
+#ifdef HAVE_CONFIG_H
+# include "config.h"
+#endif
+
+#include "wimlib/decompressor_ops.h"
+#include "wimlib/decompress_common.h"
+#include "wimlib/error.h"
+#include "wimlib/xpress_constants.h"
+
+/* This value is chosen for fast decompression. */
+#define XPRESS_TABLEBITS 12
+
+/* Decode the matches and literal bytes in a region of XPRESS-encoded data. */
+static int
+xpress_decode_window(struct input_bitstream *istream, const u16 *decode_table,
+ u8 *window, unsigned window_size)
+{
+ u8 *window_ptr = window;
+ u8 *window_end = &window[window_size];
+ unsigned sym;
+ unsigned match_len;
+ unsigned offset_high_bit;
+ unsigned match_offset;
+
+ while (window_ptr != window_end) {
+
+ sym = read_huffsym(istream, decode_table,
+ XPRESS_TABLEBITS, XPRESS_MAX_CODEWORD_LEN);
+ if (sym < XPRESS_NUM_CHARS) {
+ /* Literal */
+ *window_ptr++ = sym;
+ continue;
+ }
+
+ /* Match */
+ match_len = sym & 0xf;
+ offset_high_bit = (sym >> 4) & 0xf;
+
+ bitstream_ensure_bits(istream, 16);
+
+ match_offset = (1 << offset_high_bit) |
+ bitstream_pop_bits(istream, offset_high_bit);
+
+ if (match_len == 0xf) {
+ match_len += bitstream_read_byte(istream);
+ if (match_len == 0xf + 0xff)
+ match_len = bitstream_read_u16(istream);
+ }
+ match_len += XPRESS_MIN_MATCH_LEN;
+
+ if (unlikely(match_offset > window_ptr - window))
+ return -1;
+
+ if (unlikely(match_len > window_end - window_ptr))
+ return -1;
+
+ lz_copy(window_ptr, match_len, match_offset, window_end,
+ XPRESS_MIN_MATCH_LEN);
+
+ window_ptr += match_len;
+ }
+ return 0;
+}
+
+static int
+xpress_decompress(const void *compressed_data, size_t compressed_size,
+ void *uncompressed_data, size_t uncompressed_size, void *_ctx)
+{
+ const u8 *cdata = compressed_data;
+ u8 lens[XPRESS_NUM_SYMBOLS];
+ u8 *lens_p;
+ u16 decode_table[(1 << XPRESS_TABLEBITS) + 2 * XPRESS_NUM_SYMBOLS]
+ _aligned_attribute(DECODE_TABLE_ALIGNMENT);
+ struct input_bitstream istream;
+
+ /* XPRESS uses only one Huffman code. It contains 512 symbols, and the
+ * code lengths of these symbols are given literally as 4-bit integers
+ * in the first 256 bytes of the compressed data. */
+ if (compressed_size < XPRESS_NUM_SYMBOLS / 2)
+ return -1;
+
+ lens_p = lens;
+ for (unsigned i = 0; i < XPRESS_NUM_SYMBOLS / 2; i++) {
+ *lens_p++ = cdata[i] & 0xf;
+ *lens_p++ = cdata[i] >> 4;
+ }
+
+ if (make_huffman_decode_table(decode_table, XPRESS_NUM_SYMBOLS,
+ XPRESS_TABLEBITS, lens,
+ XPRESS_MAX_CODEWORD_LEN))
+ return -1;
+
+ init_input_bitstream(&istream, cdata + XPRESS_NUM_SYMBOLS / 2,
+ compressed_size - XPRESS_NUM_SYMBOLS / 2);
+
+ return xpress_decode_window(&istream, decode_table,
+ uncompressed_data, uncompressed_size);
+}
+
+static int
+xpress_create_decompressor(size_t max_block_size, void **dec_ret)
+{
+ if (max_block_size > XPRESS_MAX_OFFSET + 1)
+ return WIMLIB_ERR_INVALID_PARAM;
+
+ return 0;
+}
+
+const struct decompressor_ops xpress_decompressor_ops = {
+ .create_decompressor = xpress_create_decompressor,
+ .decompress = xpress_decompress,
+};
git://wimlib.net / wimlib / blobdiff