X-Git-Url: https://wimlib.net/git/?p=wimlib;a=blobdiff_plain;f=include%2Fwimlib%2Fdecompress_common.h;h=d20085db61736fdb5b0dfddf1f87619215115525;hp=4b5a7e3cdf89a34c407ab123ad7e603bf90d9acd;hb=4ee103c6e2a2988e1fb358bfa2dc38dcb621505a;hpb=883833a4b3dabec325edf1ca938000f91d587c00

diff --git a/include/wimlib/decompress_common.h b/include/wimlib/decompress_common.h
index 4b5a7e3c..d20085db 100644
--- a/include/wimlib/decompress_common.h
+++ b/include/wimlib/decompress_common.h
@@ -2,208 +2,545 @@
  * decompress_common.h
  *
  * Header for decompression code shared by multiple compression formats.
+ *
+ * The following copying information applies to this specific source code file:
+ *
+ * Written in 2012-2016 by Eric Biggers <ebiggers3@gmail.com>
+ *
+ * To the extent possible under law, the author(s) have dedicated all copyright
+ * and related and neighboring rights to this software to the public domain
+ * worldwide via the Creative Commons Zero 1.0 Universal Public Domain
+ * Dedication (the "CC0").
+ *
+ * This software 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 CC0 for more details.
+ *
+ * You should have received a copy of the CC0 along with this software; if not
+ * see <http://creativecommons.org/publicdomain/zero/1.0/>.
  */
 
 #ifndef _WIMLIB_DECOMPRESS_COMMON_H
 #define _WIMLIB_DECOMPRESS_COMMON_H
 
-#include "wimlib/assert.h"
+#include <string.h>
+
 #include "wimlib/compiler.h"
-#include "wimlib/error.h"
-#include "wimlib/endianness.h"
 #include "wimlib/types.h"
+#include "wimlib/unaligned.h"
 
-#ifndef INPUT_IDX_T_DEFINED
-#define INPUT_IDX_T_DEFINED
-typedef u32 input_idx_t;
-#endif
+/******************************************************************************/
+/*                   Input bitstream for XPRESS and LZX                       */
+/*----------------------------------------------------------------------------*/
 
-/* Structure to encapsulate a block of in-memory data that is being interpreted
- * as a stream of bits.
- *
- * This is geared specifically towards the XPRESS and LZX compression formats
- * with regards to the actual ordering the bits within the byte sequence.  */
+/* Structure that encapsulates a block of in-memory data being interpreted as a
+ * stream of bits, optionally with interwoven literal bytes.  Bits are assumed
+ * to be stored in little endian 16-bit coding units, with the bits ordered high
+ * to low.  */
 struct input_bitstream {
 
-	/* A variable of length at least 32 bits that is used to hold bits that
-	 * have been read from the stream.  The bits are ordered from high-order
-	 * to low-order, and the next bit is always the high-order bit.  */
+	/* Bits that have been read from the input buffer.  The bits are
+	 * left-justified; the next bit is always bit 31.  */
 	u32 bitbuf;
 
-	/* Number of bits in @bitbuf that are valid.  */
-	unsigned bitsleft;
+	/* Number of bits currently held in @bitbuf.  */
+	u32 bitsleft;
 
-	/* Pointer to the next byte to be retrieved from the input.  */
-	const u8 *data;
+	/* Pointer to the next byte to be retrieved from the input buffer.  */
+	const u8 *next;
 
-	/* Number of bytes of data that are left.  */
-	input_idx_t data_bytes_left;
+	/* Pointer past the end of the input buffer.  */
+	const u8 *end;
 };
 
-/* Initializes a bitstream to receive its input from @data. */
+/* Initialize a bitstream to read from the specified input buffer.  */
 static inline void
-init_input_bitstream(struct input_bitstream *istream,
-		     const void *data, input_idx_t num_data_bytes)
+init_input_bitstream(struct input_bitstream *is, const void *buffer, u32 size)
 {
-	istream->bitbuf          = 0;
-	istream->bitsleft        = 0;
-	istream->data            = data;
-	istream->data_bytes_left = num_data_bytes;
+	is->bitbuf = 0;
+	is->bitsleft = 0;
+	is->next = buffer;
+	is->end = is->next + size;
 }
 
-/* Ensures that the bit buffer variable for the bitstream contains @num_bits
- * bits.
- *
- * If there are at least @num_bits bits remaining in the bitstream, 0 is
- * returned.  Otherwise, -1 is returned.  */
-static inline int
-bitstream_ensure_bits(struct input_bitstream *istream, unsigned num_bits)
+/* Note: for performance reasons, the following methods don't return error codes
+ * to the caller if the input buffer is overrun.  Instead, they just assume that
+ * all overrun data is zeroes.  This has no effect on well-formed compressed
+ * data.  The only disadvantage is that bad compressed data may go undetected,
+ * but even this is irrelevant if higher level code checksums the uncompressed
+ * data anyway.  */
+
+/* Ensure the bit buffer variable for the bitstream contains at least @num_bits
+ * bits.  Following this, bitstream_peek_bits() and/or bitstream_remove_bits()
+ * may be called on the bitstream to peek or remove up to @num_bits bits.  */
+static inline void
+bitstream_ensure_bits(struct input_bitstream *is, const unsigned num_bits)
 {
-	for (int nbits = num_bits; (int)istream->bitsleft < nbits; nbits -= 16) {
-		u16 nextword;
-		unsigned shift;
+	/* This currently works for at most 17 bits.  */
+
+	if (is->bitsleft >= num_bits)
+		return;
 
-		if (unlikely(istream->data_bytes_left < 2))
-			return -1;
+	if (unlikely(is->end - is->next < 2))
+		goto overflow;
 
-		wimlib_assert2(istream->bitsleft <= sizeof(istream->bitbuf) * 8 - 16);
+	is->bitbuf |= (u32)get_unaligned_le16(is->next) << (16 - is->bitsleft);
+	is->next += 2;
+	is->bitsleft += 16;
 
-		nextword = le16_to_cpu(*(const le16*)istream->data);
-		shift = sizeof(istream->bitbuf) * 8 - 16 - istream->bitsleft;
-		istream->bitbuf |= (u32)nextword << shift;
-		istream->data += 2;
-		istream->bitsleft += 16;
-		istream->data_bytes_left -= 2;
+	if (unlikely(num_bits == 17 && is->bitsleft == 16)) {
+		if (unlikely(is->end - is->next < 2))
+			goto overflow;
 
+		is->bitbuf |= (u32)get_unaligned_le16(is->next);
+		is->next += 2;
+		is->bitsleft = 32;
 	}
-	return 0;
+
+	return;
+
+overflow:
+	is->bitsleft = 32;
 }
 
-/* Returns the next @num_bits bits in the buffer variable, which must contain at
- * least @num_bits bits, for the bitstream.  */
+/* Return the next @num_bits bits from the bitstream, without removing them.
+ * There must be at least @num_bits remaining in the buffer variable, from a
+ * previous call to bitstream_ensure_bits().  */
 static inline u32
-bitstream_peek_bits(const struct input_bitstream *istream, unsigned num_bits)
+bitstream_peek_bits(const struct input_bitstream *is, const unsigned num_bits)
 {
-	wimlib_assert2(istream->bitsleft >= num_bits);
-
-	if (unlikely(num_bits == 0))
-		return 0;
-
-	return istream->bitbuf >> (sizeof(istream->bitbuf) * 8 - num_bits);
+	return (is->bitbuf >> 1) >> (sizeof(is->bitbuf) * 8 - num_bits - 1);
 }
 
-/* Removes @num_bits bits from the buffer variable, which must contain at least
- * @num_bits bits, for the bitstream.  */
+/* Remove @num_bits from the bitstream.  There must be at least @num_bits
+ * remaining in the buffer variable, from a previous call to
+ * bitstream_ensure_bits().  */
 static inline void
-bitstream_remove_bits(struct input_bitstream *istream, unsigned num_bits)
+bitstream_remove_bits(struct input_bitstream *is, unsigned num_bits)
 {
-	wimlib_assert2(istream->bitsleft >= num_bits);
+	is->bitbuf <<= num_bits;
+	is->bitsleft -= num_bits;
+}
 
-	istream->bitbuf <<= num_bits;
-	istream->bitsleft -= num_bits;
+/* Remove and return @num_bits bits from the bitstream.  There must be at least
+ * @num_bits remaining in the buffer variable, from a previous call to
+ * bitstream_ensure_bits().  */
+static inline u32
+bitstream_pop_bits(struct input_bitstream *is, unsigned num_bits)
+{
+	u32 bits = bitstream_peek_bits(is, num_bits);
+	bitstream_remove_bits(is, num_bits);
+	return bits;
 }
 
-/* Gets and removes @num_bits bits from the buffer variable, which must contain
- * at least @num_bits bits, for the bitstream.  */
+/* Read and return the next @num_bits bits from the bitstream.  */
 static inline u32
-bitstream_pop_bits(struct input_bitstream *istream, unsigned num_bits)
+bitstream_read_bits(struct input_bitstream *is, unsigned num_bits)
 {
-	u32 n = bitstream_peek_bits(istream, num_bits);
-	bitstream_remove_bits(istream, num_bits);
-	return n;
+	bitstream_ensure_bits(is, num_bits);
+	return bitstream_pop_bits(is, num_bits);
 }
 
-/* Reads @num_bits bits from the input bitstream.  On success, returns 0 and
- * returns the requested bits in @n.  If there are fewer than @num_bits
- * remaining in the bitstream, -1 is returned. */
-static inline int
-bitstream_read_bits(struct input_bitstream *istream, unsigned num_bits, u32 *n)
+/* Read and return the next literal byte embedded in the bitstream.  */
+static inline u8
+bitstream_read_byte(struct input_bitstream *is)
 {
-	if (unlikely(bitstream_ensure_bits(istream, num_bits)))
-		return -1;
+	if (unlikely(is->end == is->next))
+		return 0;
+	return *is->next++;
+}
 
-	*n = bitstream_pop_bits(istream, num_bits);
-	return 0;
+/* Read and return the next 16-bit integer embedded in the bitstream.  */
+static inline u16
+bitstream_read_u16(struct input_bitstream *is)
+{
+	u16 v;
+
+	if (unlikely(is->end - is->next < 2))
+		return 0;
+	v = get_unaligned_le16(is->next);
+	is->next += 2;
+	return v;
 }
 
-/* Return the next literal byte embedded in the bitstream, or -1 if the input
- * was exhausted.  */
+/* Read and return the next 32-bit integer embedded in the bitstream.  */
+static inline u32
+bitstream_read_u32(struct input_bitstream *is)
+{
+	u32 v;
+
+	if (unlikely(is->end - is->next < 4))
+		return 0;
+	v = get_unaligned_le32(is->next);
+	is->next += 4;
+	return v;
+}
+
+/* Read into @dst_buffer an array of literal bytes embedded in the bitstream.
+ * Return 0 if there were enough bytes remaining in the input, otherwise -1. */
 static inline int
-bitstream_read_byte(struct input_bitstream *istream)
+bitstream_read_bytes(struct input_bitstream *is, void *dst_buffer, size_t count)
 {
-	if (unlikely(istream->data_bytes_left < 1))
+	if (unlikely(is->end - is->next < count))
 		return -1;
+	memcpy(dst_buffer, is->next, count);
+	is->next += count;
+	return 0;
+}
 
-	istream->data_bytes_left--;
-	return *istream->data++;
+/* Align the input bitstream on a coding-unit boundary.  */
+static inline void
+bitstream_align(struct input_bitstream *is)
+{
+	is->bitsleft = 0;
+	is->bitbuf = 0;
 }
 
-/* Reads @num_bits bits from the buffer variable for a bistream without checking
- * to see if that many bits are in the buffer or not.  */
-static inline u32
-bitstream_read_bits_nocheck(struct input_bitstream *istream, unsigned num_bits)
+/******************************************************************************/
+/*                             Huffman decoding                               */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * Required alignment for the Huffman decode tables.  We require this alignment
+ * so that we can fill the entries with vector or word instructions and not have
+ * to deal with misaligned buffers.
+ */
+#define DECODE_TABLE_ALIGNMENT 16
+
+/*
+ * Each decode table entry is 16 bits divided into two fields: 'symbol' (high 12
+ * bits) and 'length' (low 4 bits).  The precise meaning of these fields depends
+ * on the type of entry:
+ *
+ * Root table entries which are *not* subtable pointers:
+ *	symbol: symbol to decode
+ *	length: codeword length in bits
+ *
+ * Root table entries which are subtable pointers:
+ *	symbol: index of start of subtable
+ *	length: number of bits with which the subtable is indexed
+ *
+ * Subtable entries:
+ *	symbol: symbol to decode
+ *	length: codeword length in bits, minus the number of bits with which the
+ *		root table is indexed
+ */
+#define DECODE_TABLE_SYMBOL_SHIFT  4
+#define DECODE_TABLE_MAX_SYMBOL	   ((1 << (16 - DECODE_TABLE_SYMBOL_SHIFT)) - 1)
+#define DECODE_TABLE_MAX_LENGTH    ((1 << DECODE_TABLE_SYMBOL_SHIFT) - 1)
+#define DECODE_TABLE_LENGTH_MASK   DECODE_TABLE_MAX_LENGTH
+#define MAKE_DECODE_TABLE_ENTRY(symbol, length) \
+	(((symbol) << DECODE_TABLE_SYMBOL_SHIFT) | (length))
+
+/*
+ * Read and return the next Huffman-encoded symbol from the given bitstream
+ * using the given decode table.
+ *
+ * If the input data is exhausted, then the Huffman symbol will be decoded as if
+ * the missing bits were all zeroes.
+ *
+ * XXX: This is mostly duplicated in lzms_decode_huffman_symbol() in
+ * lzms_decompress.c; keep them in sync!
+ */
+static inline unsigned
+read_huffsym(struct input_bitstream *is, const u16 decode_table[],
+	     unsigned table_bits, unsigned max_codeword_len)
 {
-	u32 n = bitstream_peek_bits(istream, num_bits);
-	bitstream_remove_bits(istream, num_bits);
-	return n;
+	unsigned entry;
+	unsigned symbol;
+	unsigned length;
+
+	/* Preload the bitbuffer with 'max_codeword_len' bits so that we're
+	 * guaranteed to be able to fully decode a codeword. */
+	bitstream_ensure_bits(is, max_codeword_len);
+
+	/* Index the root table by the next 'table_bits' bits of input. */
+	entry = decode_table[bitstream_peek_bits(is, table_bits)];
+
+	/* Extract the "symbol" and "length" from the entry. */
+	symbol = entry >> DECODE_TABLE_SYMBOL_SHIFT;
+	length = entry & DECODE_TABLE_LENGTH_MASK;
+
+	/* If the root table is indexed by the full 'max_codeword_len' bits,
+	 * then there cannot be any subtables, and this will be known at compile
+	 * time.  Otherwise, we must check whether the decoded symbol is really
+	 * a subtable pointer.  If so, we must discard the bits with which the
+	 * root table was indexed, then index the subtable by the next 'length'
+	 * bits of input to get the real entry. */
+	if (max_codeword_len > table_bits &&
+	    entry >= (1U << (table_bits + DECODE_TABLE_SYMBOL_SHIFT)))
+	{
+		/* Subtable required */
+		bitstream_remove_bits(is, table_bits);
+		entry = decode_table[symbol + bitstream_peek_bits(is, length)];
+		symbol = entry >> DECODE_TABLE_SYMBOL_SHIFT;
+		length = entry & DECODE_TABLE_LENGTH_MASK;
+	}
+
+	/* Discard the bits (or the remaining bits, if a subtable was required)
+	 * of the codeword. */
+	bitstream_remove_bits(is, length);
+
+	/* Return the decoded symbol. */
+	return symbol;
 }
 
+/*
+ * The DECODE_TABLE_ENOUGH() macro evaluates to the maximum number of decode
+ * table entries, including all subtable entries, that may be required for
+ * decoding a given Huffman code.  This depends on three parameters:
+ *
+ *	num_syms: the maximum number of symbols in the code
+ *	table_bits: the number of bits with which the root table will be indexed
+ *	max_codeword_len: the maximum allowed codeword length in the code
+ *
+ * Given these parameters, the utility program 'enough' from zlib, when passed
+ * the three arguments 'num_syms', 'table_bits', and 'max_codeword_len', will
+ * compute the maximum number of entries required.  This has already been done
+ * for the combinations we need and incorporated into the macro below so that
+ * the mapping can be done at compilation time.  If an unknown combination is
+ * used, then a compilation error will result.  To fix this, use 'enough' to
+ * find the missing value and add it below.  If that still doesn't fix the
+ * compilation error, then most likely a constraint would be violated by the
+ * requested parameters, so they cannot be used, at least without other changes
+ * to the decode table --- see DECODE_TABLE_SIZE().
+ */
+#define DECODE_TABLE_ENOUGH(num_syms, table_bits, max_codeword_len) ( \
+	((num_syms) == 8 && (table_bits) == 7 && (max_codeword_len) == 15) ? 128 : \
+	((num_syms) == 8 && (table_bits) == 5 && (max_codeword_len) == 7) ? 36 : \
+	((num_syms) == 8 && (table_bits) == 6 && (max_codeword_len) == 7) ? 66 : \
+	((num_syms) == 8 && (table_bits) == 7 && (max_codeword_len) == 7) ? 128 : \
+	((num_syms) == 20 && (table_bits) == 5 && (max_codeword_len) == 15) ? 1062 : \
+	((num_syms) == 20 && (table_bits) == 6 && (max_codeword_len) == 15) ? 582 : \
+	((num_syms) == 20 && (table_bits) == 7 && (max_codeword_len) == 15) ? 390 : \
+	((num_syms) == 54 && (table_bits) == 9 && (max_codeword_len) == 15) ? 618 : \
+	((num_syms) == 54 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1098 : \
+	((num_syms) == 249 && (table_bits) == 9 && (max_codeword_len) == 16) ? 878 : \
+	((num_syms) == 249 && (table_bits) == 10 && (max_codeword_len) == 16) ? 1326 : \
+	((num_syms) == 249 && (table_bits) == 11 && (max_codeword_len) == 16) ? 2318 : \
+	((num_syms) == 256 && (table_bits) == 9 && (max_codeword_len) == 15) ? 822 : \
+	((num_syms) == 256 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1302 : \
+	((num_syms) == 256 && (table_bits) == 11 && (max_codeword_len) == 15) ? 2310 : \
+	((num_syms) == 512 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1558 : \
+	((num_syms) == 512 && (table_bits) == 11 && (max_codeword_len) == 15) ? 2566 : \
+	((num_syms) == 512 && (table_bits) == 12 && (max_codeword_len) == 15) ? 4606 : \
+	((num_syms) == 656 && (table_bits) == 10 && (max_codeword_len) == 16) ? 1734 : \
+	((num_syms) == 656 && (table_bits) == 11 && (max_codeword_len) == 16) ? 2726 : \
+	((num_syms) == 656 && (table_bits) == 12 && (max_codeword_len) == 16) ? 4758 : \
+	((num_syms) == 799 && (table_bits) == 9 && (max_codeword_len) == 15) ? 1366 : \
+	((num_syms) == 799 && (table_bits) == 10 && (max_codeword_len) == 15) ? 1846 : \
+	((num_syms) == 799 && (table_bits) == 11 && (max_codeword_len) == 15) ? 2854 : \
+	-1)
+
+/* Wrapper around DECODE_TABLE_ENOUGH() that does additional compile-time
+ * validation. */
+#define DECODE_TABLE_SIZE(num_syms, table_bits, max_codeword_len) (	\
+									\
+	/* All values must be positive. */				\
+	STATIC_ASSERT_ZERO((num_syms) > 0) +				\
+	STATIC_ASSERT_ZERO((table_bits) > 0) +				\
+	STATIC_ASSERT_ZERO((max_codeword_len) > 0) +			\
+									\
+	/* There cannot be more symbols than possible codewords. */	\
+	STATIC_ASSERT_ZERO((num_syms) <= 1U << (max_codeword_len)) +	\
+									\
+	/* There is no reason for the root table to be indexed with
+	 * more bits than the maximum codeword length. */		\
+	STATIC_ASSERT_ZERO((table_bits) <= (max_codeword_len)) +	\
+									\
+	/* The maximum symbol value must fit in the 'symbol' field. */	\
+	STATIC_ASSERT_ZERO((num_syms) - 1 <= DECODE_TABLE_MAX_SYMBOL) +	\
+									\
+	/* The maximum codeword length in the root table must fit in
+	 * the 'length' field. */					\
+	STATIC_ASSERT_ZERO((table_bits) <= DECODE_TABLE_MAX_LENGTH) +	\
+									\
+	/* The maximum codeword length in a subtable must fit in the
+	 * 'length' field. */						\
+	STATIC_ASSERT_ZERO((max_codeword_len) - (table_bits) <=		\
+			   DECODE_TABLE_MAX_LENGTH) +			\
+									\
+	/* The minimum subtable index must be greater than the maximum
+	 * symbol value.  If this were not the case, then there would
+	 * be no way to tell whether a given root table entry is a
+	 * "subtable pointer" or not.  (An alternate solution would be
+	 * to reserve a flag bit specifically for this purpose.) */	\
+	STATIC_ASSERT_ZERO((1U << table_bits) > (num_syms) - 1) +	\
+									\
+	/* The needed 'enough' value must have been defined. */		\
+	STATIC_ASSERT_ZERO(DECODE_TABLE_ENOUGH(				\
+				(num_syms), (table_bits),		\
+				(max_codeword_len)) > 0) +		\
+									\
+	/* The maximum subtable index must fit in the 'symbol' field. */\
+	STATIC_ASSERT_ZERO(DECODE_TABLE_ENOUGH(				\
+				(num_syms), (table_bits),		\
+				(max_codeword_len)) - 1 <=		\
+					DECODE_TABLE_MAX_SYMBOL) +	\
+									\
+	/* Finally, make the macro evaluate to the needed maximum
+	 * number of decode table entries. */				\
+	DECODE_TABLE_ENOUGH((num_syms), (table_bits),			\
+			    (max_codeword_len))				\
+)
+
+/*
+ * Declare the decode table for a Huffman code, given several compile-time
+ * constants that describe the code.  See DECODE_TABLE_ENOUGH() for details.
+ *
+ * Decode tables must be aligned to a DECODE_TABLE_ALIGNMENT-byte boundary.
+ * This implies that if a decode table is nested inside a dynamically allocated
+ * structure, then the outer structure must be allocated on a
+ * DECODE_TABLE_ALIGNMENT-byte aligned boundary as well.
+ */
+#define DECODE_TABLE(name, num_syms, table_bits, max_codeword_len) \
+	u16 name[DECODE_TABLE_SIZE((num_syms), (table_bits), \
+				   (max_codeword_len))]	\
+		_aligned_attribute(DECODE_TABLE_ALIGNMENT)
+
+/*
+ * Declare the temporary "working_space" array needed for building the decode
+ * table for a Huffman code.
+ */
+#define DECODE_TABLE_WORKING_SPACE(name, num_syms, max_codeword_len)	\
+	u16 name[2 * ((max_codeword_len) + 1)  + (num_syms)];
+
 extern int
-read_huffsym_near_end_of_input(struct input_bitstream *istream,
-			       const u16 decode_table[],
-			       const u8 lens[],
-			       unsigned num_syms,
-			       unsigned table_bits,
-			       unsigned *n);
-
-/* Read a Huffman-encoded symbol from a bitstream.  */
+make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
+			  unsigned table_bits, const u8 lens[],
+			  unsigned max_codeword_len, u16 working_space[]);
+
+/******************************************************************************/
+/*                             LZ match copying                               */
+/*----------------------------------------------------------------------------*/
+
+static inline void
+copy_word_unaligned(const void *src, void *dst)
+{
+	store_word_unaligned(load_word_unaligned(src), dst);
+}
+
+static inline machine_word_t
+repeat_u16(u16 b)
+{
+	machine_word_t v = b;
+
+	STATIC_ASSERT(WORDBITS == 32 || WORDBITS == 64);
+	v |= v << 16;
+	v |= v << ((WORDBITS == 64) ? 32 : 0);
+	return v;
+}
+
+static inline machine_word_t
+repeat_byte(u8 b)
+{
+	return repeat_u16(((u16)b << 8) | b);
+}
+
+/*
+ * Copy an LZ77 match of 'length' bytes from the match source at 'out_next -
+ * offset' to the match destination at 'out_next'.  The source and destination
+ * may overlap.
+ *
+ * This handles validating the length and offset.  It is validated that the
+ * beginning of the match source is '>= out_begin' and that end of the match
+ * destination is '<= out_end'.  The return value is 0 if the match was valid
+ * (and was copied), otherwise -1.
+ *
+ * 'min_length' is a hint which specifies the minimum possible match length.
+ * This should be a compile-time constant.
+ */
 static inline int
-read_huffsym(struct input_bitstream * restrict istream,
-	     const u16 decode_table[restrict],
-	     const u8 lens[restrict],
-	     unsigned num_syms,
-	     unsigned table_bits,
-	     unsigned *restrict n,
-	     unsigned max_codeword_len)
-{
-	/* If there are fewer bits remaining in the input than the maximum
-	 * codeword length, use the slow path that has extra checks.  */
-	if (unlikely(bitstream_ensure_bits(istream, max_codeword_len))) {
-		return read_huffsym_near_end_of_input(istream, decode_table,
-						      lens, num_syms,
-						      table_bits, n);
+lz_copy(u32 length, u32 offset, u8 *out_begin, u8 *out_next, u8 *out_end,
+	u32 min_length)
+{
+	const u8 *src;
+	u8 *end;
+
+	/* Validate the offset. */
+	if (unlikely(offset > out_next - out_begin))
+		return -1;
+
+	/*
+	 * Fast path: copy a match which is no longer than a few words, is not
+	 * overlapped such that copying a word at a time would produce incorrect
+	 * results, and is not too close to the end of the buffer.  Note that
+	 * this might copy more than the length of the match, but that's okay in
+	 * this scenario.
+	 */
+	src = out_next - offset;
+	if (UNALIGNED_ACCESS_IS_FAST && length <= 3 * WORDBYTES &&
+	    offset >= WORDBYTES && out_end - out_next >= 3 * WORDBYTES)
+	{
+		copy_word_unaligned(src + WORDBYTES*0, out_next + WORDBYTES*0);
+		copy_word_unaligned(src + WORDBYTES*1, out_next + WORDBYTES*1);
+		copy_word_unaligned(src + WORDBYTES*2, out_next + WORDBYTES*2);
+		return 0;
 	}
 
-	/* Use the next table_bits of the input as an index into the
-	 * decode_table.  */
-	u16 key_bits = bitstream_peek_bits(istream, table_bits);
-
-	u16 sym = decode_table[key_bits];
-
-	if (likely(sym < num_syms)) {
-		/* Fast case: The decode table directly provided the symbol.  */
-		bitstream_remove_bits(istream, lens[sym]);
-	} else {
-		/* Slow case: The symbol took too many bits to include directly
-		 * in the decode table, so search for it in a binary tree at the
-		 * end of the decode table.  */
-		bitstream_remove_bits(istream, table_bits);
-		do {
-			key_bits = sym + bitstream_peek_bits(istream, 1);
-			bitstream_remove_bits(istream, 1);
-		} while ((sym = decode_table[key_bits]) >= num_syms);
+	/* Validate the length.  This isn't needed in the fast path above, due
+	 * to the additional conditions tested, but we do need it here. */
+	if (unlikely(length > out_end - out_next))
+		return -1;
+	end = out_next + length;
+
+	/*
+	 * Try to copy one word at a time.  On i386 and x86_64 this is faster
+	 * than copying one byte at a time, unless the data is near-random and
+	 * all the matches have very short lengths.  Note that since this
+	 * requires unaligned memory accesses, it won't necessarily be faster on
+	 * every architecture.
+	 *
+	 * Also note that we might copy more than the length of the match.  For
+	 * example, if a word is 8 bytes and the match is of length 5, then
+	 * we'll simply copy 8 bytes.  This is okay as long as we don't write
+	 * beyond the end of the output buffer, hence the check for (out_end -
+	 * end >= WORDBYTES - 1).
+	 */
+	if (UNALIGNED_ACCESS_IS_FAST && likely(out_end - end >= WORDBYTES - 1))
+	{
+		if (offset >= WORDBYTES) {
+			/* The source and destination words don't overlap. */
+			do {
+				copy_word_unaligned(src, out_next);
+				src += WORDBYTES;
+				out_next += WORDBYTES;
+			} while (out_next < end);
+			return 0;
+		} else if (offset == 1) {
+			/* Offset 1 matches are equivalent to run-length
+			 * encoding of the previous byte.  This case is common
+			 * if the data contains many repeated bytes. */
+			machine_word_t v = repeat_byte(*(out_next - 1));
+			do {
+				store_word_unaligned(v, out_next);
+				src += WORDBYTES;
+				out_next += WORDBYTES;
+			} while (out_next < end);
+			return 0;
+		}
+		/*
+		 * We don't bother with special cases for other 'offset <
+		 * WORDBYTES', which are usually rarer than 'offset == 1'.
+		 * Extra checks will just slow things down.  Actually, it's
+		 * possible to handle all the 'offset < WORDBYTES' cases using
+		 * the same code, but it still becomes more complicated doesn't
+		 * seem any faster overall; it definitely slows down the more
+		 * common 'offset == 1' case.
+		 */
 	}
-	*n = sym;
+
+	/* Fall back to a bytewise copy.  */
+	if (min_length >= 2)
+		*out_next++ = *src++;
+	if (min_length >= 3)
+		*out_next++ = *src++;
+	if (min_length >= 4)
+		*out_next++ = *src++;
+	do {
+		*out_next++ = *src++;
+	} while (out_next != end);
 	return 0;
 }
 
-extern int
-make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
-			  unsigned num_bits, const u8 lengths[],
-			  unsigned max_codeword_len);
-
-/* Minimum alignment for the decode_table parameter to
- * make_huffman_decode_table().  */
-#define DECODE_TABLE_ALIGNMENT 16
-
-#endif /* _WIMLIB_DECOMPRESS_H */
+#endif /* _WIMLIB_DECOMPRESS_COMMON_H */