/* Must be at least 32 bits. */
typedef unsigned long input_bitbuf_t;
-/* Structure to provide a bitstream. */
+/* 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. */
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; the next bit is always the high-order bit. */
+ * to low-order, and the next bit is always the high-order bit. */
input_bitbuf_t bitbuf;
/* Pointer to the next byte to be retrieved from the input. */
istream->data_bytes_left = num_data_bytes;
}
-/* Ensures that the bit buffer contains @num_bits bits. */
+/* 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)
{
- wimlib_assert(num_bits <= 16);
+ wimlib_assert2(num_bits <= 16);
+
+ int ret = 0;
/* Unfortunately this needs to be different for the different
* compression types. LZX requires reading no more than the number of
* important because this may change the location of a literal byte
* read with bitstream_read_byte(). */
#ifdef XPRESS_DECOMP
- while (istream->bitsleft < 16) {
+ if (istream->bitsleft < 16) {
#else
- while (istream->bitsleft < num_bits) {
+ if (istream->bitsleft < num_bits) {
#endif
- if (istream->data_bytes_left < 2)
- return 1;
-
- unsigned shift = sizeof(input_bitbuf_t) * 8 - 16 -
- istream->bitsleft;
- istream->bitbuf |= (input_bitbuf_t)le16_to_cpu(
- *(u16*)istream->data) << shift;
- istream->data += 2;
- istream->bitsleft += 16;
- istream->data_bytes_left -= 2;
+ if (istream->data_bytes_left >= 2) {
+ unsigned shift = sizeof(input_bitbuf_t) * 8 - 16 -
+ istream->bitsleft;
+ istream->bitbuf |= (input_bitbuf_t)le16_to_cpu(
+ *(u16*)istream->data) << shift;
+ istream->data += 2;
+ istream->bitsleft += 16;
+ istream->data_bytes_left -= 2;
+ } else {
+ ret = -1;
+ }
}
- return 0;
+ wimlib_assert2(ret != 0 || istream->bitsleft >= num_bits);
+ return ret;
}
-/* Returns the next @num_bits bits in the bit buffer. It must contain at least
- * @num_bits bits to call this function. */
+/* Returns the next @num_bits bits in the buffer variable, which must contain at
+ * least @num_bits bits, for the bitstream. */
static inline unsigned
bitstream_peek_bits(const struct input_bitstream *istream, unsigned num_bits)
{
+ wimlib_assert2(istream->bitsleft >= num_bits);
+ int ret;
if (num_bits == 0)
- return 0;
- return istream->bitbuf >> (sizeof(input_bitbuf_t) * 8 - num_bits);
+ ret = 0;
+ else
+ ret = istream->bitbuf >> (sizeof(input_bitbuf_t) * 8 - num_bits);
+ return ret;
}
-/* Removes @num_bits bits from the bit buffer. It must contain at least
- * @num_bits bits to call this function. */
+/* Removes @num_bits bits from the buffer variable, which must contain at least
+ * @num_bits bits, for the bitstream. */
static inline void bitstream_remove_bits(struct input_bitstream *istream,
unsigned num_bits)
{
+ wimlib_assert2(istream->bitsleft >= num_bits);
istream->bitbuf <<= num_bits;
istream->bitsleft -= num_bits;
}
-/* Reads and returns @num_bits bits from the input bitstream. */
+/* Reads @num_bits bits from the input bitstream. @num_bits must be 16 or fewer.
+ * 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, unsigned *n)
{
- int ret;
- ret = bitstream_ensure_bits(istream, num_bits);
- if (ret != 0) {
+ wimlib_assert2(num_bits <= 16);
+ int ret = bitstream_ensure_bits(istream, num_bits);
+ if (ret == 0) {
+ *n = bitstream_peek_bits(istream, num_bits);
+ bitstream_remove_bits(istream, num_bits);
+ } else {
ERROR("bitstream_read_bits(): Input buffer exhausted");
- return ret;
}
- *n = bitstream_peek_bits(istream, num_bits);
- bitstream_remove_bits(istream, num_bits);
- return 0;
+ return ret;
}
-/* In the XPRESS format there can be literal length bytes embedded in the
- * compressed bitstream. These bytes are basically separate from the bitstream,
- * as they come AFTER the bits that are currently in the buffer variable (based
- * on reading 16 bits at a time), even though the buffer variable may not be
- * empty.
+/* In the XPRESS format there can be literal bytes embedded in the bitstream.
+ * These bytes are basically separate from the bitstream, as they come AFTER the
+ * bits that are currently in the buffer variable (based on reading 16 bits at a
+ * time), even though the buffer variable may not be empty.
*
- * This function returns the next such literal length byte in the input
- * bitstream. Returns -1 if we are at the end of the bitstream. */
+ * This function returns the next such literal byte, or -1 if there are no more.
+ */
static inline int bitstream_read_byte(struct input_bitstream *istream)
{
- wimlib_assert(istream->bitsleft < 32);
+ wimlib_assert2(istream->bitsleft < 32);
+ int ret;
if (istream->data_bytes_left == 0) {
ERROR("bitstream_read_byte(): Input buffer exhausted");
- return -1;
+ ret = -1;
+ } else {
+ istream->data_bytes_left--;
+ ret = *istream->data++;
}
- istream->data_bytes_left--;
- return *istream->data++;
+ return ret;
}
-/* Reads @num_bits bits from the bit buffer without checking to see if that many
- * bits are in the buffer or not. */
+/* 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 unsigned
bitstream_read_bits_nocheck(struct input_bitstream *istream, unsigned num_bits)
{
return n;
}
-/* Removes the bits that have been read into the bit buffer variable. */
-static inline void flush_input_bitstream(struct input_bitstream *istream)
-{
- bitstream_remove_bits(istream, istream->bitsleft);
- istream->bitsleft = 0;
- istream->bitbuf = 0;
-}
+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);
-extern int bitstream_read_bytes(struct input_bitstream *istream, size_t n,
- void *dest);
-
-extern int align_input_bitstream(struct input_bitstream *istream,
- bool skip_word_if_aligned);
+/*
+ * Reads a Huffman-encoded symbol from a bitstream.
+ *
+ * This function may be called hundreds of millions of times when extracting a
+ * large WIM file. I'm not sure it could be made much faster that it is,
+ * especially since there isn't enough time to make a big table that allows
+ * decoding multiple symbols per lookup. But if extracting files to a hard
+ * disk, the I/O will be the bottleneck anyway.
+ *
+ * @buf: The input buffer from which the symbol will be read.
+ * @decode_table: The fast Huffman decoding table for the Huffman tree.
+ * @lengths: The table that gives the length of the code for each
+ * symbol.
+ * @num_symbols: The number of symbols in the Huffman code.
+ * @table_bits: Huffman codes this length or less can be looked up
+ * directory in the decode_table, as the
+ * decode_table contains 2**table_bits entries.
+ */
+static inline int read_huffsym(struct input_bitstream *istream,
+ const u16 decode_table[],
+ const u8 lens[],
+ unsigned num_syms,
+ unsigned table_bits,
+ unsigned *n,
+ unsigned max_codeword_len)
+{
+ int ret;
-extern int read_huffsym(struct input_bitstream *stream,
- const u16 decode_table[],
- const u8 lengths[],
- unsigned num_symbols,
- unsigned table_bits,
- unsigned *n,
- unsigned max_codeword_len);
+ /* In the most common case, there are at least max_codeword_len bits
+ * remaining in the stream. */
+ if (bitstream_ensure_bits(istream, max_codeword_len) == 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 the entry in the decode table is not a valid symbol, it is
+ * the offset of the root of its Huffman subtree. */
+ if (sym >= num_syms) {
+ bitstream_remove_bits(istream, table_bits);
+ do {
+ key_bits = sym + bitstream_peek_bits(istream, 1);
+ bitstream_remove_bits(istream, 1);
+
+ wimlib_assert2(key_bits < num_syms * 2 +
+ (1 << table_bits));
+ } while ((sym = decode_table[key_bits]) >= num_syms);
+ } else {
+ wimlib_assert2(lens[sym] <= table_bits);
+ bitstream_remove_bits(istream, lens[sym]);
+ }
+ *n = sym;
+ ret = 0;
+ } else {
+ /* Otherwise, we must be careful to use only the bits that are
+ * actually remaining. */
+ ret = read_huffsym_near_end_of_input(istream, decode_table,
+ lens, num_syms,
+ table_bits, n);
+ }
+ return ret;
+}
extern int make_huffman_decode_table(u16 decode_table[], unsigned num_syms,
unsigned num_bits, const u8 lengths[],
git://wimlib.net / wimlib / blobdiff