return 0;
}
-/*
- * Decode a match and copy its bytes into the decompression window.
- *
- * Return the length of the match in bytes, or 0 if the match underflowed the
- * window or overflowed the current block.
- */
-static u32
-lzx_decode_match(unsigned main_symbol, int block_type,
- u32 bytes_remaining, u8 *window, u32 window_pos,
- const struct lzx_tables *tables,
- struct lzx_lru_queue *queue,
- struct input_bitstream *istream)
-{
- unsigned length_header;
- unsigned position_slot;
- u32 match_len;
- u32 match_offset;
- unsigned num_extra_bits;
- u32 verbatim_bits;
- u32 aligned_bits;
-
- /* The main symbol is offset by 256 because values under 256 indicate a
- * literal value. */
- main_symbol -= 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_symbol & LZX_NUM_PRIMARY_LENS;
- position_slot = main_symbol >> 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 code, 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)
- match_len += read_huffsym_using_lencode(istream, tables);
-
- /* 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. */
- if (position_slot <= 2) {
- /* Note: This isn't a real LRU queue, since using the R2 offset
- * doesn't bump the R1 offset down to R2. This quirk allows all
- * 3 recent offsets to be handled by the same code. (For R0,
- * the swap is a no-op.) */
- match_offset = queue->R[position_slot];
- queue->R[position_slot] = queue->R[0];
- queue->R[0] = match_offset;
- } else {
- /* 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 offset
- * code 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. ) */
- verbatim_bits = bitstream_read_bits(istream,
- num_extra_bits - 3);
- verbatim_bits <<= 3;
- aligned_bits = read_huffsym_using_alignedcode(istream,
- tables);
- } 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. */
- verbatim_bits = bitstream_read_bits(istream, num_extra_bits);
- 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;
- }
-
- /* Validate the match, then copy it to the current position. */
-
- if (unlikely(match_len > bytes_remaining))
- return 0;
-
- if (unlikely(match_offset > window_pos))
- return 0;
-
- lz_copy(&window[window_pos], match_len, match_offset,
- &window[window_pos + bytes_remaining]);
-
- return match_len;
-}
-
/*
* Decompress an LZX-compressed block of data.
*
struct lzx_lru_queue *queue,
struct input_bitstream *istream)
{
- u32 block_end;
- unsigned main_symbol;
+ u8 *window_ptr = &window[window_pos];
+ u8 *window_end = window_ptr + block_size;
+ unsigned mainsym;
u32 match_len;
+ unsigned position_slot;
+ u32 match_offset;
+ unsigned num_extra_bits;
+
+ while (window_ptr != window_end) {
- block_end = window_pos + block_size;
- while (window_pos < block_end) {
- main_symbol = read_huffsym_using_maincode(istream, tables);
- if (main_symbol < LZX_NUM_CHARS) {
+ mainsym = read_huffsym_using_maincode(istream, tables);
+ if (mainsym < LZX_NUM_CHARS) {
/* Literal */
- window[window_pos++] = main_symbol;
+ *window_ptr++ = mainsym;
+ continue;
+ }
+
+ /* Match */
+
+ /* Decode the length header and position slot. */
+ mainsym -= LZX_NUM_CHARS;
+ match_len = mainsym & 0x7;
+ position_slot = mainsym >> 3;
+
+ /* If needed, read a length symbol to decode the full length. */
+ if (match_len == 0x7)
+ match_len += read_huffsym_using_lencode(istream, tables);
+ match_len += LZX_MIN_MATCH_LEN;
+
+ if (position_slot <= 2) {
+ /* Repeat offset */
+
+ /* Note: This isn't a real LRU queue, since using the R2
+ * offset doesn't bump the R1 offset down to R2. This
+ * quirk allows all 3 recent offsets to be handled by
+ * the same code. (For R0, the swap is a no-op.) */
+ match_offset = queue->R[position_slot];
+ queue->R[position_slot] = queue->R[0];
+ queue->R[0] = match_offset;
} else {
- /* Match */
- match_len = lzx_decode_match(main_symbol,
- block_type,
- block_end - window_pos,
- window,
- window_pos,
- tables,
- queue,
- istream);
- if (unlikely(match_len == 0))
- return -1;
- window_pos += match_len;
+ /* Explicit offset */
+
+ /* Look up the number of extra bits that need to be read
+ * to decode offsets with this position slot. */
+ num_extra_bits = lzx_get_num_extra_bits(position_slot);
+
+ /* Start with the position slot base value. */
+ match_offset = lzx_position_base[position_slot];
+
+ /* In aligned offset blocks, the low-order 3 bits of
+ * each offset are encoded using the aligned offset
+ * code. Otherwise, all the extra bits are literal. */
+ if (block_type == LZX_BLOCKTYPE_ALIGNED && num_extra_bits >= 3) {
+ match_offset += bitstream_read_bits(istream, num_extra_bits - 3) << 3;
+ match_offset += read_huffsym_using_alignedcode(istream, tables);
+ } else {
+ match_offset += bitstream_read_bits(istream, num_extra_bits);
+ }
+
+ /* Adjust the offset. */
+ match_offset -= LZX_OFFSET_OFFSET;
+
+ /* Update the match offset LRU queue. */
+ queue->R[2] = queue->R[1];
+ queue->R[1] = queue->R[0];
+ queue->R[0] = match_offset;
}
+
+ /* Validate the match, then copy it to the current position. */
+
+ if (unlikely(match_len > window_end - window_ptr))
+ return -1;
+
+ if (unlikely(match_offset > window_ptr - window))
+ return -1;
+
+ lz_copy(window_ptr, match_len, match_offset, window_end);
+
+ window_ptr += match_len;
}
return 0;
}
git://wimlib.net / wimlib / commitdiff