* Huffman codes that were computed for the block.
*
* Note: the algorithm does not yet attempt to split the input into multiple LZX
- * blocks, instead using a series of blocks of LZX_DIV_BLOCK_SIZE bytes.
+ * blocks; it instead uses a series of blocks of LZX_DIV_BLOCK_SIZE bytes.
*
* Fast algorithm
* --------------
* chunks.
*
* We reserve a few extra bytes to potentially allow reading off the end
- * of the array in the match-finding code for optimization purposes.
- */
+ * of the array in the match-finding code for optimization purposes
+ * (currently only needed for the hash chain match-finder). */
u8 *window;
/* Number of bytes of data to be compressed, which is the number of
}
/*
- * Output an LZX match.
+ * Output a precomputed LZX match.
*
- * @out: The bitstream to write the match to.
- * @block_type: The type of the LZX block (LZX_BLOCKTYPE_ALIGNED or LZX_BLOCKTYPE_VERBATIM)
- * @match: The match.
- * @codes: Pointer to a structure that contains the codewords for the
- * main, length, and aligned offset Huffman codes.
+ * @out:
+ * The bitstream to which to write the match.
+ * @block_type:
+ * The type of the LZX block (LZX_BLOCKTYPE_ALIGNED or
+ * LZX_BLOCKTYPE_VERBATIM)
+ * @match:
+ * The match, as a (length, offset) pair.
+ * @codes:
+ * Pointer to a structure that contains the codewords for the main, length,
+ * and aligned offset Huffman codes for the current LZX compressed block.
*/
static void
lzx_write_match(struct output_bitstream *out, int block_type,
}
}
+/* Output an LZX literal (encoded with the main Huffman code). */
+static void
+lzx_write_literal(struct output_bitstream *out, u8 literal,
+ const struct lzx_codes *codes)
+{
+ bitstream_put_bits(out,
+ codes->codewords.main[literal],
+ codes->lens.main[literal]);
+}
+
static unsigned
lzx_build_precode(const u8 lens[restrict],
const u8 prev_lens[restrict],
}
/*
- * Writes a compressed Huffman code to the output, preceded by the precode for
- * it.
+ * Output a Huffman code in the compressed form used in LZX.
+ *
+ * The Huffman code is represented in the output as a logical series of codeword
+ * lengths from which the Huffman code, which must be in canonical form, can be
+ * reconstructed.
+ *
+ * The codeword lengths are themselves compressed using a separate Huffman code,
+ * the "precode", which contains a symbol for each possible codeword length in
+ * the larger code as well as several special symbols to represent repeated
+ * codeword lengths (a form of run-length encoding). The precode is itself
+ * constructed in canonical form, and its codeword lengths are represented
+ * literally in 20 4-bit fields that immediately precede the compressed codeword
+ * lengths of the larger code.
*
- * The Huffman code is represented in the output as a series of path lengths
- * from which the canonical Huffman code can be reconstructed. The path lengths
- * themselves are compressed using a separate Huffman code, the precode, which
- * consists of LZX_PRECODE_NUM_SYMBOLS (= 20) symbols that cover all possible
- * code lengths, plus extra codes for repeated lengths. The path lengths of the
- * precode precede the path lengths of the larger code and are uncompressed,
- * consisting of 20 entries of 4 bits each.
+ * Furthermore, the codeword lengths of the larger code are actually represented
+ * as deltas from the codeword lengths of the corresponding code in the previous
+ * block.
*
- * @out: Bitstream to write the code to.
- * @lens: The code lengths for the Huffman code, indexed by symbol.
- * @prev_lens: Code lengths for this Huffman code, indexed by symbol,
- * in the *previous block*, or all zeroes if this is the
- * first block.
- * @num_syms: The number of symbols in the code.
+ * @out:
+ * Bitstream to which to write the compressed Huffman code.
+ * @lens:
+ * The codeword lengths, indexed by symbol, in the Huffman code.
+ * @prev_lens:
+ * The codeword lengths, indexed by symbol, in the corresponding Huffman
+ * code in the previous block, or all zeroes if this is the first block.
+ * @num_syms:
+ * The number of symbols in the Huffman code.
*/
static void
lzx_write_compressed_code(struct output_bitstream *out,
}
/*
- * Writes all compressed matches and literal bytes in an LZX block to the the
- * output bitstream.
+ * Write all matches and literal bytes (which were precomputed) in an LZX
+ * compressed block to the output bitstream in the final compressed
+ * representation.
*
* @ostream
* The output bitstream.
* @block_type
- * The type of the block (LZX_BLOCKTYPE_ALIGNED or LZX_BLOCKTYPE_VERBATIM).
+ * The chosen type of the LZX compressed block (LZX_BLOCKTYPE_ALIGNED or
+ * LZX_BLOCKTYPE_VERBATIM).
* @match_tab
- * The array of matches/literals that will be output (length @match_count).
+ * The array of matches/literals to output.
* @match_count
- * Number of matches/literals to be output.
+ * Number of matches/literals to output (length of @match_tab).
* @codes
- * Pointer to a structure that contains the codewords for the main, length,
- * and aligned offset Huffman codes.
+ * The main, length, and aligned offset Huffman codes for the current
+ * LZX compressed block.
*/
static void
lzx_write_matches_and_literals(struct output_bitstream *ostream,
for (unsigned i = 0; i < match_count; i++) {
struct lzx_match match = match_tab[i];
- /* High bit of the match indicates whether the match is an
- * actual match (1) or a literal uncompressed byte (0) */
- if (match.data & 0x80000000) {
- /* match */
- lzx_write_match(ostream, block_type,
- match, codes);
- } else {
- /* literal byte */
- bitstream_put_bits(ostream,
- codes->codewords.main[match.data],
- codes->lens.main[match.data]);
- }
+ /* The high bit of the 32-bit intermediate representation
+ * indicates whether the item is an actual LZ-style match (1) or
+ * a literal byte (0). */
+ if (match.data & 0x80000000)
+ lzx_write_match(ostream, block_type, match, codes);
+ else
+ lzx_write_literal(ostream, match.data, codes);
}
}
/* Retrieve a list of matches available at the next position in the input.
*
- * The matches are written to ctx->matches in decreasing order of length, and
- * the return value is the number of matches found. */
+ * A pointer to the matches array is written into @matches_ret, and the return
+ * value is the number of matches found. */
static u32
lzx_lz_get_matches_caching(struct lzx_compressor *ctx,
const struct lzx_lru_queue *queue,
&ctx->queue);
}
-/*
- * Set default symbol costs.
- */
+/* Set default symbol costs for the LZX Huffman codes. */
static void
lzx_set_default_costs(struct lzx_costs * costs, unsigned num_main_syms)
{
unsigned i;
- /* Literal symbols */
+ /* Main code (part 1): Literal symbols */
for (i = 0; i < LZX_NUM_CHARS; i++)
costs->main[i] = 8;
- /* Match header symbols */
+ /* Main code (part 2): Match header symbols */
for (; i < num_main_syms; i++)
costs->main[i] = 10;
- /* Length symbols */
+ /* Length code */
for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++)
costs->len[i] = 8;
- /* Aligned offset symbols */
+ /* Aligned offset code */
for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++)
costs->aligned[i] = 3;
}
-/* Given the frequencies of symbols in a compressed block and the corresponding
- * Huffman codes, return LZX_BLOCKTYPE_ALIGNED or LZX_BLOCKTYPE_VERBATIM if an
- * aligned offset or verbatim block, respectively, will take fewer bits to
- * output. */
+/* Given the frequencies of symbols in an LZX-compressed block and the
+ * corresponding Huffman codes, return LZX_BLOCKTYPE_ALIGNED or
+ * LZX_BLOCKTYPE_VERBATIM if an aligned offset or verbatim block, respectively,
+ * will take fewer bits to output. */
static int
lzx_choose_verbatim_or_aligned(const struct lzx_freqs * freqs,
const struct lzx_codes * codes)
/* Verbatim blocks have a constant 3 bits per position footer. Aligned
* offset blocks have an aligned offset symbol per position footer, plus
- * an extra 24 bits to output the lengths necessary to reconstruct the
- * aligned offset code itself. */
+ * an extra 24 bits per block to output the lengths necessary to
+ * reconstruct the aligned offset code itself. */
for (unsigned i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
verbatim_cost += 3 * freqs->aligned[i];
aligned_cost += codes->lens.aligned[i] * freqs->aligned[i];
}
/* Find a near-optimal sequence of matches/literals with which to output the
- * specified LZX block, then set its type to that which has the minimum cost to
- * output. */
+ * specified LZX block, then set the block's type to that which has the minimum
+ * cost to output (either verbatim or aligned). */
static void
lzx_optimize_block(struct lzx_compressor *ctx, struct lzx_block_spec *spec,
unsigned num_passes)
/* Set up a default cost model. */
lzx_set_default_costs(&ctx->costs, ctx->num_main_syms);
+ /* TODO: The compression ratio could be slightly improved by performing
+ * data-dependent block splitting instead of using fixed-size blocks.
+ * Doing so well is a computationally hard problem, however. */
ctx->num_blocks = DIV_ROUND_UP(ctx->window_size, LZX_DIV_BLOCK_SIZE);
for (unsigned i = 0; i < ctx->num_blocks; i++) {
unsigned pos = LZX_DIV_BLOCK_SIZE * i;
return true;
}
-
const struct compressor_ops lzx_compressor_ops = {
.params_valid = lzx_params_valid,
.get_needed_memory = lzx_get_needed_memory,
git://wimlib.net / wimlib / blobdiff