/*
* lzx_compress.c
*
- * A compressor for the LZX compression format, as used in WIM files.
+ * A compressor for the LZX compression format, as used in WIM archives.
*/
/*
- * Copyright (C) 2012, 2013, 2014, 2015 Eric Biggers
+ * Copyright (C) 2012-2016 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
* This file contains a compressor for the LZX ("Lempel-Ziv eXtended")
* compression format, as used in the WIM (Windows IMaging) file format.
*
- * Two different parsing algorithms are implemented: "near-optimal" and "lazy".
- * "Near-optimal" is significantly slower than "lazy", but results in a better
- * compression ratio. The "near-optimal" algorithm is used at the default
- * compression level.
+ * Two different LZX-compatible algorithms are implemented: "near-optimal" and
+ * "lazy". "Near-optimal" is significantly slower than "lazy", but results in a
+ * better compression ratio. The "near-optimal" algorithm is used at the
+ * default compression level.
*
* This file may need some slight modifications to be used outside of the WIM
* format. In particular, in other situations the LZX block header might be
* slightly different, and sliding window support might be required.
*
- * Note: LZX is a compression format derived from DEFLATE, the format used by
- * zlib and gzip. Both LZX and DEFLATE use LZ77 matching and Huffman coding.
- * Certain details are quite similar, such as the method for storing Huffman
- * codes. However, the main differences are:
+ * LZX is a compression format derived from DEFLATE, the format used by zlib and
+ * gzip. Both LZX and DEFLATE use LZ77 matching and Huffman coding. Certain
+ * details are quite similar, such as the method for storing Huffman codes.
+ * However, the main differences are:
*
* - LZX preprocesses the data to attempt to make x86 machine code slightly more
* compressible before attempting to compress it further.
* ("verbatim" and "aligned").
*
* - LZX has a minimum match length of 2 rather than 3. Length 2 matches can be
- * useful, but generally only if the parser is smart about choosing them.
+ * useful, but generally only if the compressor is smart about choosing them.
*
* - In LZX, offset slots 0 through 2 actually represent entries in an LRU queue
* of match offsets. This is very useful for certain types of files, such as
* binary files that have repeating records.
*/
-#ifdef HAVE_CONFIG_H
-# include "config.h"
-#endif
+/******************************************************************************/
+/* General parameters */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * The compressor uses the faster algorithm at levels <= MAX_FAST_LEVEL. It
+ * uses the slower algorithm at levels > MAX_FAST_LEVEL.
+ */
+#define MAX_FAST_LEVEL 34
+
+/*
+ * The compressor-side limits on the codeword lengths (in bits) for each Huffman
+ * code. To make outputting bits slightly faster, some of these limits are
+ * lower than the limits defined by the LZX format. This does not significantly
+ * affect the compression ratio.
+ */
+#define MAIN_CODEWORD_LIMIT 16
+#define LENGTH_CODEWORD_LIMIT 12
+#define ALIGNED_CODEWORD_LIMIT 7
+#define PRE_CODEWORD_LIMIT 7
+
+
+/******************************************************************************/
+/* Block splitting parameters */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * The compressor always outputs blocks of at least this size in bytes, except
+ * for the last block which may need to be smaller.
+ */
+#define MIN_BLOCK_SIZE 6500
/*
- * Start a new LZX block (with new Huffman codes) after this many bytes.
+ * The compressor attempts to end a block when it reaches this size in bytes.
+ * The final size might be slightly larger due to matches extending beyond the
+ * end of the block. Specifically:
*
- * Note: actual block sizes may slightly exceed this value.
+ * - The near-optimal compressor may choose a match of up to LZX_MAX_MATCH_LEN
+ * bytes starting at position 'SOFT_MAX_BLOCK_SIZE - 1'.
*
- * TODO: recursive splitting and cost evaluation might be good for an extremely
- * high compression mode, but otherwise it is almost always far too slow for how
- * much it helps. Perhaps some sort of heuristic would be useful?
+ * - The lazy compressor may choose a sequence of literals starting at position
+ * 'SOFT_MAX_BLOCK_SIZE - 1' when it sees a sequence of increasingly better
+ * matches. The final match may be up to LZX_MAX_MATCH_LEN bytes. The
+ * length of the literal sequence is approximately limited by the "nice match
+ * length" parameter.
*/
-#define LZX_DIV_BLOCK_SIZE 32768
+#define SOFT_MAX_BLOCK_SIZE 100000
/*
- * LZX_CACHE_PER_POS is the number of lz_match structures to reserve in the
- * match cache for each byte position. This value should be high enough so that
- * nearly the time, all matches found in a given block can fit in the match
- * cache. However, fallback behavior (immediately terminating the block) on
- * cache overflow is still required.
+ * The number of observed items (matches and literals) that represents
+ * sufficient data for the compressor to decide whether the current block should
+ * be ended or not.
*/
-#define LZX_CACHE_PER_POS 7
+#define NUM_OBSERVATIONS_PER_BLOCK_CHECK 400
+
+
+/******************************************************************************/
+/* Parameters for slower algorithm */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * The log base 2 of the number of entries in the hash table for finding length
+ * 2 matches. This could be as high as 16, but using a smaller hash table
+ * speeds up compression due to reduced cache pressure.
+ */
+#define BT_MATCHFINDER_HASH2_ORDER 12
/*
- * LZX_CACHE_LENGTH is the number of lz_match structures in the match cache,
- * excluding the extra "overflow" entries. The per-position multiplier is '1 +
- * LZX_CACHE_PER_POS' instead of 'LZX_CACHE_PER_POS' because there is an
- * overhead of one lz_match per position, used to hold the match count at that
- * position.
+ * The number of lz_match structures in the match cache, excluding the extra
+ * "overflow" entries. This value should be high enough so that nearly the
+ * time, all matches found in a given block can fit in the match cache.
+ * However, fallback behavior (immediately terminating the block) on cache
+ * overflow is still required.
*/
-#define LZX_CACHE_LENGTH (LZX_DIV_BLOCK_SIZE * (1 + LZX_CACHE_PER_POS))
+#define CACHE_LENGTH (SOFT_MAX_BLOCK_SIZE * 5)
/*
- * LZX_MAX_MATCHES_PER_POS is an upper bound on the number of matches that can
- * ever be saved in the match cache for a single position. Since each match we
- * save for a single position has a distinct length, we can use the number of
- * possible match lengths in LZX as this bound. This bound is guaranteed to be
- * valid in all cases, although if 'nice_match_length < LZX_MAX_MATCH_LEN', then
- * it will never actually be reached.
+ * An upper bound on the number of matches that can ever be saved in the match
+ * cache for a single position. Since each match we save for a single position
+ * has a distinct length, we can use the number of possible match lengths in LZX
+ * as this bound. This bound is guaranteed to be valid in all cases, although
+ * if 'nice_match_length < LZX_MAX_MATCH_LEN', then it will never actually be
+ * reached.
*/
-#define LZX_MAX_MATCHES_PER_POS LZX_NUM_LENS
+#define MAX_MATCHES_PER_POS LZX_NUM_LENS
/*
- * LZX_BIT_COST is a scaling factor that represents the cost to output one bit.
- * This makes it possible to consider fractional bit costs.
+ * A scaling factor that makes it possible to consider fractional bit costs. A
+ * single bit has a cost of BIT_COST.
*
* Note: this is only useful as a statistical trick for when the true costs are
- * unknown. In reality, each token in LZX requires a whole number of bits to
+ * unknown. Ultimately, each token in LZX requires a whole number of bits to
* output.
*/
-#define LZX_BIT_COST 16
+#define BIT_COST 64
/*
- * Should the compressor take into account the costs of aligned offset symbols?
+ * Should the compressor take into account the costs of aligned offset symbols
+ * instead of assuming that all are equally likely?
*/
-#define LZX_CONSIDER_ALIGNED_COSTS 1
+#define CONSIDER_ALIGNED_COSTS 1
/*
- * LZX_MAX_FAST_LEVEL is the maximum compression level at which we use the
- * faster algorithm.
+ * Should the "minimum" cost path search algorithm consider "gap" matches, where
+ * a normal match is followed by a literal, then by a match with the same
+ * offset? This is one specific, somewhat common situation in which the true
+ * minimum cost path is often different from the path found by looking only one
+ * edge ahead.
*/
-#define LZX_MAX_FAST_LEVEL 34
+#define CONSIDER_GAP_MATCHES 1
-/*
- * BT_MATCHFINDER_HASH2_ORDER is the log base 2 of the number of entries in the
- * hash table for finding length 2 matches. This could be as high as 16, but
- * using a smaller hash table speeds up compression due to reduced cache
- * pressure.
- */
-#define BT_MATCHFINDER_HASH2_ORDER 12
+/******************************************************************************/
+/* Includes */
+/*----------------------------------------------------------------------------*/
-/*
- * These are the compressor-side limits on the codeword lengths for each Huffman
- * code. To make outputting bits slightly faster, some of these limits are
- * lower than the limits defined by the LZX format. This does not significantly
- * affect the compression ratio, at least for the block sizes we use.
- */
-#define MAIN_CODEWORD_LIMIT 12 /* 64-bit: can buffer 4 main symbols */
-#define LENGTH_CODEWORD_LIMIT 12
-#define ALIGNED_CODEWORD_LIMIT 7
-#define PRE_CODEWORD_LIMIT 7
+#ifdef HAVE_CONFIG_H
+# include "config.h"
+#endif
#include "wimlib/compress_common.h"
#include "wimlib/compressor_ops.h"
#include "wimlib/unaligned.h"
#include "wimlib/util.h"
-/* Matchfinders with 16-bit positions */
+/* Note: BT_MATCHFINDER_HASH2_ORDER must be defined before including
+ * bt_matchfinder.h. */
+
+/* Matchfinders with 16-bit positions */
#define mf_pos_t u16
#define MF_SUFFIX _16
#include "wimlib/bt_matchfinder.h"
#include "wimlib/hc_matchfinder.h"
-/* Matchfinders with 32-bit positions */
+/* Matchfinders with 32-bit positions */
#undef mf_pos_t
#undef MF_SUFFIX
#define mf_pos_t u32
#include "wimlib/bt_matchfinder.h"
#include "wimlib/hc_matchfinder.h"
-struct lzx_output_bitstream;
+/******************************************************************************/
+/* Compressor structure */
+/*----------------------------------------------------------------------------*/
-/* Codewords for the LZX Huffman codes. */
+/* Codewords for the Huffman codes */
struct lzx_codewords {
u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
u32 len[LZX_LENCODE_NUM_SYMBOLS];
u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};
-/* Codeword lengths (in bits) for the LZX Huffman codes.
- * A zero length means the corresponding codeword has zero frequency. */
+/*
+ * Codeword lengths, in bits, for the Huffman codes.
+ *
+ * A codeword length of 0 means the corresponding codeword has zero frequency.
+ *
+ * The main and length codes each have one extra entry for use as a sentinel.
+ * See lzx_write_compressed_code().
+ */
struct lzx_lens {
u8 main[LZX_MAINCODE_MAX_NUM_SYMBOLS + 1];
u8 len[LZX_LENCODE_NUM_SYMBOLS + 1];
u8 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};
-/* Cost model for near-optimal parsing */
-struct lzx_costs {
-
- /* 'match_cost[offset_slot][len - LZX_MIN_MATCH_LEN]' is the cost for a
- * length 'len' match that has an offset belonging to 'offset_slot'. */
- u32 match_cost[LZX_MAX_OFFSET_SLOTS][LZX_NUM_LENS];
-
- /* Cost for each symbol in the main code */
- u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
-
- /* Cost for each symbol in the length code */
- u32 len[LZX_LENCODE_NUM_SYMBOLS];
-
-#if LZX_CONSIDER_ALIGNED_COSTS
- /* Cost for each symbol in the aligned code */
- u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
-#endif
-};
-
-/* Codewords and lengths for the LZX Huffman codes. */
+/* Codewords and lengths for the Huffman codes */
struct lzx_codes {
struct lzx_codewords codewords;
struct lzx_lens lens;
};
-/* Symbol frequency counters for the LZX Huffman codes. */
+/* Symbol frequency counters for the Huffman-encoded alphabets */
struct lzx_freqs {
u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
u32 len[LZX_LENCODE_NUM_SYMBOLS];
u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
};
+/* Block split statistics. See the "Block splitting algorithm" section later in
+ * this file for details. */
+#define NUM_LITERAL_OBSERVATION_TYPES 8
+#define NUM_MATCH_OBSERVATION_TYPES 2
+#define NUM_OBSERVATION_TYPES (NUM_LITERAL_OBSERVATION_TYPES + \
+ NUM_MATCH_OBSERVATION_TYPES)
+struct lzx_block_split_stats {
+ u32 new_observations[NUM_OBSERVATION_TYPES];
+ u32 observations[NUM_OBSERVATION_TYPES];
+ u32 num_new_observations;
+ u32 num_observations;
+};
+
/*
* Represents a run of literals followed by a match or end-of-block. This
- * struct is needed to temporarily store items chosen by the parser, since items
- * cannot be written until all items for the block have been chosen and the
- * block's Huffman codes have been computed.
+ * structure is needed to temporarily store items chosen by the compressor,
+ * since items cannot be written until all items for the block have been chosen
+ * and the block's Huffman codes have been computed.
*/
struct lzx_sequence {
u16 adjusted_length;
/* If bit 31 is clear, then this field contains the match header in bits
- * 0-8 and the match offset minus LZX_OFFSET_ADJUSTMENT in bits 9-30.
- * Otherwise, this sequence's literal run was the last literal run in
- * the block, so there is no match that follows it. */
+ * 0-8, and either the match offset plus LZX_OFFSET_ADJUSTMENT or a
+ * recent offset code in bits 9-30. Otherwise (if bit 31 is set), this
+ * sequence's literal run was the last literal run in the block, so
+ * there is no match that follows it. */
u32 adjusted_offset_and_match_hdr;
};
u32 cost;
/*
- * The match or literal that was taken to reach this position. This can
- * change as progressively lower cost paths are found to reach this
- * position.
+ * The best arrival to this node, i.e. the match or literal that was
+ * used to arrive to this position at the given 'cost'. This can change
+ * as progressively lower cost paths are found to reach this position.
*
- * This variable is divided into two bitfields.
+ * For non-gap matches, this variable is divided into two bitfields
+ * whose meanings depend on the item type:
*
* Literals:
* Low bits are 0, high bits are the literal.
*
* Explicit offset matches:
- * Low bits are the match length, high bits are the offset plus 2.
+ * Low bits are the match length, high bits are the offset plus
+ * LZX_OFFSET_ADJUSTMENT.
*
* Repeat offset matches:
* Low bits are the match length, high bits are the queue index.
+ *
+ * For gap matches, identified by OPTIMUM_GAP_MATCH set, special
+ * behavior applies --- see the code.
*/
u32 item;
#define OPTIMUM_OFFSET_SHIFT 9
#define OPTIMUM_LEN_MASK ((1 << OPTIMUM_OFFSET_SHIFT) - 1)
-} _aligned_attribute(8);
+#if CONSIDER_GAP_MATCHES
+# define OPTIMUM_GAP_MATCH 0x80000000
+#endif
-/*
- * Least-recently-used queue for match offsets.
- *
- * This is represented as a 64-bit integer for efficiency. There are three
- * offsets of 21 bits each. Bit 64 is garbage.
- */
-struct lzx_lru_queue {
- u64 R;
-};
+} _aligned_attribute(8);
-#define LZX_QUEUE64_OFFSET_SHIFT 21
-#define LZX_QUEUE64_OFFSET_MASK (((u64)1 << LZX_QUEUE64_OFFSET_SHIFT) - 1)
+/* The cost model for near-optimal parsing */
+struct lzx_costs {
-#define LZX_QUEUE64_R0_SHIFT (0 * LZX_QUEUE64_OFFSET_SHIFT)
-#define LZX_QUEUE64_R1_SHIFT (1 * LZX_QUEUE64_OFFSET_SHIFT)
-#define LZX_QUEUE64_R2_SHIFT (2 * LZX_QUEUE64_OFFSET_SHIFT)
+ /*
+ * 'match_cost[offset_slot][len - LZX_MIN_MATCH_LEN]' is the cost of a
+ * length 'len' match which has an offset belonging to 'offset_slot'.
+ * The cost includes the main symbol, the length symbol if required, and
+ * the extra offset bits if any, excluding any entropy-coded bits
+ * (aligned offset bits). It does *not* include the cost of the aligned
+ * offset symbol which may be required.
+ */
+ u16 match_cost[LZX_MAX_OFFSET_SLOTS][LZX_NUM_LENS];
-#define LZX_QUEUE64_R0_MASK (LZX_QUEUE64_OFFSET_MASK << LZX_QUEUE64_R0_SHIFT)
-#define LZX_QUEUE64_R1_MASK (LZX_QUEUE64_OFFSET_MASK << LZX_QUEUE64_R1_SHIFT)
-#define LZX_QUEUE64_R2_MASK (LZX_QUEUE64_OFFSET_MASK << LZX_QUEUE64_R2_SHIFT)
+ /* Cost of each symbol in the main code */
+ u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS];
-static inline void
-lzx_lru_queue_init(struct lzx_lru_queue *queue)
-{
- queue->R = ((u64)1 << LZX_QUEUE64_R0_SHIFT) |
- ((u64)1 << LZX_QUEUE64_R1_SHIFT) |
- ((u64)1 << LZX_QUEUE64_R2_SHIFT);
-}
+ /* Cost of each symbol in the length code */
+ u32 len[LZX_LENCODE_NUM_SYMBOLS];
-static inline u64
-lzx_lru_queue_R0(struct lzx_lru_queue queue)
-{
- return (queue.R >> LZX_QUEUE64_R0_SHIFT) & LZX_QUEUE64_OFFSET_MASK;
-}
+#if CONSIDER_ALIGNED_COSTS
+ /* Cost of each symbol in the aligned offset code */
+ u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS];
+#endif
+};
-static inline u64
-lzx_lru_queue_R1(struct lzx_lru_queue queue)
-{
- return (queue.R >> LZX_QUEUE64_R1_SHIFT) & LZX_QUEUE64_OFFSET_MASK;
-}
+struct lzx_output_bitstream;
-static inline u64
-lzx_lru_queue_R2(struct lzx_lru_queue queue)
-{
- return (queue.R >> LZX_QUEUE64_R2_SHIFT) & LZX_QUEUE64_OFFSET_MASK;
-}
+/* The main LZX compressor structure */
+struct lzx_compressor {
-/* Push a match offset onto the front (most recently used) end of the queue. */
-static inline struct lzx_lru_queue
-lzx_lru_queue_push(struct lzx_lru_queue queue, u32 offset)
-{
- return (struct lzx_lru_queue) {
- .R = (queue.R << LZX_QUEUE64_OFFSET_SHIFT) | offset,
- };
-}
+ /* The buffer for preprocessed input data, if not using destructive
+ * compression */
+ void *in_buffer;
-/* Pop a match offset off the front (most recently used) end of the queue. */
-static inline u32
-lzx_lru_queue_pop(struct lzx_lru_queue *queue_p)
-{
- u32 offset = queue_p->R & LZX_QUEUE64_OFFSET_MASK;
- queue_p->R >>= LZX_QUEUE64_OFFSET_SHIFT;
- return offset;
-}
+ /* If true, then the compressor need not preserve the input buffer if it
+ * compresses the data successfully */
+ bool destructive;
-/* Swap a match offset to the front of the queue. */
-static inline struct lzx_lru_queue
-lzx_lru_queue_swap(struct lzx_lru_queue queue, unsigned idx)
-{
- if (idx == 0)
- return queue;
-
- if (idx == 1)
- return (struct lzx_lru_queue) {
- .R = (lzx_lru_queue_R1(queue) << LZX_QUEUE64_R0_SHIFT) |
- (lzx_lru_queue_R0(queue) << LZX_QUEUE64_R1_SHIFT) |
- (queue.R & LZX_QUEUE64_R2_MASK),
- };
+ /* Pointer to the compress() implementation chosen at allocation time */
+ void (*impl)(struct lzx_compressor *, const u8 *, size_t,
+ struct lzx_output_bitstream *);
- return (struct lzx_lru_queue) {
- .R = (lzx_lru_queue_R2(queue) << LZX_QUEUE64_R0_SHIFT) |
- (queue.R & LZX_QUEUE64_R1_MASK) |
- (lzx_lru_queue_R0(queue) << LZX_QUEUE64_R2_SHIFT),
- };
-}
+ /* The log base 2 of the window size for match offset encoding purposes.
+ * This will be >= LZX_MIN_WINDOW_ORDER and <= LZX_MAX_WINDOW_ORDER. */
+ unsigned window_order;
-/* The main LZX compressor structure */
-struct lzx_compressor {
+ /* The number of symbols in the main alphabet. This depends on the
+ * window order, since the window order determines the maximum possible
+ * match offset. */
+ unsigned num_main_syms;
- /* The "nice" match length: if a match of this length is found, then
- * choose it immediately without further consideration. */
+ /* The "nice" match length: if a match of this length is found, then it
+ * is chosen immediately without further consideration. */
unsigned nice_match_length;
- /* The maximum search depth: consider at most this many potential
- * matches at each position. */
+ /* The maximum search depth: at most this many potential matches are
+ * considered at each position. */
unsigned max_search_depth;
- /* The log base 2 of the LZX window size for LZ match offset encoding
- * purposes. This will be >= LZX_MIN_WINDOW_ORDER and <=
- * LZX_MAX_WINDOW_ORDER. */
- unsigned window_order;
-
- /* The number of symbols in the main alphabet. This depends on
- * @window_order, since @window_order determines the maximum possible
- * offset. */
- unsigned num_main_syms;
-
- /* Number of optimization passes per block */
+ /* The number of optimization passes per block */
unsigned num_optim_passes;
- /* The preprocessed buffer of data being compressed */
- u8 *in_buffer;
-
- /* The number of bytes of data to be compressed, which is the number of
- * bytes of data in @in_buffer that are actually valid. */
- size_t in_nbytes;
-
- /* Pointer to the compress() implementation chosen at allocation time */
- void (*impl)(struct lzx_compressor *, struct lzx_output_bitstream *);
-
- /* If true, the compressor need not preserve the input buffer if it
- * compresses the data successfully. */
- bool destructive;
-
- /* The Huffman symbol frequency counters for the current block. */
+ /* The symbol frequency counters for the current block */
struct lzx_freqs freqs;
+ /* Block split statistics for the current block */
+ struct lzx_block_split_stats split_stats;
+
/* The Huffman codes for the current and previous blocks. The one with
* index 'codes_index' is for the current block, and the other one is
- * for the previous block. */
+ * for the previous block. */
struct lzx_codes codes[2];
unsigned codes_index;
- /* The matches and literals that the parser has chosen for the current
- * block. The required length of this array is limited by the maximum
- * number of matches that can ever be chosen for a single block, plus
- * one for the special entry at the end. */
+ /* The matches and literals that the compressor has chosen for the
+ * current block. The required length of this array is limited by the
+ * maximum number of matches that can ever be chosen for a single block,
+ * plus one for the special entry at the end. */
struct lzx_sequence chosen_sequences[
- DIV_ROUND_UP(LZX_DIV_BLOCK_SIZE, LZX_MIN_MATCH_LEN) + 1];
-
- /* Tables for mapping adjusted offsets to offset slots */
+ DIV_ROUND_UP(SOFT_MAX_BLOCK_SIZE, LZX_MIN_MATCH_LEN) + 1];
- /* offset slots [0, 29] */
- u8 offset_slot_tab_1[32768];
-
- /* offset slots [30, 49] */
- u8 offset_slot_tab_2[128];
+ /* Tables for mapping adjusted offsets to offset slots */
+ u8 offset_slot_tab_1[32768]; /* offset slots [0, 29] */
+ u8 offset_slot_tab_2[128]; /* offset slots [30, 49] */
union {
- /* Data for greedy or lazy parsing */
+ /* Data for lzx_compress_lazy() */
struct {
- /* Hash chains matchfinder (MUST BE LAST!!!) */
+ /* Hash chains matchfinder (MUST BE LAST!!!) */
union {
struct hc_matchfinder_16 hc_mf_16;
struct hc_matchfinder_32 hc_mf_32;
};
};
- /* Data for near-optimal parsing */
+ /* Data for lzx_compress_near_optimal() */
struct {
/*
- * The graph nodes for the current block.
- *
- * We need at least 'LZX_DIV_BLOCK_SIZE +
- * LZX_MAX_MATCH_LEN - 1' nodes because that is the
- * maximum block size that may be used. Add 1 because
- * we need a node to represent end-of-block.
+ * Array of nodes, one per position, for running the
+ * minimum-cost path algorithm.
*
- * It is possible that nodes past end-of-block are
- * accessed during match consideration, but this can
- * only occur if the block was truncated at
- * LZX_DIV_BLOCK_SIZE. So the same bound still applies.
- * Note that since nodes past the end of the block will
- * never actually have an effect on the items that are
- * chosen for the block, it makes no difference what
- * their costs are initialized to (if anything).
+ * This array must be large enough to accommodate the
+ * worst-case number of nodes, which occurs if the
+ * compressor finds a match of length LZX_MAX_MATCH_LEN
+ * at position 'SOFT_MAX_BLOCK_SIZE - 1', producing a
+ * block of size 'SOFT_MAX_BLOCK_SIZE - 1 +
+ * LZX_MAX_MATCH_LEN'. Add one for the end-of-block
+ * node.
*/
- struct lzx_optimum_node optimum_nodes[LZX_DIV_BLOCK_SIZE +
- LZX_MAX_MATCH_LEN - 1 + 1];
+ struct lzx_optimum_node optimum_nodes[
+ SOFT_MAX_BLOCK_SIZE - 1 +
+ LZX_MAX_MATCH_LEN + 1];
- /* The cost model for the current block */
+ /* The cost model for the current optimization pass */
struct lzx_costs costs;
/*
* Note: in rare cases, there will be a very high number
* of matches in the block and this array will overflow.
* If this happens, we force the end of the current
- * block. LZX_CACHE_LENGTH is the length at which we
+ * block. CACHE_LENGTH is the length at which we
* actually check for overflow. The extra slots beyond
* this are enough to absorb the worst case overflow,
- * which occurs if starting at
- * &match_cache[LZX_CACHE_LENGTH - 1], we write the
- * match count header, then write
- * LZX_MAX_MATCHES_PER_POS matches, then skip searching
- * for matches at 'LZX_MAX_MATCH_LEN - 1' positions and
+ * which occurs if starting at &match_cache[CACHE_LENGTH
+ * - 1], we write the match count header, then write
+ * MAX_MATCHES_PER_POS matches, then skip searching for
+ * matches at 'LZX_MAX_MATCH_LEN - 1' positions and
* write the match count header for each.
*/
- struct lz_match match_cache[LZX_CACHE_LENGTH +
- LZX_MAX_MATCHES_PER_POS +
+ struct lz_match match_cache[CACHE_LENGTH +
+ MAX_MATCHES_PER_POS +
LZX_MAX_MATCH_LEN - 1];
- /* Binary trees matchfinder (MUST BE LAST!!!) */
+ /* Binary trees matchfinder (MUST BE LAST!!!) */
union {
struct bt_matchfinder_16 bt_mf_16;
struct bt_matchfinder_32 bt_mf_32;
};
};
+/******************************************************************************/
+/* Matchfinder utilities */
+/*----------------------------------------------------------------------------*/
+
/*
* Will a matchfinder using 16-bit positions be sufficient for compressing
* buffers of up to the specified size? The limit could be 65536 bytes, but we
return max_bufsize <= 32768;
}
+/*
+ * Return the offset slot for the specified adjusted match offset.
+ */
+static inline unsigned
+lzx_get_offset_slot(struct lzx_compressor *c, u32 adjusted_offset,
+ bool is_16_bit)
+{
+ if (is_16_bit || adjusted_offset < ARRAY_LEN(c->offset_slot_tab_1))
+ return c->offset_slot_tab_1[adjusted_offset];
+ return c->offset_slot_tab_2[adjusted_offset >> 14];
+}
+
/*
* The following macros call either the 16-bit or the 32-bit version of a
* matchfinder function based on the value of 'is_16_bit', which will be known
((is_16_bit) ? CONCAT(funcname, _16)(&(c)->bt_mf_16, ##__VA_ARGS__) : \
CONCAT(funcname, _32)(&(c)->bt_mf_32, ##__VA_ARGS__));
+/******************************************************************************/
+/* Output bitstream */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * The LZX bitstream is encoded as a sequence of little endian 16-bit coding
+ * units. Bits are ordered from most significant to least significant within
+ * each coding unit.
+ */
+
/*
* Structure to keep track of the current state of sending bits to the
* compressed output buffer.
- *
- * The LZX bitstream is encoded as a sequence of 16-bit coding units.
*/
struct lzx_output_bitstream {
- /* Bits that haven't yet been written to the output buffer. */
+ /* Bits that haven't yet been written to the output buffer */
machine_word_t bitbuf;
- /* Number of bits currently held in @bitbuf. */
- u32 bitcount;
+ /* Number of bits currently held in @bitbuf */
+ machine_word_t bitcount;
- /* Pointer to the start of the output buffer. */
+ /* Pointer to the start of the output buffer */
u8 *start;
/* Pointer to the position in the output buffer at which the next coding
- * unit should be written. */
+ * unit should be written */
u8 *next;
- /* Pointer just past the end of the output buffer, rounded down to a
- * 2-byte boundary. */
+ /* Pointer to just past the end of the output buffer, rounded down by
+ * one byte if needed to make 'end - start' a multiple of 2 */
u8 *end;
};
-/* Can the specified number of bits always be added to 'bitbuf' after any
+/* Can the specified number of bits always be added to 'bitbuf' after all
* pending 16-bit coding units have been flushed? */
-#define CAN_BUFFER(n) ((n) <= (8 * sizeof(machine_word_t)) - 15)
+#define CAN_BUFFER(n) ((n) <= WORDBITS - 15)
-/*
- * Initialize the output bitstream.
- *
- * @os
- * The output bitstream structure to initialize.
- * @buffer
- * The buffer being written to.
- * @size
- * Size of @buffer, in bytes.
- */
+/* Initialize the output bitstream to write to the specified buffer. */
static void
lzx_init_output(struct lzx_output_bitstream *os, void *buffer, size_t size)
{
os->bitbuf = 0;
os->bitcount = 0;
os->start = buffer;
- os->next = os->start;
- os->end = os->start + (size & ~1);
+ os->next = buffer;
+ os->end = (u8 *)buffer + (size & ~1);
}
-/* Add some bits to the bitbuffer variable of the output bitstream. The caller
- * must make sure there is enough room. */
+/*
+ * Add some bits to the bitbuffer variable of the output bitstream. The caller
+ * must make sure there is enough room.
+ */
static inline void
lzx_add_bits(struct lzx_output_bitstream *os, u32 bits, unsigned num_bits)
{
os->bitcount += num_bits;
}
-/* Flush bits from the bitbuffer variable to the output buffer. 'max_num_bits'
+/*
+ * Flush bits from the bitbuffer variable to the output buffer. 'max_num_bits'
* specifies the maximum number of bits that may have been added since the last
- * flush. */
+ * flush.
+ */
static inline void
lzx_flush_bits(struct lzx_output_bitstream *os, unsigned max_num_bits)
{
+ /* Masking the number of bits to shift is only needed to avoid undefined
+ * behavior; we don't actually care about the results of bad shifts. On
+ * x86, the explicit masking generates no extra code. */
+ const u32 shift_mask = WORDBITS - 1;
+
if (os->end - os->next < 6)
return;
- put_unaligned_u16_le(os->bitbuf >> (os->bitcount - 16), os->next + 0);
+ put_unaligned_le16(os->bitbuf >> ((os->bitcount - 16) &
+ shift_mask), os->next + 0);
if (max_num_bits > 16)
- put_unaligned_u16_le(os->bitbuf >> (os->bitcount - 32), os->next + 2);
+ put_unaligned_le16(os->bitbuf >> ((os->bitcount - 32) &
+ shift_mask), os->next + 2);
if (max_num_bits > 32)
- put_unaligned_u16_le(os->bitbuf >> (os->bitcount - 48), os->next + 4);
+ put_unaligned_le16(os->bitbuf >> ((os->bitcount - 48) &
+ shift_mask), os->next + 4);
os->next += (os->bitcount >> 4) << 1;
os->bitcount &= 15;
}
* Flush the last coding unit to the output buffer if needed. Return the total
* number of bytes written to the output buffer, or 0 if an overflow occurred.
*/
-static u32
+static size_t
lzx_flush_output(struct lzx_output_bitstream *os)
{
if (os->end - os->next < 6)
return 0;
if (os->bitcount != 0) {
- put_unaligned_u16_le(os->bitbuf << (16 - os->bitcount), os->next);
+ put_unaligned_le16(os->bitbuf << (16 - os->bitcount), os->next);
os->next += 2;
}
return os->next - os->start;
}
-/* Build the main, length, and aligned offset Huffman codes used in LZX.
- *
- * This takes as input the frequency tables for each code and produces as output
- * a set of tables that map symbols to codewords and codeword lengths. */
+/******************************************************************************/
+/* Preparing Huffman codes */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * Build the Huffman codes. This takes as input the frequency tables for each
+ * code and produces as output a set of tables that map symbols to codewords and
+ * codeword lengths.
+ */
static void
-lzx_make_huffman_codes(struct lzx_compressor *c)
+lzx_build_huffman_codes(struct lzx_compressor *c)
{
const struct lzx_freqs *freqs = &c->freqs;
struct lzx_codes *codes = &c->codes[c->codes_index];
STATIC_ASSERT(MAIN_CODEWORD_LIMIT >= 9 &&
MAIN_CODEWORD_LIMIT <= LZX_MAX_MAIN_CODEWORD_LEN);
- STATIC_ASSERT(LENGTH_CODEWORD_LIMIT >= 9 &&
- LENGTH_CODEWORD_LIMIT <= LZX_MAX_LEN_CODEWORD_LEN);
- STATIC_ASSERT(ALIGNED_CODEWORD_LIMIT >= LZX_NUM_ALIGNED_OFFSET_BITS &&
- ALIGNED_CODEWORD_LIMIT <= LZX_MAX_ALIGNED_CODEWORD_LEN);
-
make_canonical_huffman_code(c->num_main_syms,
MAIN_CODEWORD_LIMIT,
freqs->main,
codes->lens.main,
codes->codewords.main);
+ STATIC_ASSERT(LENGTH_CODEWORD_LIMIT >= 8 &&
+ LENGTH_CODEWORD_LIMIT <= LZX_MAX_LEN_CODEWORD_LEN);
make_canonical_huffman_code(LZX_LENCODE_NUM_SYMBOLS,
LENGTH_CODEWORD_LIMIT,
freqs->len,
codes->lens.len,
codes->codewords.len);
+ STATIC_ASSERT(ALIGNED_CODEWORD_LIMIT >= LZX_NUM_ALIGNED_OFFSET_BITS &&
+ ALIGNED_CODEWORD_LIMIT <= LZX_MAX_ALIGNED_CODEWORD_LEN);
make_canonical_huffman_code(LZX_ALIGNEDCODE_NUM_SYMBOLS,
ALIGNED_CODEWORD_LIMIT,
freqs->aligned,
codes->codewords.aligned);
}
-/* Reset the symbol frequencies for the LZX Huffman codes. */
+/* Reset the symbol frequencies for the current block. */
static void
lzx_reset_symbol_frequencies(struct lzx_compressor *c)
{
/* Symbol 18: RLE 20 to 51 zeroes at a time. */
while ((run_end - run_start) >= 20) {
- extra_bits = min((run_end - run_start) - 20, 0x1f);
+ extra_bits = min((run_end - run_start) - 20, 0x1F);
precode_freqs[18]++;
*itemptr++ = 18 | (extra_bits << 5);
run_start += 20 + extra_bits;
/* Symbol 17: RLE 4 to 19 zeroes at a time. */
if ((run_end - run_start) >= 4) {
- extra_bits = min((run_end - run_start) - 4, 0xf);
+ extra_bits = min((run_end - run_start) - 4, 0xF);
precode_freqs[17]++;
*itemptr++ = 17 | (extra_bits << 5);
run_start += 4 + extra_bits;
return itemptr - precode_items;
}
+/******************************************************************************/
+/* Outputting compressed data */
+/*----------------------------------------------------------------------------*/
+
/*
* Output a Huffman code in the compressed form used in LZX.
*
/* Build the precode. */
STATIC_ASSERT(PRE_CODEWORD_LIMIT >= 5 &&
PRE_CODEWORD_LIMIT <= LZX_MAX_PRE_CODEWORD_LEN);
- make_canonical_huffman_code(LZX_PRECODE_NUM_SYMBOLS,
- PRE_CODEWORD_LIMIT,
+ make_canonical_huffman_code(LZX_PRECODE_NUM_SYMBOLS, PRE_CODEWORD_LIMIT,
precode_freqs, precode_lens,
precode_codewords);
* @sequences
* The matches and literals to output, given as a series of sequences.
* @codes
- * The main, length, and aligned offset Huffman codes for the current
- * LZX compressed block.
+ * The main, length, and aligned offset Huffman codes for the block.
*/
static void
lzx_write_sequences(struct lzx_output_bitstream *os, int block_type,
if (litrunlen) { /* Is the literal run nonempty? */
/* Verify optimization is enabled on 64-bit */
- STATIC_ASSERT(sizeof(machine_word_t) < 8 ||
- CAN_BUFFER(4 * MAIN_CODEWORD_LIMIT));
+ STATIC_ASSERT(WORDBITS < 64 ||
+ CAN_BUFFER(3 * MAIN_CODEWORD_LIMIT));
- if (CAN_BUFFER(4 * MAIN_CODEWORD_LIMIT)) {
+ if (CAN_BUFFER(3 * MAIN_CODEWORD_LIMIT)) {
- /* 64-bit: write 4 literals at a time. */
- while (litrunlen >= 4) {
+ /* 64-bit: write 3 literals at a time. */
+ while (litrunlen >= 3) {
unsigned lit0 = block_data[0];
unsigned lit1 = block_data[1];
unsigned lit2 = block_data[2];
- unsigned lit3 = block_data[3];
- lzx_add_bits(os, codes->codewords.main[lit0], codes->lens.main[lit0]);
- lzx_add_bits(os, codes->codewords.main[lit1], codes->lens.main[lit1]);
- lzx_add_bits(os, codes->codewords.main[lit2], codes->lens.main[lit2]);
- lzx_add_bits(os, codes->codewords.main[lit3], codes->lens.main[lit3]);
- lzx_flush_bits(os, 4 * MAIN_CODEWORD_LIMIT);
- block_data += 4;
- litrunlen -= 4;
+ lzx_add_bits(os, codes->codewords.main[lit0],
+ codes->lens.main[lit0]);
+ lzx_add_bits(os, codes->codewords.main[lit1],
+ codes->lens.main[lit1]);
+ lzx_add_bits(os, codes->codewords.main[lit2],
+ codes->lens.main[lit2]);
+ lzx_flush_bits(os, 3 * MAIN_CODEWORD_LIMIT);
+ block_data += 3;
+ litrunlen -= 3;
}
if (litrunlen--) {
unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
+ lzx_add_bits(os, codes->codewords.main[lit],
+ codes->lens.main[lit]);
if (litrunlen--) {
unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
- if (litrunlen--) {
- unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
- lzx_flush_bits(os, 3 * MAIN_CODEWORD_LIMIT);
- } else {
- lzx_flush_bits(os, 2 * MAIN_CODEWORD_LIMIT);
- }
+ lzx_add_bits(os, codes->codewords.main[lit],
+ codes->lens.main[lit]);
+ lzx_flush_bits(os, 2 * MAIN_CODEWORD_LIMIT);
} else {
lzx_flush_bits(os, 1 * MAIN_CODEWORD_LIMIT);
}
/* 32-bit: write 1 literal at a time. */
do {
unsigned lit = *block_data++;
- lzx_add_bits(os, codes->codewords.main[lit], codes->lens.main[lit]);
+ lzx_add_bits(os, codes->codewords.main[lit],
+ codes->lens.main[lit]);
lzx_flush_bits(os, MAIN_CODEWORD_LIMIT);
} while (--litrunlen);
}
adjusted_offset = seq->adjusted_offset_and_match_hdr >> 9;
num_extra_bits = lzx_extra_offset_bits[offset_slot];
- extra_bits = adjusted_offset - lzx_offset_slot_base[offset_slot];
+ extra_bits = adjusted_offset - (lzx_offset_slot_base[offset_slot] +
+ LZX_OFFSET_ADJUSTMENT);
#define MAX_MATCH_BITS (MAIN_CODEWORD_LIMIT + LENGTH_CODEWORD_LIMIT + \
14 + ALIGNED_CODEWORD_LIMIT)
/* Verify optimization is enabled on 64-bit */
- STATIC_ASSERT(sizeof(machine_word_t) < 8 || CAN_BUFFER(MAX_MATCH_BITS));
+ STATIC_ASSERT(WORDBITS < 64 || CAN_BUFFER(MAX_MATCH_BITS));
/* Output the main symbol for the match. */
/* If needed, output the length symbol for the match. */
if (adjusted_length >= LZX_NUM_PRIMARY_LENS) {
- lzx_add_bits(os, codes->codewords.len[adjusted_length - LZX_NUM_PRIMARY_LENS],
- codes->lens.len[adjusted_length - LZX_NUM_PRIMARY_LENS]);
+ lzx_add_bits(os, codes->codewords.len[adjusted_length -
+ LZX_NUM_PRIMARY_LENS],
+ codes->lens.len[adjusted_length -
+ LZX_NUM_PRIMARY_LENS]);
if (!CAN_BUFFER(MAX_MATCH_BITS))
lzx_flush_bits(os, LENGTH_CODEWORD_LIMIT);
}
if (!CAN_BUFFER(MAX_MATCH_BITS))
lzx_flush_bits(os, 14);
- lzx_add_bits(os, codes->codewords.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK],
- codes->lens.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK]);
+ lzx_add_bits(os, codes->codewords.aligned[adjusted_offset &
+ LZX_ALIGNED_OFFSET_BITMASK],
+ codes->lens.aligned[adjusted_offset &
+ LZX_ALIGNED_OFFSET_BITMASK]);
if (!CAN_BUFFER(MAX_MATCH_BITS))
lzx_flush_bits(os, ALIGNED_CODEWORD_LIMIT);
} else {
* LZX_BLOCKTYPE_* constants. */
lzx_write_bits(os, block_type, 3);
- /* Output the block size.
+ /*
+ * Output the block size.
*
- * The original LZX format seemed to always encode the block size in 3
- * bytes. However, the implementation in WIMGAPI, as used in WIM files,
- * uses the first bit to indicate whether the block is the default size
- * (32768) or a different size given explicitly by the next 16 bits.
+ * The original LZX format encoded the block size in 24 bits. However,
+ * the LZX format used in WIM archives uses 1 bit to specify whether the
+ * block has the default size of 32768 bytes, then optionally 16 bits to
+ * specify a non-default size. This works fine for Microsoft's WIM
+ * software (WIMGAPI), which never compresses more than 32768 bytes at a
+ * time with LZX. However, as an extension, our LZX compressor supports
+ * compressing up to 2097152 bytes, with a corresponding increase in
+ * window size. It is possible for blocks in these larger buffers to
+ * exceed 65535 bytes; such blocks cannot have their size represented in
+ * 16 bits.
*
- * By default, this compressor uses a window size of 32768 and therefore
- * follows the WIMGAPI behavior. However, this compressor also supports
- * window sizes greater than 32768 bytes, which do not appear to be
- * supported by WIMGAPI. In such cases, we retain the default size bit
- * to mean a size of 32768 bytes but output non-default block size in 24
- * bits rather than 16. The compatibility of this behavior is unknown
- * because WIMs created with chunk size greater than 32768 can seemingly
- * only be opened by wimlib anyway. */
+ * The chosen solution was to use 24 bits for the block size when
+ * possibly required --- specifically, when the compressor has been
+ * allocated to be capable of compressing more than 32768 bytes at once
+ * (which also causes the number of main symbols to be increased).
+ */
if (block_size == LZX_DEFAULT_BLOCK_SIZE) {
lzx_write_bits(os, 1, 1);
} else {
lzx_write_sequences(os, block_type, block_begin, sequences, codes);
}
-/* Given the frequencies of symbols in an LZX-compressed block and the
+/*
+ * 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. */
+ * will take fewer bits to output.
+ */
static int
lzx_choose_verbatim_or_aligned(const struct lzx_freqs * freqs,
const struct lzx_codes * codes)
{
- u32 aligned_cost = 0;
u32 verbatim_cost = 0;
+ u32 aligned_cost = 0;
- /* A verbatim block requires 3 bits in each place that an aligned symbol
- * would be used in an aligned offset block. */
+ /* A verbatim block requires 3 bits in each place that an aligned offset
+ * symbol would be used in an aligned offset block. */
for (unsigned i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
verbatim_cost += LZX_NUM_ALIGNED_OFFSET_BITS * freqs->aligned[i];
aligned_cost += codes->lens.aligned[i] * freqs->aligned[i];
}
- /* Account for output of the aligned offset code. */
- aligned_cost += LZX_ALIGNEDCODE_ELEMENT_SIZE * LZX_ALIGNEDCODE_NUM_SYMBOLS;
+ /* Account for the cost of sending the codeword lengths of the aligned
+ * offset code. */
+ aligned_cost += LZX_ALIGNEDCODE_ELEMENT_SIZE *
+ LZX_ALIGNEDCODE_NUM_SYMBOLS;
if (aligned_cost < verbatim_cost)
return LZX_BLOCKTYPE_ALIGNED;
}
/*
- * Return the offset slot for the specified adjusted match offset, using the
- * compressor's acceleration tables to speed up the mapping.
- */
-static inline unsigned
-lzx_comp_get_offset_slot(struct lzx_compressor *c, u32 adjusted_offset,
- bool is_16_bit)
-{
- if (is_16_bit || adjusted_offset < ARRAY_LEN(c->offset_slot_tab_1))
- return c->offset_slot_tab_1[adjusted_offset];
- return c->offset_slot_tab_2[adjusted_offset >> 14];
-}
-
-/*
- * Finish an LZX block:
+ * Flush an LZX block:
*
- * - build the Huffman codes
- * - decide whether to output the block as VERBATIM or ALIGNED
- * - output the block
- * - swap the indices of the current and previous Huffman codes
+ * 1. Build the Huffman codes.
+ * 2. Decide whether to output the block as VERBATIM or ALIGNED.
+ * 3. Write the block.
+ * 4. Swap the indices of the current and previous Huffman codes.
+ *
+ * Note: we never output UNCOMPRESSED blocks. This probably should be
+ * implemented sometime, but it doesn't make much difference.
*/
static void
-lzx_finish_block(struct lzx_compressor *c, struct lzx_output_bitstream *os,
- const u8 *block_begin, u32 block_size, u32 seq_idx)
+lzx_flush_block(struct lzx_compressor *c, struct lzx_output_bitstream *os,
+ const u8 *block_begin, u32 block_size, u32 seq_idx)
{
int block_type;
- lzx_make_huffman_codes(c);
+ lzx_build_huffman_codes(c);
block_type = lzx_choose_verbatim_or_aligned(&c->freqs,
&c->codes[c->codes_index]);
c->codes_index ^= 1;
}
-/* Tally the Huffman symbol for a literal and increment the literal run length.
+/******************************************************************************/
+/* Block splitting algorithm */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * The problem of block splitting is to decide when it is worthwhile to start a
+ * new block with new entropy codes. There is a theoretically optimal solution:
+ * recursively consider every possible block split, considering the exact cost
+ * of each block, and choose the minimum cost approach. But this is far too
+ * slow. Instead, as an approximation, we can count symbols and after every N
+ * symbols, compare the expected distribution of symbols based on the previous
+ * data with the actual distribution. If they differ "by enough", then start a
+ * new block.
+ *
+ * As an optimization and heuristic, we don't distinguish between every symbol
+ * but rather we combine many symbols into a single "observation type". For
+ * literals we only look at the high bits and low bits, and for matches we only
+ * look at whether the match is long or not. The assumption is that for typical
+ * "real" data, places that are good block boundaries will tend to be noticable
+ * based only on changes in these aggregate frequencies, without looking for
+ * subtle differences in individual symbols. For example, a change from ASCII
+ * bytes to non-ASCII bytes, or from few matches (generally less compressible)
+ * to many matches (generally more compressible), would be easily noticed based
+ * on the aggregates.
+ *
+ * For determining whether the frequency distributions are "different enough" to
+ * start a new block, the simply heuristic of splitting when the sum of absolute
+ * differences exceeds a constant seems to be good enough.
+ *
+ * Finally, for an approximation, it is not strictly necessary that the exact
+ * symbols being used are considered. With "near-optimal parsing", for example,
+ * the actual symbols that will be used are unknown until after the block
+ * boundary is chosen and the block has been optimized. Since the final choices
+ * cannot be used, we can use preliminary "greedy" choices instead.
*/
-static inline void
-lzx_record_literal(struct lzx_compressor *c, unsigned literal, u32 *litrunlen_p)
+
+/* Initialize the block split statistics when starting a new block. */
+static void
+lzx_init_block_split_stats(struct lzx_block_split_stats *stats)
{
- c->freqs.main[literal]++;
- ++*litrunlen_p;
+ memset(stats, 0, sizeof(*stats));
}
-/* Tally the Huffman symbol for a match, save the match data and the length of
- * the preceding literal run in the next lzx_sequence, and update the recent
- * offsets queue. */
+/* Literal observation. Heuristic: use the top 2 bits and low 1 bits of the
+ * literal, for 8 possible literal observation types. */
static inline void
-lzx_record_match(struct lzx_compressor *c, unsigned length, u32 offset_data,
- u32 recent_offsets[LZX_NUM_RECENT_OFFSETS], bool is_16_bit,
- u32 *litrunlen_p, struct lzx_sequence **next_seq_p)
+lzx_observe_literal(struct lzx_block_split_stats *stats, u8 lit)
{
- u32 litrunlen = *litrunlen_p;
- struct lzx_sequence *next_seq = *next_seq_p;
- unsigned offset_slot;
- unsigned v;
+ stats->new_observations[((lit >> 5) & 0x6) | (lit & 1)]++;
+ stats->num_new_observations++;
+}
- v = length - LZX_MIN_MATCH_LEN;
+/* Match observation. Heuristic: use one observation type for "short match" and
+ * one observation type for "long match". */
+static inline void
+lzx_observe_match(struct lzx_block_split_stats *stats, unsigned length)
+{
+ stats->new_observations[NUM_LITERAL_OBSERVATION_TYPES + (length >= 5)]++;
+ stats->num_new_observations++;
+}
- /* Save the literal run length and adjusted length. */
- next_seq->litrunlen = litrunlen;
- next_seq->adjusted_length = v;
+static bool
+lzx_should_end_block(struct lzx_block_split_stats *stats)
+{
+ if (stats->num_observations > 0) {
+
+ /* Note: to avoid slow divisions, we do not divide by
+ * 'num_observations', but rather do all math with the numbers
+ * multiplied by 'num_observations'. */
+ u32 total_delta = 0;
+ for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
+ u32 expected = stats->observations[i] *
+ stats->num_new_observations;
+ u32 actual = stats->new_observations[i] *
+ stats->num_observations;
+ u32 delta = (actual > expected) ? actual - expected :
+ expected - actual;
+ total_delta += delta;
+ }
- /* Compute the length header and tally the length symbol if needed */
- if (v >= LZX_NUM_PRIMARY_LENS) {
- c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
- v = LZX_NUM_PRIMARY_LENS;
+ /* Ready to end the block? */
+ if (total_delta >=
+ stats->num_new_observations * 7 / 8 * stats->num_observations)
+ return true;
}
- /* Compute the offset slot */
- offset_slot = lzx_comp_get_offset_slot(c, offset_data, is_16_bit);
+ for (int i = 0; i < NUM_OBSERVATION_TYPES; i++) {
+ stats->num_observations += stats->new_observations[i];
+ stats->observations[i] += stats->new_observations[i];
+ stats->new_observations[i] = 0;
+ }
+ stats->num_new_observations = 0;
+ return false;
+}
- /* Compute the match header. */
- v += offset_slot * LZX_NUM_LEN_HEADERS;
+/******************************************************************************/
+/* Slower ("near-optimal") compression algorithm */
+/*----------------------------------------------------------------------------*/
- /* Save the adjusted offset and match header. */
- next_seq->adjusted_offset_and_match_hdr = (offset_data << 9) | v;
+/*
+ * Least-recently-used queue for match offsets.
+ *
+ * This is represented as a 64-bit integer for efficiency. There are three
+ * offsets of 21 bits each. Bit 64 is garbage.
+ */
+struct lzx_lru_queue {
+ u64 R;
+} _aligned_attribute(8);
- /* Tally the main symbol. */
- c->freqs.main[LZX_NUM_CHARS + v]++;
+#define LZX_QUEUE_OFFSET_SHIFT 21
+#define LZX_QUEUE_OFFSET_MASK (((u64)1 << LZX_QUEUE_OFFSET_SHIFT) - 1)
- /* Update the recent offsets queue. */
- if (offset_data < LZX_NUM_RECENT_OFFSETS) {
- /* Repeat offset match */
- swap(recent_offsets[0], recent_offsets[offset_data]);
- } else {
- /* Explicit offset match */
+#define LZX_QUEUE_R0_SHIFT (0 * LZX_QUEUE_OFFSET_SHIFT)
+#define LZX_QUEUE_R1_SHIFT (1 * LZX_QUEUE_OFFSET_SHIFT)
+#define LZX_QUEUE_R2_SHIFT (2 * LZX_QUEUE_OFFSET_SHIFT)
- /* Tally the aligned offset symbol if needed */
- if (offset_data >= 16)
- c->freqs.aligned[offset_data & LZX_ALIGNED_OFFSET_BITMASK]++;
+#define LZX_QUEUE_R0_MASK (LZX_QUEUE_OFFSET_MASK << LZX_QUEUE_R0_SHIFT)
+#define LZX_QUEUE_R1_MASK (LZX_QUEUE_OFFSET_MASK << LZX_QUEUE_R1_SHIFT)
+#define LZX_QUEUE_R2_MASK (LZX_QUEUE_OFFSET_MASK << LZX_QUEUE_R2_SHIFT)
- recent_offsets[2] = recent_offsets[1];
- recent_offsets[1] = recent_offsets[0];
- recent_offsets[0] = offset_data - LZX_OFFSET_ADJUSTMENT;
- }
+#define LZX_QUEUE_INITIALIZER { \
+ ((u64)1 << LZX_QUEUE_R0_SHIFT) | \
+ ((u64)1 << LZX_QUEUE_R1_SHIFT) | \
+ ((u64)1 << LZX_QUEUE_R2_SHIFT) }
- /* Reset the literal run length and advance to the next sequence. */
- *next_seq_p = next_seq + 1;
- *litrunlen_p = 0;
+static inline u64
+lzx_lru_queue_R0(struct lzx_lru_queue queue)
+{
+ return (queue.R >> LZX_QUEUE_R0_SHIFT) & LZX_QUEUE_OFFSET_MASK;
}
-/* Finish the last lzx_sequence. The last lzx_sequence is just a literal run;
- * there is no match. This literal run may be empty. */
-static inline void
-lzx_finish_sequence(struct lzx_sequence *last_seq, u32 litrunlen)
+static inline u64
+lzx_lru_queue_R1(struct lzx_lru_queue queue)
{
- last_seq->litrunlen = litrunlen;
-
- /* Special value to mark last sequence */
- last_seq->adjusted_offset_and_match_hdr = 0x80000000;
+ return (queue.R >> LZX_QUEUE_R1_SHIFT) & LZX_QUEUE_OFFSET_MASK;
}
-/*
- * Given the minimum-cost path computed through the item graph for the current
- * block, walk the path and count how many of each symbol in each Huffman-coded
- * alphabet would be required to output the items (matches and literals) along
- * the path.
- *
- * Note that the path will be walked backwards (from the end of the block to the
- * beginning of the block), but this doesn't matter because this function only
- * computes frequencies.
- */
-static inline void
-lzx_tally_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit)
+static inline u64
+lzx_lru_queue_R2(struct lzx_lru_queue queue)
{
- u32 node_idx = block_size;
- for (;;) {
- u32 len;
- u32 offset_data;
- unsigned v;
- unsigned offset_slot;
+ return (queue.R >> LZX_QUEUE_R2_SHIFT) & LZX_QUEUE_OFFSET_MASK;
+}
- /* Tally literals until either a match or the beginning of the
- * block is reached. */
- for (;;) {
- u32 item = c->optimum_nodes[node_idx].item;
-
- len = item & OPTIMUM_LEN_MASK;
- offset_data = item >> OPTIMUM_OFFSET_SHIFT;
-
- if (len != 0) /* Not a literal? */
- break;
-
- /* Tally the main symbol for the literal. */
- c->freqs.main[offset_data]++;
-
- if (--node_idx == 0) /* Beginning of block was reached? */
- return;
- }
-
- node_idx -= len;
-
- /* Tally a match. */
-
- /* Tally the aligned offset symbol if needed. */
- if (offset_data >= 16)
- c->freqs.aligned[offset_data & LZX_ALIGNED_OFFSET_BITMASK]++;
-
- /* Tally the length symbol if needed. */
- v = len - LZX_MIN_MATCH_LEN;;
- if (v >= LZX_NUM_PRIMARY_LENS) {
- c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
- v = LZX_NUM_PRIMARY_LENS;
- }
+/* Push a match offset onto the front (most recently used) end of the queue. */
+static inline struct lzx_lru_queue
+lzx_lru_queue_push(struct lzx_lru_queue queue, u32 offset)
+{
+ return (struct lzx_lru_queue) {
+ .R = (queue.R << LZX_QUEUE_OFFSET_SHIFT) | offset,
+ };
+}
- /* Tally the main symbol. */
- offset_slot = lzx_comp_get_offset_slot(c, offset_data, is_16_bit);
- v += offset_slot * LZX_NUM_LEN_HEADERS;
- c->freqs.main[LZX_NUM_CHARS + v]++;
+/* Swap a match offset to the front of the queue. */
+static inline struct lzx_lru_queue
+lzx_lru_queue_swap(struct lzx_lru_queue queue, unsigned idx)
+{
+ unsigned shift = idx * 21;
+ const u64 mask = LZX_QUEUE_R0_MASK;
+ const u64 mask_high = mask << shift;
- if (node_idx == 0) /* Beginning of block was reached? */
- return;
- }
+ return (struct lzx_lru_queue) {
+ (queue.R & ~(mask | mask_high)) |
+ ((queue.R & mask_high) >> shift) |
+ ((queue.R & mask) << shift)
+ };
}
-/*
- * Like lzx_tally_item_list(), but this function also generates the list of
- * lzx_sequences for the minimum-cost path and writes it to c->chosen_sequences,
- * ready to be output to the bitstream after the Huffman codes are computed.
- * The lzx_sequences will be written to decreasing memory addresses as the path
- * is walked backwards, which means they will end up in the expected
- * first-to-last order. The return value is the index in c->chosen_sequences at
- * which the lzx_sequences begin.
- */
static inline u32
-lzx_record_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit)
+lzx_walk_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit,
+ bool record)
{
u32 node_idx = block_size;
u32 seq_idx = ARRAY_LEN(c->chosen_sequences) - 1;
u32 lit_start_node;
- /* Special value to mark last sequence */
- c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr = 0x80000000;
+ if (record) {
+ /* Special value to mark last sequence */
+ c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr = 0x80000000;
+ lit_start_node = node_idx;
+ }
- lit_start_node = node_idx;
for (;;) {
+ u32 item;
u32 len;
- u32 offset_data;
+ u32 adjusted_offset;
unsigned v;
unsigned offset_slot;
- /* Record literals until either a match or the beginning of the
- * block is reached. */
+ /* Tally literals until either a match or the beginning of the
+ * block is reached. Note: the item in the node at the
+ * beginning of the block has all bits set, causing this loop to
+ * end when it is reached. */
for (;;) {
- u32 item = c->optimum_nodes[node_idx].item;
+ item = c->optimum_nodes[node_idx].item;
+ if (item & OPTIMUM_LEN_MASK)
+ break;
+ c->freqs.main[item >> OPTIMUM_OFFSET_SHIFT]++;
+ node_idx--;
+ }
- len = item & OPTIMUM_LEN_MASK;
- offset_data = item >> OPTIMUM_OFFSET_SHIFT;
+ #if CONSIDER_GAP_MATCHES
+ if (item & OPTIMUM_GAP_MATCH) {
- if (len != 0) /* Not a literal? */
+ if (node_idx == 0)
break;
- /* Tally the main symbol for the literal. */
- c->freqs.main[offset_data]++;
+ /* Record the literal run length for the next sequence
+ * (the "previous sequence" when walking backwards). */
+ len = item & OPTIMUM_LEN_MASK;
+ if (record) {
+ c->chosen_sequences[seq_idx--].litrunlen =
+ lit_start_node - node_idx;
+ lit_start_node = node_idx - len;
+ }
- if (--node_idx == 0) /* Beginning of block was reached? */
- goto out;
+ /* Tally the rep0 match after the gap. */
+ v = len - LZX_MIN_MATCH_LEN;
+ if (record)
+ c->chosen_sequences[seq_idx].adjusted_length = v;
+ if (v >= LZX_NUM_PRIMARY_LENS) {
+ c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
+ v = LZX_NUM_PRIMARY_LENS;
+ }
+ c->freqs.main[LZX_NUM_CHARS + v]++;
+ if (record)
+ c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr = v;
+
+ /* Tally the literal in the gap. */
+ c->freqs.main[(u8)(item >> OPTIMUM_OFFSET_SHIFT)]++;
+
+ /* Fall through and tally the match before the gap.
+ * (It was temporarily saved in the 'cost' field of the
+ * previous node, which was free to reuse.) */
+ item = c->optimum_nodes[--node_idx].cost;
+ node_idx -= len;
+ }
+ #else /* CONSIDER_GAP_MATCHES */
+ if (node_idx == 0)
+ break;
+ #endif /* !CONSIDER_GAP_MATCHES */
+
+ len = item & OPTIMUM_LEN_MASK;
+ adjusted_offset = item >> OPTIMUM_OFFSET_SHIFT;
+
+ /* Record the literal run length for the next sequence (the
+ * "previous sequence" when walking backwards). */
+ if (record) {
+ c->chosen_sequences[seq_idx--].litrunlen =
+ lit_start_node - node_idx;
+ node_idx -= len;
+ lit_start_node = node_idx;
+ } else {
+ node_idx -= len;
}
- /* Save the literal run length for the next sequence (the
- * "previous sequence" when walking backwards). */
- c->chosen_sequences[seq_idx--].litrunlen = lit_start_node - node_idx;
- node_idx -= len;
- lit_start_node = node_idx;
-
- /* Record a match. */
+ /* Record a match. */
- /* Tally the aligned offset symbol if needed. */
- if (offset_data >= 16)
- c->freqs.aligned[offset_data & LZX_ALIGNED_OFFSET_BITMASK]++;
+ /* Tally the aligned offset symbol if needed. */
+ if (adjusted_offset >= 16)
+ c->freqs.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK]++;
- /* Save the adjusted length. */
+ /* Record the adjusted length. */
v = len - LZX_MIN_MATCH_LEN;
- c->chosen_sequences[seq_idx].adjusted_length = v;
+ if (record)
+ c->chosen_sequences[seq_idx].adjusted_length = v;
- /* Tally the length symbol if needed. */
+ /* Tally the length symbol if needed. */
if (v >= LZX_NUM_PRIMARY_LENS) {
c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
v = LZX_NUM_PRIMARY_LENS;
}
- /* Tally the main symbol. */
- offset_slot = lzx_comp_get_offset_slot(c, offset_data, is_16_bit);
+ /* Tally the main symbol. */
+ offset_slot = lzx_get_offset_slot(c, adjusted_offset, is_16_bit);
v += offset_slot * LZX_NUM_LEN_HEADERS;
c->freqs.main[LZX_NUM_CHARS + v]++;
- /* Save the adjusted offset and match header. */
- c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr =
- (offset_data << 9) | v;
-
- if (node_idx == 0) /* Beginning of block was reached? */
- goto out;
+ /* Record the adjusted offset and match header. */
+ if (record) {
+ c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr =
+ (adjusted_offset << 9) | v;
+ }
}
-out:
- /* Save the literal run length for the first sequence. */
- c->chosen_sequences[seq_idx].litrunlen = lit_start_node - node_idx;
+ /* Record the literal run length for the first sequence. */
+ if (record)
+ c->chosen_sequences[seq_idx].litrunlen = lit_start_node - node_idx;
- /* Return the index in c->chosen_sequences at which the lzx_sequences
- * begin. */
+ /* Return the index in chosen_sequences at which the sequences begin. */
return seq_idx;
}
+/*
+ * Given the minimum-cost path computed through the item graph for the current
+ * block, walk the path and count how many of each symbol in each Huffman-coded
+ * alphabet would be required to output the items (matches and literals) along
+ * the path.
+ *
+ * Note that the path will be walked backwards (from the end of the block to the
+ * beginning of the block), but this doesn't matter because this function only
+ * computes frequencies.
+ */
+static inline void
+lzx_tally_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit)
+{
+ lzx_walk_item_list(c, block_size, is_16_bit, false);
+}
+
+/*
+ * Like lzx_tally_item_list(), but this function also generates the list of
+ * lzx_sequences for the minimum-cost path and writes it to c->chosen_sequences,
+ * ready to be output to the bitstream after the Huffman codes are computed.
+ * The lzx_sequences will be written to decreasing memory addresses as the path
+ * is walked backwards, which means they will end up in the expected
+ * first-to-last order. The return value is the index in c->chosen_sequences at
+ * which the lzx_sequences begin.
+ */
+static inline u32
+lzx_record_item_list(struct lzx_compressor *c, u32 block_size, bool is_16_bit)
+{
+ return lzx_walk_item_list(c, block_size, is_16_bit, true);
+}
+
/*
* Find an inexpensive path through the graph of possible match/literal choices
* for the current block. The nodes of the graph are
* c->optimum_nodes[0...block_size]. They correspond directly to the bytes in
* the current block, plus one extra node for end-of-block. The edges of the
* graph are matches and literals. The goal is to find the minimum cost path
- * from 'c->optimum_nodes[0]' to 'c->optimum_nodes[block_size]'.
+ * from 'c->optimum_nodes[0]' to 'c->optimum_nodes[block_size]', given the cost
+ * model 'c->costs'.
*
* The algorithm works forwards, starting at 'c->optimum_nodes[0]' and
* proceeding forwards one node at a time. At each node, a selection of matches
* (len >= 2), as well as the literal byte (len = 1), is considered. An item of
* length 'len' provides a new path to reach the node 'len' bytes later. If
* such a path is the lowest cost found so far to reach that later node, then
- * that later node is updated with the new path.
+ * that later node is updated with the new cost and the "arrival" which provided
+ * that cost.
*
* Note that although this algorithm is based on minimum cost path search, due
* to various simplifying assumptions the result is not guaranteed to be the
* path to contain a prefix, ending at a position, where that path prefix is
* *not* the minimum cost path to that position. This can happen if such a path
* prefix results in a different adaptive state which results in lower costs
- * later. The algorithm does not solve this problem; it only considers the
- * lowest cost to reach each individual position.
+ * later. The algorithm does not solve this problem in general; it only looks
+ * one step ahead, with the exception of special consideration for "gap
+ * matches".
*/
static inline struct lzx_lru_queue
lzx_find_min_cost_path(struct lzx_compressor * const restrict c,
bool is_16_bit)
{
struct lzx_optimum_node *cur_node = c->optimum_nodes;
- struct lzx_optimum_node * const end_node = &c->optimum_nodes[block_size];
+ struct lzx_optimum_node * const end_node = cur_node + block_size;
struct lz_match *cache_ptr = c->match_cache;
const u8 *in_next = block_begin;
const u8 * const block_end = block_begin + block_size;
- /* Instead of storing the match offset LRU queues in the
+ /*
+ * Instead of storing the match offset LRU queues in the
* 'lzx_optimum_node' structures, we save memory (and cache lines) by
* storing them in a smaller array. This works because the algorithm
* only requires a limited history of the adaptive state. Once a given
- * state is more than LZX_MAX_MATCH_LEN bytes behind the current node,
- * it is no longer needed. */
+ * state is more than LZX_MAX_MATCH_LEN bytes behind the current node
+ * (more if gap match consideration is enabled; we just round up to 512
+ * so it's a power of 2), it is no longer needed.
+ *
+ * The QUEUE() macro finds the queue for the given node. This macro has
+ * been optimized by taking advantage of 'struct lzx_lru_queue' and
+ * 'struct lzx_optimum_node' both being 8 bytes in size and alignment.
+ */
struct lzx_lru_queue queues[512];
-
STATIC_ASSERT(ARRAY_LEN(queues) >= LZX_MAX_MATCH_LEN + 1);
-#define QUEUE(in) (queues[(uintptr_t)(in) % ARRAY_LEN(queues)])
+ STATIC_ASSERT(sizeof(c->optimum_nodes[0]) == sizeof(queues[0]));
+#define QUEUE(node) \
+ (*(struct lzx_lru_queue *)((char *)queues + \
+ ((uintptr_t)(node) % (ARRAY_LEN(queues) * sizeof(queues[0])))))
+ /*(queues[(uintptr_t)(node) / sizeof(*(node)) % ARRAY_LEN(queues)])*/
+
+#if CONSIDER_GAP_MATCHES
+ u32 matches_before_gap[ARRAY_LEN(queues)];
+#define MATCH_BEFORE_GAP(node) \
+ (matches_before_gap[(uintptr_t)(node) / sizeof(*(node)) % \
+ ARRAY_LEN(matches_before_gap)])
+#endif
- /* Initially, the cost to reach each node is "infinity". */
+ /*
+ * Initially, the cost to reach each node is "infinity".
+ *
+ * The first node actually should have cost 0, but "infinity"
+ * (0xFFFFFFFF) works just as well because it immediately overflows.
+ *
+ * The following statement also intentionally sets the 'item' of the
+ * first node, which would otherwise have no meaning, to 0xFFFFFFFF for
+ * use as a sentinel. See lzx_walk_item_list().
+ */
memset(c->optimum_nodes, 0xFF,
(block_size + 1) * sizeof(c->optimum_nodes[0]));
- QUEUE(block_begin) = initial_queue;
+ /* Initialize the recent offsets queue for the first node. */
+ QUEUE(cur_node) = initial_queue;
+
+ do { /* For each node in the block in position order... */
- /* The following loop runs 'block_size' iterations, one per node. */
- do {
unsigned num_matches;
unsigned literal;
u32 cost;
unsigned max_len = min(block_end - in_next, LZX_MAX_MATCH_LEN);
const u8 *matchptr;
- /* Consider R0 match */
- matchptr = in_next - lzx_lru_queue_R0(QUEUE(in_next));
+ /* Consider rep0 matches. */
+ matchptr = in_next - lzx_lru_queue_R0(QUEUE(cur_node));
if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next))
- goto R0_done;
+ goto rep0_done;
STATIC_ASSERT(LZX_MIN_MATCH_LEN == 2);
do {
u32 cost = cur_node->cost +
}
} while (in_next[next_len - 1] == matchptr[next_len - 1]);
- R0_done:
+ rep0_done:
- /* Consider R1 match */
- matchptr = in_next - lzx_lru_queue_R1(QUEUE(in_next));
+ /* Consider rep1 matches. */
+ matchptr = in_next - lzx_lru_queue_R1(QUEUE(cur_node));
if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next))
- goto R1_done;
+ goto rep1_done;
if (matchptr[next_len - 1] != in_next[next_len - 1])
- goto R1_done;
+ goto rep1_done;
for (unsigned len = 2; len < next_len - 1; len++)
if (matchptr[len] != in_next[len])
- goto R1_done;
+ goto rep1_done;
do {
u32 cost = cur_node->cost +
c->costs.match_cost[1][
}
} while (in_next[next_len - 1] == matchptr[next_len - 1]);
- R1_done:
+ rep1_done:
- /* Consider R2 match */
- matchptr = in_next - lzx_lru_queue_R2(QUEUE(in_next));
+ /* Consider rep2 matches. */
+ matchptr = in_next - lzx_lru_queue_R2(QUEUE(cur_node));
if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next))
- goto R2_done;
+ goto rep2_done;
if (matchptr[next_len - 1] != in_next[next_len - 1])
- goto R2_done;
+ goto rep2_done;
for (unsigned len = 2; len < next_len - 1; len++)
if (matchptr[len] != in_next[len])
- goto R2_done;
+ goto rep2_done;
do {
u32 cost = cur_node->cost +
c->costs.match_cost[2][
}
} while (in_next[next_len - 1] == matchptr[next_len - 1]);
- R2_done:
+ rep2_done:
while (next_len > cache_ptr->length)
if (++cache_ptr == end_matches)
goto done_matches;
- /* Consider explicit offset matches */
- do {
+ /* Consider explicit offset matches. */
+ for (;;) {
u32 offset = cache_ptr->offset;
- u32 offset_data = offset + LZX_OFFSET_ADJUSTMENT;
- unsigned offset_slot = lzx_comp_get_offset_slot(c, offset_data,
- is_16_bit);
+ u32 adjusted_offset = offset + LZX_OFFSET_ADJUSTMENT;
+ unsigned offset_slot = lzx_get_offset_slot(c, adjusted_offset, is_16_bit);
u32 base_cost = cur_node->cost;
+ u32 cost;
- #if LZX_CONSIDER_ALIGNED_COSTS
- if (offset_data >= 16)
- base_cost += c->costs.aligned[offset_data &
+ #if CONSIDER_ALIGNED_COSTS
+ if (offset >= 16 - LZX_OFFSET_ADJUSTMENT)
+ base_cost += c->costs.aligned[adjusted_offset &
LZX_ALIGNED_OFFSET_BITMASK];
#endif
-
do {
- u32 cost = base_cost +
- c->costs.match_cost[offset_slot][
+ cost = base_cost +
+ c->costs.match_cost[offset_slot][
next_len - LZX_MIN_MATCH_LEN];
if (cost < (cur_node + next_len)->cost) {
(cur_node + next_len)->cost = cost;
(cur_node + next_len)->item =
- (offset_data << OPTIMUM_OFFSET_SHIFT) | next_len;
+ (adjusted_offset << OPTIMUM_OFFSET_SHIFT) | next_len;
}
} while (++next_len <= cache_ptr->length);
- } while (++cache_ptr != end_matches);
+
+ if (++cache_ptr == end_matches) {
+ #if CONSIDER_GAP_MATCHES
+ /* Also consider the longest explicit
+ * offset match as a "gap match": match
+ * + lit + rep0. */
+ s32 remaining = (block_end - in_next) - (s32)next_len;
+ if (likely(remaining >= 2)) {
+ const u8 *strptr = in_next + next_len;
+ const u8 *matchptr = strptr - offset;
+ if (load_u16_unaligned(strptr) == load_u16_unaligned(matchptr)) {
+ STATIC_ASSERT(ARRAY_LEN(queues) - LZX_MAX_MATCH_LEN - 2 >= 250);
+ STATIC_ASSERT(ARRAY_LEN(queues) == ARRAY_LEN(matches_before_gap));
+ unsigned limit = min(remaining,
+ min(ARRAY_LEN(queues) - LZX_MAX_MATCH_LEN - 2,
+ LZX_MAX_MATCH_LEN));
+ unsigned rep0_len = lz_extend(strptr, matchptr, 2, limit);
+ u8 lit = strptr[-1];
+ cost += c->costs.main[lit] +
+ c->costs.match_cost[0][rep0_len - LZX_MIN_MATCH_LEN];
+ unsigned total_len = next_len + rep0_len;
+ if (cost < (cur_node + total_len)->cost) {
+ (cur_node + total_len)->cost = cost;
+ (cur_node + total_len)->item =
+ OPTIMUM_GAP_MATCH |
+ ((u32)lit << OPTIMUM_OFFSET_SHIFT) |
+ rep0_len;
+ MATCH_BEFORE_GAP(cur_node + total_len) =
+ (adjusted_offset << OPTIMUM_OFFSET_SHIFT) |
+ (next_len - 1);
+ }
+ }
+ }
+ #endif /* CONSIDER_GAP_MATCHES */
+ break;
+ }
+ }
}
done_matches:
* To avoid an extra branch, actually checking the preferability
* of coding the literal is integrated into the queue update
- * code below. */
+ * code below. */
literal = *in_next++;
cost = cur_node->cost + c->costs.main[literal];
- /* Advance to the next position. */
+ /* Advance to the next position. */
cur_node++;
/* The lowest-cost path to the current position is now known.
* Finalize the recent offsets queue that results from taking
- * this lowest-cost path. */
+ * this lowest-cost path. */
if (cost <= cur_node->cost) {
- /* Literal: queue remains unchanged. */
+ /* Literal: queue remains unchanged. */
cur_node->cost = cost;
cur_node->item = (u32)literal << OPTIMUM_OFFSET_SHIFT;
- QUEUE(in_next) = QUEUE(in_next - 1);
+ QUEUE(cur_node) = QUEUE(cur_node - 1);
} else {
- /* Match: queue update is needed. */
+ /* Match: queue update is needed. */
unsigned len = cur_node->item & OPTIMUM_LEN_MASK;
- u32 offset_data = cur_node->item >> OPTIMUM_OFFSET_SHIFT;
- if (offset_data >= LZX_NUM_RECENT_OFFSETS) {
- /* Explicit offset match: insert offset at front */
- QUEUE(in_next) =
- lzx_lru_queue_push(QUEUE(in_next - len),
- offset_data - LZX_OFFSET_ADJUSTMENT);
- } else {
- /* Repeat offset match: swap offset to front */
- QUEUE(in_next) =
- lzx_lru_queue_swap(QUEUE(in_next - len),
- offset_data);
+ #if CONSIDER_GAP_MATCHES
+ s32 adjusted_offset = (s32)cur_node->item >> OPTIMUM_OFFSET_SHIFT;
+ STATIC_ASSERT(OPTIMUM_GAP_MATCH == 0x80000000); /* assuming sign extension */
+ #else
+ u32 adjusted_offset = cur_node->item >> OPTIMUM_OFFSET_SHIFT;
+ #endif
+
+ if (adjusted_offset >= LZX_NUM_RECENT_OFFSETS) {
+ /* Explicit offset match: insert offset at front. */
+ QUEUE(cur_node) =
+ lzx_lru_queue_push(QUEUE(cur_node - len),
+ adjusted_offset - LZX_OFFSET_ADJUSTMENT);
+ }
+ #if CONSIDER_GAP_MATCHES
+ else if (adjusted_offset < 0) {
+ /* "Gap match": Explicit offset match, then a
+ * literal, then rep0 match. Save the explicit
+ * offset match information in the cost field of
+ * the previous node, which isn't needed
+ * anymore. Then insert the offset at the front
+ * of the queue. */
+ u32 match_before_gap = MATCH_BEFORE_GAP(cur_node);
+ (cur_node - 1)->cost = match_before_gap;
+ QUEUE(cur_node) =
+ lzx_lru_queue_push(QUEUE(cur_node - len - 1 -
+ (match_before_gap & OPTIMUM_LEN_MASK)),
+ (match_before_gap >> OPTIMUM_OFFSET_SHIFT) -
+ LZX_OFFSET_ADJUSTMENT);
+ }
+ #endif
+ else {
+ /* Repeat offset match: swap offset to front. */
+ QUEUE(cur_node) =
+ lzx_lru_queue_swap(QUEUE(cur_node - len),
+ adjusted_offset);
}
}
} while (cur_node != end_node);
- /* Return the match offset queue at the end of the minimum cost path. */
- return QUEUE(block_end);
+ /* Return the recent offsets queue at the end of the path. */
+ return QUEUE(cur_node);
}
-/* Given the costs for the main and length codewords, compute 'match_costs'. */
+/*
+ * Given the costs for the main and length codewords (c->costs.main and
+ * c->costs.len), initialize the match cost array (c->costs.match_cost) which
+ * directly provides the cost of every possible (length, offset slot) pair.
+ */
static void
lzx_compute_match_costs(struct lzx_compressor *c)
{
- unsigned num_offset_slots = (c->num_main_syms - LZX_NUM_CHARS) / LZX_NUM_LEN_HEADERS;
+ unsigned num_offset_slots = (c->num_main_syms - LZX_NUM_CHARS) /
+ LZX_NUM_LEN_HEADERS;
struct lzx_costs *costs = &c->costs;
+ unsigned main_symbol = LZX_NUM_CHARS;
- for (unsigned offset_slot = 0; offset_slot < num_offset_slots; offset_slot++) {
-
- u32 extra_cost = (u32)lzx_extra_offset_bits[offset_slot] * LZX_BIT_COST;
- unsigned main_symbol = LZX_NUM_CHARS + (offset_slot * LZX_NUM_LEN_HEADERS);
+ for (unsigned offset_slot = 0; offset_slot < num_offset_slots;
+ offset_slot++)
+ {
+ u32 extra_cost = lzx_extra_offset_bits[offset_slot] * BIT_COST;
unsigned i;
- #if LZX_CONSIDER_ALIGNED_COSTS
+ #if CONSIDER_ALIGNED_COSTS
if (offset_slot >= 8)
- extra_cost -= LZX_NUM_ALIGNED_OFFSET_BITS * LZX_BIT_COST;
+ extra_cost -= LZX_NUM_ALIGNED_OFFSET_BITS * BIT_COST;
#endif
- for (i = 0; i < LZX_NUM_PRIMARY_LENS; i++)
+ for (i = 0; i < LZX_NUM_PRIMARY_LENS; i++) {
costs->match_cost[offset_slot][i] =
costs->main[main_symbol++] + extra_cost;
+ }
- extra_cost += costs->main[main_symbol];
+ extra_cost += costs->main[main_symbol++];
- for (; i < LZX_NUM_LENS; i++)
+ for (; i < LZX_NUM_LENS; i++) {
costs->match_cost[offset_slot][i] =
- costs->len[i - LZX_NUM_PRIMARY_LENS] + extra_cost;
+ costs->len[i - LZX_NUM_PRIMARY_LENS] +
+ extra_cost;
+ }
}
}
-/* Set default LZX Huffman symbol costs to bootstrap the iterative optimization
- * algorithm. */
-static void
-lzx_set_default_costs(struct lzx_compressor *c, const u8 *block, u32 block_size)
+/*
+ * Fast approximation for log2f(x). This is not as accurate as the standard C
+ * version. It does not need to be perfectly accurate because it is only used
+ * for estimating symbol costs, which is very approximate anyway.
+ */
+static float
+log2f_fast(float x)
{
- u32 i;
- bool have_byte[256];
- unsigned num_used_bytes;
+ union {
+ float f;
+ s32 i;
+ } u = { .f = x };
+
+ /* Extract the exponent and subtract 127 to remove the bias. This gives
+ * the integer part of the result. */
+ float res = ((u.i >> 23) & 0xFF) - 127;
- /* The costs below are hard coded to use a scaling factor of 16. */
- STATIC_ASSERT(LZX_BIT_COST == 16);
+ /* Set the exponent to 0 (plus bias of 127). This transforms the number
+ * to the range [1, 2) while retaining the same mantissa. */
+ u.i = (u.i & ~(0xFF << 23)) | (127 << 23);
/*
- * Heuristics:
+ * Approximate the log2 of the transformed number using a degree 2
+ * interpolating polynomial for log2(x) over the interval [1, 2). Then
+ * add this to the extracted exponent to produce the final approximation
+ * of log2(x).
*
- * - Use smaller initial costs for literal symbols when the input buffer
- * contains fewer distinct bytes.
+ * The coefficients of the interpolating polynomial used here were found
+ * using the script tools/log2_interpolation.r.
+ */
+ return res - 1.653124006f + u.f * (1.9941812f - u.f * 0.3347490189f);
+
+}
+
+/*
+ * Return the estimated cost of a symbol which has been estimated to have the
+ * given probability.
+ */
+static u32
+lzx_cost_for_probability(float prob)
+{
+ /*
+ * The basic formula is:
+ *
+ * entropy = -log2(probability)
*
- * - Assume that match symbols are more costly than literal symbols.
+ * Use this to get the cost in fractional bits. Then multiply by our
+ * scaling factor of BIT_COST and truncate to a u32.
*
- * - Assume that length symbols for shorter lengths are less costly than
- * length symbols for longer lengths.
+ * In addition, the minimum cost is BIT_COST (one bit) because the
+ * entropy coding method will be Huffman codes.
*/
+ u32 cost = -log2f_fast(prob) * BIT_COST;
+ return max(cost, BIT_COST);
+}
- for (i = 0; i < 256; i++)
- have_byte[i] = false;
+/*
+ * Mapping: number of used literals => heuristic probability of a literal times
+ * 6870. Generated by running this R command:
+ *
+ * cat(paste(round(6870*2^-((304+(0:256))/64)), collapse=", "))
+ */
+static const u8 literal_scaled_probs[257] = {
+ 255, 253, 250, 247, 244, 242, 239, 237, 234, 232, 229, 227, 224, 222,
+ 219, 217, 215, 212, 210, 208, 206, 203, 201, 199, 197, 195, 193, 191,
+ 189, 186, 184, 182, 181, 179, 177, 175, 173, 171, 169, 167, 166, 164,
+ 162, 160, 159, 157, 155, 153, 152, 150, 149, 147, 145, 144, 142, 141,
+ 139, 138, 136, 135, 133, 132, 130, 129, 128, 126, 125, 124, 122, 121,
+ 120, 118, 117, 116, 115, 113, 112, 111, 110, 109, 107, 106, 105, 104,
+ 103, 102, 101, 100, 98, 97, 96, 95, 94, 93, 92, 91, 90, 89, 88, 87, 86,
+ 86, 85, 84, 83, 82, 81, 80, 79, 78, 78, 77, 76, 75, 74, 73, 73, 72, 71,
+ 70, 70, 69, 68, 67, 67, 66, 65, 65, 64, 63, 62, 62, 61, 60, 60, 59, 59,
+ 58, 57, 57, 56, 55, 55, 54, 54, 53, 53, 52, 51, 51, 50, 50, 49, 49, 48,
+ 48, 47, 47, 46, 46, 45, 45, 44, 44, 43, 43, 42, 42, 41, 41, 40, 40, 40,
+ 39, 39, 38, 38, 38, 37, 37, 36, 36, 36, 35, 35, 34, 34, 34, 33, 33, 33,
+ 32, 32, 32, 31, 31, 31, 30, 30, 30, 29, 29, 29, 28, 28, 28, 27, 27, 27,
+ 27, 26, 26, 26, 25, 25, 25, 25, 24, 24, 24, 24, 23, 23, 23, 23, 22, 22,
+ 22, 22, 21, 21, 21, 21, 20, 20, 20, 20, 20, 19, 19, 19, 19, 19, 18, 18,
+ 18, 18, 18, 17, 17, 17, 17, 17, 16, 16, 16, 16
+};
- for (i = 0; i < block_size; i++)
- have_byte[block[i]] = true;
+/*
+ * Mapping: length symbol => default cost of that symbol. This is derived from
+ * sample data but has been slightly edited to add more bias towards the
+ * shortest lengths, which are the most common.
+ */
+static const u16 lzx_default_len_costs[LZX_LENCODE_NUM_SYMBOLS] = {
+ 300, 310, 320, 330, 360, 396, 399, 416, 451, 448, 463, 466, 505, 492,
+ 503, 514, 547, 531, 566, 561, 589, 563, 592, 586, 623, 602, 639, 627,
+ 659, 643, 657, 650, 685, 662, 661, 672, 685, 686, 696, 680, 657, 682,
+ 666, 699, 674, 699, 679, 709, 688, 712, 692, 714, 694, 716, 698, 712,
+ 706, 727, 714, 727, 713, 723, 712, 718, 719, 719, 720, 735, 725, 735,
+ 728, 740, 727, 739, 727, 742, 716, 733, 733, 740, 738, 746, 737, 747,
+ 738, 745, 736, 748, 742, 749, 745, 749, 743, 748, 741, 752, 745, 752,
+ 747, 750, 747, 752, 748, 753, 750, 752, 753, 753, 749, 744, 752, 755,
+ 753, 756, 745, 748, 746, 745, 723, 757, 755, 758, 755, 758, 752, 757,
+ 754, 757, 755, 759, 755, 758, 753, 755, 755, 758, 757, 761, 755, 750,
+ 758, 759, 759, 760, 758, 751, 757, 757, 759, 759, 758, 759, 758, 761,
+ 750, 761, 758, 760, 759, 761, 758, 761, 760, 752, 759, 760, 759, 759,
+ 757, 762, 760, 761, 761, 748, 761, 760, 762, 763, 752, 762, 762, 763,
+ 762, 762, 763, 763, 762, 763, 762, 763, 762, 763, 763, 764, 763, 762,
+ 763, 762, 762, 762, 764, 764, 763, 764, 763, 763, 763, 762, 763, 763,
+ 762, 764, 764, 763, 762, 763, 763, 763, 763, 762, 764, 763, 762, 764,
+ 764, 763, 763, 765, 764, 764, 762, 763, 764, 765, 763, 764, 763, 764,
+ 762, 764, 764, 754, 763, 764, 763, 763, 762, 763, 584,
+};
- num_used_bytes = 0;
- for (i = 0; i < 256; i++)
- num_used_bytes += have_byte[i];
+/* Set default costs to bootstrap the iterative optimization algorithm. */
+static void
+lzx_set_default_costs(struct lzx_compressor *c)
+{
+ unsigned i;
+ u32 num_literals = 0;
+ u32 num_used_literals = 0;
+ float inv_num_matches = 1.0f / c->freqs.main[LZX_NUM_CHARS];
+ float inv_num_items;
+ float prob_match = 1.0f;
+ u32 match_cost;
+ float base_literal_prob;
+
+ /* Some numbers here have been hardcoded to assume a bit cost of 64. */
+ STATIC_ASSERT(BIT_COST == 64);
+
+ /* Estimate the number of literals that will used. 'num_literals' is
+ * the total number, whereas 'num_used_literals' is the number of
+ * distinct symbols. */
+ for (i = 0; i < LZX_NUM_CHARS; i++) {
+ num_literals += c->freqs.main[i];
+ num_used_literals += (c->freqs.main[i] != 0);
+ }
- for (i = 0; i < 256; i++)
- c->costs.main[i] = 140 - (256 - num_used_bytes) / 4;
+ /* Note: all match headers were tallied as symbol 'LZX_NUM_CHARS'. We
+ * don't attempt to estimate which ones will be used. */
+
+ inv_num_items = 1.0f / (num_literals + c->freqs.main[LZX_NUM_CHARS]);
+ base_literal_prob = literal_scaled_probs[num_used_literals] *
+ (1.0f / 6870.0f);
+
+ /* Literal costs. We use two different methods to compute the
+ * probability of each literal and mix together their results. */
+ for (i = 0; i < LZX_NUM_CHARS; i++) {
+ u32 freq = c->freqs.main[i];
+ if (freq != 0) {
+ float prob = 0.5f * ((freq * inv_num_items) +
+ base_literal_prob);
+ c->costs.main[i] = lzx_cost_for_probability(prob);
+ prob_match -= prob;
+ } else {
+ c->costs.main[i] = 11 * BIT_COST;
+ }
+ }
+ /* Match header costs. We just assume that all match headers are
+ * equally probable, but we do take into account the relative cost of a
+ * match header vs. a literal depending on how common matches are
+ * expected to be vs. literals. */
+ prob_match = max(prob_match, 0.15f);
+ match_cost = lzx_cost_for_probability(prob_match / (c->num_main_syms -
+ LZX_NUM_CHARS));
for (; i < c->num_main_syms; i++)
- c->costs.main[i] = 170;
+ c->costs.main[i] = match_cost;
+ /* Length symbol costs. These are just set to fixed values which
+ * reflect the fact the smallest lengths are typically the most common,
+ * and therefore are typically the cheapest. */
for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++)
- c->costs.len[i] = 103 + (i / 4);
+ c->costs.len[i] = lzx_default_len_costs[i];
-#if LZX_CONSIDER_ALIGNED_COSTS
- for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++)
- c->costs.aligned[i] = LZX_NUM_ALIGNED_OFFSET_BITS * LZX_BIT_COST;
+#if CONSIDER_ALIGNED_COSTS
+ /* Aligned offset symbol costs. These are derived from the estimated
+ * probability of each aligned offset symbol. */
+ for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
+ /* We intentionally tallied the frequencies in the wrong slots,
+ * not accounting for LZX_OFFSET_ADJUSTMENT, since doing the
+ * fixup here is faster: a constant 8 subtractions here vs. one
+ * addition for every match. */
+ unsigned j = (i - LZX_OFFSET_ADJUSTMENT) & LZX_ALIGNED_OFFSET_BITMASK;
+ if (c->freqs.aligned[j] != 0) {
+ float prob = c->freqs.aligned[j] * inv_num_matches;
+ c->costs.aligned[i] = lzx_cost_for_probability(prob);
+ } else {
+ c->costs.aligned[i] =
+ (2 * LZX_NUM_ALIGNED_OFFSET_BITS) * BIT_COST;
+ }
+ }
#endif
-
- lzx_compute_match_costs(c);
}
/* Update the current cost model to reflect the computed Huffman codes. */
static void
-lzx_update_costs(struct lzx_compressor *c)
+lzx_set_costs_from_codes(struct lzx_compressor *c)
{
unsigned i;
const struct lzx_lens *lens = &c->codes[c->codes_index].lens;
for (i = 0; i < c->num_main_syms; i++) {
c->costs.main[i] = (lens->main[i] ? lens->main[i] :
- MAIN_CODEWORD_LIMIT) * LZX_BIT_COST;
+ MAIN_CODEWORD_LIMIT) * BIT_COST;
}
for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++) {
c->costs.len[i] = (lens->len[i] ? lens->len[i] :
- LENGTH_CODEWORD_LIMIT) * LZX_BIT_COST;
+ LENGTH_CODEWORD_LIMIT) * BIT_COST;
}
-#if LZX_CONSIDER_ALIGNED_COSTS
+#if CONSIDER_ALIGNED_COSTS
for (i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) {
c->costs.aligned[i] = (lens->aligned[i] ? lens->aligned[i] :
- ALIGNED_CODEWORD_LIMIT) * LZX_BIT_COST;
+ ALIGNED_CODEWORD_LIMIT) * BIT_COST;
}
#endif
-
- lzx_compute_match_costs(c);
}
+/*
+ * Choose a "near-optimal" literal/match sequence to use for the current block,
+ * then flush the block. Because the cost of each Huffman symbol is unknown
+ * until the Huffman codes have been built and the Huffman codes themselves
+ * depend on the symbol frequencies, this uses an iterative optimization
+ * algorithm to approximate an optimal solution. The first optimization pass
+ * for the block uses default costs; additional passes use costs derived from
+ * the Huffman codes computed in the previous pass.
+ */
static inline struct lzx_lru_queue
-lzx_optimize_and_write_block(struct lzx_compressor * const restrict c,
+lzx_optimize_and_flush_block(struct lzx_compressor * const restrict c,
struct lzx_output_bitstream * const restrict os,
const u8 * const restrict block_begin,
const u32 block_size,
struct lzx_lru_queue new_queue;
u32 seq_idx;
- /* The first optimization pass uses a default cost model. Each
- * additional optimization pass uses a cost model derived from the
- * Huffman code computed in the previous pass. */
+ lzx_set_default_costs(c);
- lzx_set_default_costs(c, block_begin, block_size);
- lzx_reset_symbol_frequencies(c);
- do {
+ for (;;) {
+ lzx_compute_match_costs(c);
new_queue = lzx_find_min_cost_path(c, block_begin, block_size,
initial_queue, is_16_bit);
- if (num_passes_remaining > 1) {
- lzx_tally_item_list(c, block_size, is_16_bit);
- lzx_make_huffman_codes(c);
- lzx_update_costs(c);
- lzx_reset_symbol_frequencies(c);
- }
- } while (--num_passes_remaining);
+ if (--num_passes_remaining == 0)
+ break;
+
+ /* At least one optimization pass remains. Update the costs. */
+ lzx_reset_symbol_frequencies(c);
+ lzx_tally_item_list(c, block_size, is_16_bit);
+ lzx_build_huffman_codes(c);
+ lzx_set_costs_from_codes(c);
+ }
+
+ /* Done optimizing. Generate the sequence list and flush the block. */
+ lzx_reset_symbol_frequencies(c);
seq_idx = lzx_record_item_list(c, block_size, is_16_bit);
- lzx_finish_block(c, os, block_begin, block_size, seq_idx);
+ lzx_flush_block(c, os, block_begin, block_size, seq_idx);
return new_queue;
}
* This is the "near-optimal" LZX compressor.
*
* For each block, it performs a relatively thorough graph search to find an
- * inexpensive (in terms of compressed size) way to output that block.
+ * inexpensive (in terms of compressed size) way to output the block.
*
* Note: there are actually many things this algorithm leaves on the table in
* terms of compression ratio. So although it may be "near-optimal", it is
* simpler "greedy" or "lazy" parse while still being relatively fast.
*/
static inline void
-lzx_compress_near_optimal(struct lzx_compressor *c,
- struct lzx_output_bitstream *os,
+lzx_compress_near_optimal(struct lzx_compressor * restrict c,
+ const u8 * const restrict in_begin, size_t in_nbytes,
+ struct lzx_output_bitstream * restrict os,
bool is_16_bit)
{
- const u8 * const in_begin = c->in_buffer;
const u8 * in_next = in_begin;
- const u8 * const in_end = in_begin + c->in_nbytes;
+ const u8 * const in_end = in_begin + in_nbytes;
u32 max_len = LZX_MAX_MATCH_LEN;
u32 nice_len = min(c->nice_match_length, max_len);
- u32 next_hashes[2] = {};
- struct lzx_lru_queue queue;
+ u32 next_hashes[2] = {0, 0};
+ struct lzx_lru_queue queue = LZX_QUEUE_INITIALIZER;
+ /* Initialize the matchfinder. */
CALL_BT_MF(is_16_bit, c, bt_matchfinder_init);
- lzx_lru_queue_init(&queue);
do {
- /* Starting a new block */
- const u8 * const in_block_begin = in_next;
- const u8 * const in_block_end =
- in_next + min(LZX_DIV_BLOCK_SIZE, in_end - in_next);
+ /* Starting a new block */
- /* Run the block through the matchfinder and cache the matches. */
+ const u8 * const in_block_begin = in_next;
+ const u8 * const in_max_block_end =
+ in_next + min(SOFT_MAX_BLOCK_SIZE, in_end - in_next);
struct lz_match *cache_ptr = c->match_cache;
- do {
- struct lz_match *lz_matchptr;
- u32 best_len;
-
- /* If approaching the end of the input buffer, adjust
- * 'max_len' and 'nice_len' accordingly. */
- if (unlikely(max_len > in_end - in_next)) {
- max_len = in_end - in_next;
- nice_len = min(max_len, nice_len);
- if (unlikely(max_len < BT_MATCHFINDER_REQUIRED_NBYTES)) {
- in_next++;
- cache_ptr->length = 0;
- cache_ptr++;
- continue;
- }
- }
+ const u8 *next_search_pos = in_next;
+ const u8 *next_observation = in_next;
+ const u8 *next_pause_point =
+ min(in_next + min(MIN_BLOCK_SIZE,
+ in_max_block_end - in_next),
+ in_max_block_end - min(LZX_MAX_MATCH_LEN - 1,
+ in_max_block_end - in_next));
+
+ lzx_init_block_split_stats(&c->split_stats);
+ lzx_reset_symbol_frequencies(c);
- /* Check for matches. */
- lz_matchptr = CALL_BT_MF(is_16_bit, c, bt_matchfinder_get_matches,
- in_begin,
- in_next - in_begin,
- max_len,
- nice_len,
- c->max_search_depth,
- next_hashes,
- &best_len,
- cache_ptr + 1);
- in_next++;
- cache_ptr->length = lz_matchptr - (cache_ptr + 1);
- cache_ptr = lz_matchptr;
+ if (in_next >= next_pause_point)
+ goto pause;
- /*
- * If there was a very long match found, then don't
- * cache any matches for the bytes covered by that
- * match. This avoids degenerate behavior when
- * compressing highly redundant data, where the number
- * of matches can be very large.
- *
- * This heuristic doesn't actually hurt the compression
- * ratio very much. If there's a long match, then the
- * data must be highly compressible, so it doesn't
- * matter as much what we do.
- */
- if (best_len >= nice_len) {
- --best_len;
- do {
- if (unlikely(max_len > in_end - in_next)) {
- max_len = in_end - in_next;
- nice_len = min(max_len, nice_len);
- if (unlikely(max_len < BT_MATCHFINDER_REQUIRED_NBYTES)) {
- in_next++;
- cache_ptr->length = 0;
- cache_ptr++;
- continue;
- }
+ /*
+ * Run the input buffer through the matchfinder, caching the
+ * matches, until we decide to end the block.
+ *
+ * For a tighter matchfinding loop, we compute a "pause point",
+ * which is the next position at which we may need to check
+ * whether to end the block or to decrease max_len. We then
+ * only do these extra checks upon reaching the pause point.
+ */
+ resume_matchfinding:
+ do {
+ if (in_next >= next_search_pos) {
+ /* Search for matches at this position. */
+ struct lz_match *lz_matchptr;
+ u32 best_len;
+
+ lz_matchptr = CALL_BT_MF(is_16_bit, c,
+ bt_matchfinder_get_matches,
+ in_begin,
+ in_next - in_begin,
+ max_len,
+ nice_len,
+ c->max_search_depth,
+ next_hashes,
+ &best_len,
+ cache_ptr + 1);
+ cache_ptr->length = lz_matchptr - (cache_ptr + 1);
+ cache_ptr = lz_matchptr;
+
+ /* Accumulate literal/match statistics for block
+ * splitting and for generating the initial cost
+ * model. */
+ if (in_next >= next_observation) {
+ best_len = cache_ptr[-1].length;
+ if (best_len >= 3) {
+ /* Match (len >= 3) */
+
+ /*
+ * Note: for performance reasons this has
+ * been simplified significantly:
+ *
+ * - We wait until later to account for
+ * LZX_OFFSET_ADJUSTMENT.
+ * - We don't account for repeat offsets.
+ * - We don't account for different match headers.
+ */
+ c->freqs.aligned[cache_ptr[-1].offset &
+ LZX_ALIGNED_OFFSET_BITMASK]++;
+ c->freqs.main[LZX_NUM_CHARS]++;
+
+ lzx_observe_match(&c->split_stats, best_len);
+ next_observation = in_next + best_len;
+ } else {
+ /* Literal */
+ c->freqs.main[*in_next]++;
+ lzx_observe_literal(&c->split_stats, *in_next);
+ next_observation = in_next + 1;
}
- CALL_BT_MF(is_16_bit, c, bt_matchfinder_skip_position,
- in_begin,
- in_next - in_begin,
- max_len,
- nice_len,
- c->max_search_depth,
- next_hashes);
- in_next++;
+ }
+
+ /*
+ * If there was a very long match found, then
+ * don't cache any matches for the bytes covered
+ * by that match. This avoids degenerate
+ * behavior when compressing highly redundant
+ * data, where the number of matches can be very
+ * large.
+ *
+ * This heuristic doesn't actually hurt the
+ * compression ratio *too* much. If there's a
+ * long match, then the data must be highly
+ * compressible, so it doesn't matter as much
+ * what we do.
+ */
+ if (best_len >= nice_len)
+ next_search_pos = in_next + best_len;
+ } else {
+ /* Don't search for matches at this position. */
+ CALL_BT_MF(is_16_bit, c,
+ bt_matchfinder_skip_position,
+ in_begin,
+ in_next - in_begin,
+ nice_len,
+ c->max_search_depth,
+ next_hashes);
+ cache_ptr->length = 0;
+ cache_ptr++;
+ }
+ } while (++in_next < next_pause_point &&
+ likely(cache_ptr < &c->match_cache[CACHE_LENGTH]));
+
+ pause:
+
+ /* Adjust max_len and nice_len if we're nearing the end of the
+ * input buffer. In addition, if we are so close to the end of
+ * the input buffer that there cannot be any more matches, then
+ * just advance through the last few positions and record no
+ * matches. */
+ if (unlikely(max_len > in_end - in_next)) {
+ max_len = in_end - in_next;
+ nice_len = min(max_len, nice_len);
+ if (max_len < BT_MATCHFINDER_REQUIRED_NBYTES) {
+ while (in_next != in_end) {
cache_ptr->length = 0;
cache_ptr++;
- } while (--best_len);
+ in_next++;
+ }
}
- } while (in_next < in_block_end &&
- likely(cache_ptr < &c->match_cache[LZX_CACHE_LENGTH]));
-
- /* We've finished running the block through the matchfinder.
- * Now choose a match/literal sequence and write the block. */
+ }
- queue = lzx_optimize_and_write_block(c, os, in_block_begin,
+ /* End the block if the match cache may overflow. */
+ if (unlikely(cache_ptr >= &c->match_cache[CACHE_LENGTH]))
+ goto end_block;
+
+ /* End the block if the soft maximum size has been reached. */
+ if (in_next >= in_max_block_end)
+ goto end_block;
+
+ /* End the block if the block splitting algorithm thinks this is
+ * a good place to do so. */
+ if (c->split_stats.num_new_observations >=
+ NUM_OBSERVATIONS_PER_BLOCK_CHECK &&
+ in_max_block_end - in_next >= MIN_BLOCK_SIZE &&
+ lzx_should_end_block(&c->split_stats))
+ goto end_block;
+
+ /* It's not time to end the block yet. Compute the next pause
+ * point and resume matchfinding. */
+ next_pause_point =
+ min(in_next + min(NUM_OBSERVATIONS_PER_BLOCK_CHECK * 2 -
+ c->split_stats.num_new_observations,
+ in_max_block_end - in_next),
+ in_max_block_end - min(LZX_MAX_MATCH_LEN - 1,
+ in_max_block_end - in_next));
+ goto resume_matchfinding;
+
+ end_block:
+ /* We've decided on a block boundary and cached matches. Now
+ * choose a match/literal sequence and flush the block. */
+ queue = lzx_optimize_and_flush_block(c, os, in_block_begin,
in_next - in_block_begin,
queue, is_16_bit);
} while (in_next != in_end);
}
static void
-lzx_compress_near_optimal_16(struct lzx_compressor *c,
- struct lzx_output_bitstream *os)
+lzx_compress_near_optimal_16(struct lzx_compressor *c, const u8 *in,
+ size_t in_nbytes, struct lzx_output_bitstream *os)
{
- lzx_compress_near_optimal(c, os, true);
+ lzx_compress_near_optimal(c, in, in_nbytes, os, true);
}
static void
-lzx_compress_near_optimal_32(struct lzx_compressor *c,
- struct lzx_output_bitstream *os)
+lzx_compress_near_optimal_32(struct lzx_compressor *c, const u8 *in,
+ size_t in_nbytes, struct lzx_output_bitstream *os)
{
- lzx_compress_near_optimal(c, os, false);
+ lzx_compress_near_optimal(c, in, in_nbytes, os, false);
}
+/******************************************************************************/
+/* Faster ("lazy") compression algorithm */
+/*----------------------------------------------------------------------------*/
+
/*
- * Given a pointer to the current byte sequence and the current list of recent
- * match offsets, find the longest repeat offset match.
- *
- * If no match of at least 2 bytes is found, then return 0.
+ * Called when the compressor chooses to use a literal. This tallies the
+ * Huffman symbol for the literal, increments the current literal run length,
+ * and "observes" the literal for the block split statistics.
+ */
+static inline void
+lzx_choose_literal(struct lzx_compressor *c, unsigned literal, u32 *litrunlen_p)
+{
+ lzx_observe_literal(&c->split_stats, literal);
+ c->freqs.main[literal]++;
+ ++*litrunlen_p;
+}
+
+/*
+ * Called when the compressor chooses to use a match. This tallies the Huffman
+ * symbol(s) for a match, saves the match data and the length of the preceding
+ * literal run, updates the recent offsets queue, and "observes" the match for
+ * the block split statistics.
+ */
+static inline void
+lzx_choose_match(struct lzx_compressor *c, unsigned length, u32 adjusted_offset,
+ u32 recent_offsets[LZX_NUM_RECENT_OFFSETS], bool is_16_bit,
+ u32 *litrunlen_p, struct lzx_sequence **next_seq_p)
+{
+ u32 litrunlen = *litrunlen_p;
+ struct lzx_sequence *next_seq = *next_seq_p;
+ unsigned offset_slot;
+ unsigned v;
+
+ lzx_observe_match(&c->split_stats, length);
+
+ v = length - LZX_MIN_MATCH_LEN;
+
+ /* Save the literal run length and adjusted length. */
+ next_seq->litrunlen = litrunlen;
+ next_seq->adjusted_length = v;
+
+ /* Compute the length header, then tally the length symbol if needed. */
+ if (v >= LZX_NUM_PRIMARY_LENS) {
+ c->freqs.len[v - LZX_NUM_PRIMARY_LENS]++;
+ v = LZX_NUM_PRIMARY_LENS;
+ }
+
+ /* Compute the offset slot. */
+ offset_slot = lzx_get_offset_slot(c, adjusted_offset, is_16_bit);
+
+ /* Compute the match header. */
+ v += offset_slot * LZX_NUM_LEN_HEADERS;
+
+ /* Save the adjusted offset and match header. */
+ next_seq->adjusted_offset_and_match_hdr = (adjusted_offset << 9) | v;
+
+ /* Tally the main symbol. */
+ c->freqs.main[LZX_NUM_CHARS + v]++;
+
+ /* Update the recent offsets queue. */
+ if (adjusted_offset < LZX_NUM_RECENT_OFFSETS) {
+ /* Repeat offset match. */
+ swap(recent_offsets[0], recent_offsets[adjusted_offset]);
+ } else {
+ /* Explicit offset match. */
+
+ /* Tally the aligned offset symbol if needed. */
+ if (adjusted_offset >= 16)
+ c->freqs.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK]++;
+
+ recent_offsets[2] = recent_offsets[1];
+ recent_offsets[1] = recent_offsets[0];
+ recent_offsets[0] = adjusted_offset - LZX_OFFSET_ADJUSTMENT;
+ }
+
+ /* Reset the literal run length and advance to the next sequence. */
+ *next_seq_p = next_seq + 1;
+ *litrunlen_p = 0;
+}
+
+/*
+ * Called when the compressor ends a block. This finshes the last lzx_sequence,
+ * which is just a literal run with no following match. This literal run might
+ * be empty.
+ */
+static inline void
+lzx_finish_sequence(struct lzx_sequence *last_seq, u32 litrunlen)
+{
+ last_seq->litrunlen = litrunlen;
+
+ /* Special value to mark last sequence */
+ last_seq->adjusted_offset_and_match_hdr = 0x80000000;
+}
+
+/*
+ * Find the longest repeat offset match with the current position. If a match
+ * is found, return its length and set *best_rep_idx_ret to the index of its
+ * offset in @recent_offsets. Otherwise, return 0.
*
- * If a match of at least 2 bytes is found, then return its length and set
- * *rep_max_idx_ret to the index of its offset in @queue.
-*/
+ * Don't bother with length 2 matches; consider matches of length >= 3 only.
+ * Also assume that max_len >= 3.
+ */
static unsigned
lzx_find_longest_repeat_offset_match(const u8 * const in_next,
- const u32 bytes_remaining,
- const u32 recent_offsets[LZX_NUM_RECENT_OFFSETS],
- unsigned *rep_max_idx_ret)
+ const u32 recent_offsets[],
+ const unsigned max_len,
+ unsigned *best_rep_idx_ret)
{
- STATIC_ASSERT(LZX_NUM_RECENT_OFFSETS == 3);
+ STATIC_ASSERT(LZX_NUM_RECENT_OFFSETS == 3); /* loop is unrolled */
- const unsigned max_len = min(bytes_remaining, LZX_MAX_MATCH_LEN);
- const u16 next_2_bytes = load_u16_unaligned(in_next);
+ const u32 seq3 = load_u24_unaligned(in_next);
const u8 *matchptr;
- unsigned rep_max_len;
- unsigned rep_max_idx;
+ unsigned best_rep_len = 0;
+ unsigned best_rep_idx = 0;
unsigned rep_len;
+ /* Check for rep0 match (most recent offset) */
matchptr = in_next - recent_offsets[0];
- if (load_u16_unaligned(matchptr) == next_2_bytes)
- rep_max_len = lz_extend(in_next, matchptr, 2, max_len);
- else
- rep_max_len = 0;
- rep_max_idx = 0;
+ if (load_u24_unaligned(matchptr) == seq3)
+ best_rep_len = lz_extend(in_next, matchptr, 3, max_len);
+ /* Check for rep1 match (second most recent offset) */
matchptr = in_next - recent_offsets[1];
- if (load_u16_unaligned(matchptr) == next_2_bytes) {
- rep_len = lz_extend(in_next, matchptr, 2, max_len);
- if (rep_len > rep_max_len) {
- rep_max_len = rep_len;
- rep_max_idx = 1;
+ if (load_u24_unaligned(matchptr) == seq3) {
+ rep_len = lz_extend(in_next, matchptr, 3, max_len);
+ if (rep_len > best_rep_len) {
+ best_rep_len = rep_len;
+ best_rep_idx = 1;
}
}
+ /* Check for rep2 match (third most recent offset) */
matchptr = in_next - recent_offsets[2];
- if (load_u16_unaligned(matchptr) == next_2_bytes) {
- rep_len = lz_extend(in_next, matchptr, 2, max_len);
- if (rep_len > rep_max_len) {
- rep_max_len = rep_len;
- rep_max_idx = 2;
+ if (load_u24_unaligned(matchptr) == seq3) {
+ rep_len = lz_extend(in_next, matchptr, 3, max_len);
+ if (rep_len > best_rep_len) {
+ best_rep_len = rep_len;
+ best_rep_idx = 2;
}
}
- *rep_max_idx_ret = rep_max_idx;
- return rep_max_len;
+ *best_rep_idx_ret = best_rep_idx;
+ return best_rep_len;
}
-/* Fast heuristic scoring for lazy parsing: how "good" is this match? */
+/*
+ * Fast heuristic scoring for lazy parsing: how "good" is this match?
+ * This is mainly determined by the length: longer matches are better.
+ * However, we also give a bonus to close (small offset) matches and to repeat
+ * offset matches, since those require fewer bits to encode.
+ */
+
static inline unsigned
lzx_explicit_offset_match_score(unsigned len, u32 adjusted_offset)
{
if (adjusted_offset < 4096)
score++;
-
if (adjusted_offset < 256)
score++;
return rep_len + 3;
}
-/* This is the "lazy" LZX compressor. */
+/*
+ * This is the "lazy" LZX compressor. The basic idea is that before it chooses
+ * a match, it checks to see if there's a longer match at the next position. If
+ * yes, it chooses a literal and continues to the next position. If no, it
+ * chooses the match.
+ *
+ * Some additional heuristics are used as well. Repeat offset matches are
+ * considered favorably and sometimes are chosen immediately. In addition, long
+ * matches (at least "nice_len" bytes) are chosen immediately as well. Finally,
+ * when we decide whether a match is "better" than another, we take the offset
+ * into consideration as well as the length.
+ */
static inline void
-lzx_compress_lazy(struct lzx_compressor *c, struct lzx_output_bitstream *os,
- bool is_16_bit)
+lzx_compress_lazy(struct lzx_compressor * restrict c,
+ const u8 * const restrict in_begin, size_t in_nbytes,
+ struct lzx_output_bitstream * restrict os, bool is_16_bit)
{
- const u8 * const in_begin = c->in_buffer;
const u8 * in_next = in_begin;
- const u8 * const in_end = in_begin + c->in_nbytes;
+ const u8 * const in_end = in_begin + in_nbytes;
unsigned max_len = LZX_MAX_MATCH_LEN;
unsigned nice_len = min(c->nice_match_length, max_len);
STATIC_ASSERT(LZX_NUM_RECENT_OFFSETS == 3);
- u32 recent_offsets[3] = {1, 1, 1};
- u32 next_hashes[2] = {};
+ u32 recent_offsets[LZX_NUM_RECENT_OFFSETS] = {1, 1, 1};
+ u32 next_hashes[2] = {0, 0};
+ /* Initialize the matchfinder. */
CALL_HC_MF(is_16_bit, c, hc_matchfinder_init);
do {
- /* Starting a new block */
+ /* Starting a new block */
const u8 * const in_block_begin = in_next;
- const u8 * const in_block_end =
- in_next + min(LZX_DIV_BLOCK_SIZE, in_end - in_next);
+ const u8 * const in_max_block_end =
+ in_next + min(SOFT_MAX_BLOCK_SIZE, in_end - in_next);
struct lzx_sequence *next_seq = c->chosen_sequences;
+ u32 litrunlen = 0;
unsigned cur_len;
u32 cur_offset;
- u32 cur_offset_data;
+ u32 cur_adjusted_offset;
unsigned cur_score;
unsigned next_len;
u32 next_offset;
- u32 next_offset_data;
+ u32 next_adjusted_offset;
unsigned next_score;
- unsigned rep_max_len;
- unsigned rep_max_idx;
+ unsigned best_rep_len;
+ unsigned best_rep_idx;
unsigned rep_score;
unsigned skip_len;
- u32 litrunlen = 0;
lzx_reset_symbol_frequencies(c);
+ lzx_init_block_split_stats(&c->split_stats);
do {
+ /* Adjust max_len and nice_len if we're nearing the end
+ * of the input buffer. */
if (unlikely(max_len > in_end - in_next)) {
max_len = in_end - in_next;
nice_len = min(max_len, nice_len);
}
- /* Find the longest match at the current position. */
-
- cur_len = CALL_HC_MF(is_16_bit, c, hc_matchfinder_longest_match,
+ /* Find the longest match (subject to the
+ * max_search_depth cutoff parameter) with the current
+ * position. Don't bother with length 2 matches; only
+ * look for matches of length >= 3. */
+ cur_len = CALL_HC_MF(is_16_bit, c,
+ hc_matchfinder_longest_match,
in_begin,
in_next - in_begin,
2,
c->max_search_depth,
next_hashes,
&cur_offset);
+
+ /* If there was no match found, or the only match found
+ * was a distant short match, then choose a literal. */
if (cur_len < 3 ||
(cur_len == 3 &&
cur_offset >= 8192 - LZX_OFFSET_ADJUSTMENT &&
cur_offset != recent_offsets[1] &&
cur_offset != recent_offsets[2]))
{
- /* There was no match found, or the only match found
- * was a distant length 3 match. Output a literal. */
- lzx_record_literal(c, *in_next++, &litrunlen);
+ lzx_choose_literal(c, *in_next, &litrunlen);
+ in_next++;
continue;
}
+ /* Heuristic: if this match has the most recent offset,
+ * then go ahead and choose it as a rep0 match. */
if (cur_offset == recent_offsets[0]) {
in_next++;
- cur_offset_data = 0;
skip_len = cur_len - 1;
+ cur_adjusted_offset = 0;
goto choose_cur_match;
}
- cur_offset_data = cur_offset + LZX_OFFSET_ADJUSTMENT;
- cur_score = lzx_explicit_offset_match_score(cur_len, cur_offset_data);
-
- /* Consider a repeat offset match */
- rep_max_len = lzx_find_longest_repeat_offset_match(in_next,
- in_end - in_next,
- recent_offsets,
- &rep_max_idx);
+ /* Compute the longest match's score as an explicit
+ * offset match. */
+ cur_adjusted_offset = cur_offset + LZX_OFFSET_ADJUSTMENT;
+ cur_score = lzx_explicit_offset_match_score(cur_len, cur_adjusted_offset);
+
+ /* Find the longest repeat offset match at this
+ * position. If we find one and it's "better" than the
+ * explicit offset match we found, then go ahead and
+ * choose the repeat offset match immediately. */
+ best_rep_len = lzx_find_longest_repeat_offset_match(in_next,
+ recent_offsets,
+ max_len,
+ &best_rep_idx);
in_next++;
- if (rep_max_len >= 3 &&
- (rep_score = lzx_repeat_offset_match_score(rep_max_len,
- rep_max_idx)) >= cur_score)
+ if (best_rep_len != 0 &&
+ (rep_score = lzx_repeat_offset_match_score(best_rep_len,
+ best_rep_idx)) >= cur_score)
{
- cur_len = rep_max_len;
- cur_offset_data = rep_max_idx;
- skip_len = rep_max_len - 1;
+ cur_len = best_rep_len;
+ cur_adjusted_offset = best_rep_idx;
+ skip_len = best_rep_len - 1;
goto choose_cur_match;
}
have_cur_match:
+ /*
+ * We have a match at the current position. If the
+ * match is very long, then choose it immediately.
+ * Otherwise, see if there's a better match at the next
+ * position.
+ */
- /* We have a match at the current position. */
-
- /* If we have a very long match, choose it immediately. */
if (cur_len >= nice_len) {
skip_len = cur_len - 1;
goto choose_cur_match;
}
- /* See if there's a better match at the next position. */
-
if (unlikely(max_len > in_end - in_next)) {
max_len = in_end - in_next;
nice_len = min(max_len, nice_len);
}
- next_len = CALL_HC_MF(is_16_bit, c, hc_matchfinder_longest_match,
+ next_len = CALL_HC_MF(is_16_bit, c,
+ hc_matchfinder_longest_match,
in_begin,
in_next - in_begin,
cur_len - 2,
&next_offset);
if (next_len <= cur_len - 2) {
+ /* No potentially better match was found. */
in_next++;
skip_len = cur_len - 2;
goto choose_cur_match;
}
- next_offset_data = next_offset + LZX_OFFSET_ADJUSTMENT;
- next_score = lzx_explicit_offset_match_score(next_len, next_offset_data);
+ next_adjusted_offset = next_offset + LZX_OFFSET_ADJUSTMENT;
+ next_score = lzx_explicit_offset_match_score(next_len, next_adjusted_offset);
- rep_max_len = lzx_find_longest_repeat_offset_match(in_next,
- in_end - in_next,
- recent_offsets,
- &rep_max_idx);
+ best_rep_len = lzx_find_longest_repeat_offset_match(in_next,
+ recent_offsets,
+ max_len,
+ &best_rep_idx);
in_next++;
- if (rep_max_len >= 3 &&
- (rep_score = lzx_repeat_offset_match_score(rep_max_len,
- rep_max_idx)) >= next_score)
+ if (best_rep_len != 0 &&
+ (rep_score = lzx_repeat_offset_match_score(best_rep_len,
+ best_rep_idx)) >= next_score)
{
if (rep_score > cur_score) {
/* The next match is better, and it's a
- * repeat offset match. */
- lzx_record_literal(c, *(in_next - 2),
+ * repeat offset match. */
+ lzx_choose_literal(c, *(in_next - 2),
&litrunlen);
- cur_len = rep_max_len;
- cur_offset_data = rep_max_idx;
+ cur_len = best_rep_len;
+ cur_adjusted_offset = best_rep_idx;
skip_len = cur_len - 1;
goto choose_cur_match;
}
} else {
if (next_score > cur_score) {
/* The next match is better, and it's an
- * explicit offset match. */
- lzx_record_literal(c, *(in_next - 2),
+ * explicit offset match. */
+ lzx_choose_literal(c, *(in_next - 2),
&litrunlen);
cur_len = next_len;
- cur_offset_data = next_offset_data;
+ cur_adjusted_offset = next_adjusted_offset;
cur_score = next_score;
goto have_cur_match;
}
}
- /* The original match was better. */
+ /* The original match was better; choose it. */
skip_len = cur_len - 2;
choose_cur_match:
- lzx_record_match(c, cur_len, cur_offset_data,
+ /* Choose a match and have the matchfinder skip over its
+ * remaining bytes. */
+ lzx_choose_match(c, cur_len, cur_adjusted_offset,
recent_offsets, is_16_bit,
&litrunlen, &next_seq);
- in_next = CALL_HC_MF(is_16_bit, c, hc_matchfinder_skip_positions,
+ in_next = CALL_HC_MF(is_16_bit, c,
+ hc_matchfinder_skip_positions,
in_begin,
in_next - in_begin,
in_end - in_begin,
skip_len,
next_hashes);
- } while (in_next < in_block_end);
- lzx_finish_sequence(next_seq, litrunlen);
+ /* Keep going until it's time to end the block. */
+ } while (in_next < in_max_block_end &&
+ !(c->split_stats.num_new_observations >=
+ NUM_OBSERVATIONS_PER_BLOCK_CHECK &&
+ in_next - in_block_begin >= MIN_BLOCK_SIZE &&
+ in_end - in_next >= MIN_BLOCK_SIZE &&
+ lzx_should_end_block(&c->split_stats)));
- lzx_finish_block(c, os, in_block_begin, in_next - in_block_begin, 0);
+ /* Flush the block. */
+ lzx_finish_sequence(next_seq, litrunlen);
+ lzx_flush_block(c, os, in_block_begin, in_next - in_block_begin, 0);
+ /* Keep going until we've reached the end of the input buffer. */
} while (in_next != in_end);
}
static void
-lzx_compress_lazy_16(struct lzx_compressor *c, struct lzx_output_bitstream *os)
+lzx_compress_lazy_16(struct lzx_compressor *c, const u8 *in, size_t in_nbytes,
+ struct lzx_output_bitstream *os)
{
- lzx_compress_lazy(c, os, true);
+ lzx_compress_lazy(c, in, in_nbytes, os, true);
}
static void
-lzx_compress_lazy_32(struct lzx_compressor *c, struct lzx_output_bitstream *os)
+lzx_compress_lazy_32(struct lzx_compressor *c, const u8 *in, size_t in_nbytes,
+ struct lzx_output_bitstream *os)
{
- lzx_compress_lazy(c, os, false);
+ lzx_compress_lazy(c, in, in_nbytes, os, false);
}
-/* Generate the acceleration tables for offset slots. */
+/******************************************************************************/
+/* Compressor operations */
+/*----------------------------------------------------------------------------*/
+
+/*
+ * Generate tables for mapping match offsets (actually, "adjusted" match
+ * offsets) to offset slots.
+ */
static void
lzx_init_offset_slot_tabs(struct lzx_compressor *c)
{
u32 adjusted_offset = 0;
unsigned slot = 0;
- /* slots [0, 29] */
+ /* slots [0, 29] */
for (; adjusted_offset < ARRAY_LEN(c->offset_slot_tab_1);
adjusted_offset++)
{
- if (adjusted_offset >= lzx_offset_slot_base[slot + 1])
+ if (adjusted_offset >= lzx_offset_slot_base[slot + 1] +
+ LZX_OFFSET_ADJUSTMENT)
slot++;
c->offset_slot_tab_1[adjusted_offset] = slot;
}
- /* slots [30, 49] */
+ /* slots [30, 49] */
for (; adjusted_offset < LZX_MAX_WINDOW_SIZE;
adjusted_offset += (u32)1 << 14)
{
- if (adjusted_offset >= lzx_offset_slot_base[slot + 1])
+ if (adjusted_offset >= lzx_offset_slot_base[slot + 1] +
+ LZX_OFFSET_ADJUSTMENT)
slot++;
c->offset_slot_tab_2[adjusted_offset >> 14] = slot;
}
static size_t
lzx_get_compressor_size(size_t max_bufsize, unsigned compression_level)
{
- if (compression_level <= LZX_MAX_FAST_LEVEL) {
+ if (compression_level <= MAX_FAST_LEVEL) {
if (lzx_is_16_bit(max_bufsize))
return offsetof(struct lzx_compressor, hc_mf_16) +
hc_matchfinder_size_16(max_bufsize);
}
}
+/* Compute the amount of memory needed to allocate an LZX compressor. */
static u64
lzx_get_needed_memory(size_t max_bufsize, unsigned compression_level,
bool destructive)
size += lzx_get_compressor_size(max_bufsize, compression_level);
if (!destructive)
- size += max_bufsize; /* in_buffer */
+ size += max_bufsize; /* account for in_buffer */
return size;
}
+/* Allocate an LZX compressor. */
static int
lzx_create_compressor(size_t max_bufsize, unsigned compression_level,
bool destructive, void **c_ret)
unsigned window_order;
struct lzx_compressor *c;
+ /* Validate the maximum buffer size and get the window order from it. */
window_order = lzx_get_window_order(max_bufsize);
if (window_order == 0)
return WIMLIB_ERR_INVALID_PARAM;
+ /* Allocate the compressor. */
c = MALLOC(lzx_get_compressor_size(max_bufsize, compression_level));
if (!c)
goto oom0;
- c->destructive = destructive;
-
- c->num_main_syms = lzx_get_num_main_syms(window_order);
c->window_order = window_order;
+ c->num_main_syms = lzx_get_num_main_syms(window_order);
+ c->destructive = destructive;
+ /* Allocate the buffer for preprocessed data if needed. */
if (!c->destructive) {
c->in_buffer = MALLOC(max_bufsize);
if (!c->in_buffer)
goto oom1;
}
- if (compression_level <= LZX_MAX_FAST_LEVEL) {
-
- /* Fast compression: Use lazy parsing. */
+ if (compression_level <= MAX_FAST_LEVEL) {
+ /* Fast compression: Use lazy parsing. */
if (lzx_is_16_bit(max_bufsize))
c->impl = lzx_compress_lazy_16;
else
c->impl = lzx_compress_lazy_32;
+
+ /* Scale max_search_depth and nice_match_length with the
+ * compression level. */
c->max_search_depth = (60 * compression_level) / 20;
c->nice_match_length = (80 * compression_level) / 20;
/* lzx_compress_lazy() needs max_search_depth >= 2 because it
* halves the max_search_depth when attempting a lazy match, and
- * max_search_depth cannot be 0. */
- if (c->max_search_depth < 2)
- c->max_search_depth = 2;
+ * max_search_depth must be at least 1. */
+ c->max_search_depth = max(c->max_search_depth, 2);
} else {
- /* Normal / high compression: Use near-optimal parsing. */
-
+ /* Normal / high compression: Use near-optimal parsing. */
if (lzx_is_16_bit(max_bufsize))
c->impl = lzx_compress_near_optimal_16;
else
c->impl = lzx_compress_near_optimal_32;
- /* Scale nice_match_length and max_search_depth with the
- * compression level. */
+ /* Scale max_search_depth and nice_match_length with the
+ * compression level. */
c->max_search_depth = (24 * compression_level) / 50;
c->nice_match_length = (48 * compression_level) / 50;
- /* Set a number of optimization passes appropriate for the
- * compression level. */
-
+ /* Also scale num_optim_passes with the compression level. But
+ * the more passes there are, the less they help --- so don't
+ * add them linearly. */
c->num_optim_passes = 1;
-
- if (compression_level >= 45)
- c->num_optim_passes++;
-
- /* Use more optimization passes for higher compression levels.
- * But the more passes there are, the less they help --- so
- * don't add them linearly. */
- if (compression_level >= 70) {
- c->num_optim_passes++;
- if (compression_level >= 100)
- c->num_optim_passes++;
- if (compression_level >= 150)
- c->num_optim_passes++;
- if (compression_level >= 200)
- c->num_optim_passes++;
- if (compression_level >= 300)
- c->num_optim_passes++;
- }
+ c->num_optim_passes += (compression_level >= 45);
+ c->num_optim_passes += (compression_level >= 70);
+ c->num_optim_passes += (compression_level >= 100);
+ c->num_optim_passes += (compression_level >= 150);
+ c->num_optim_passes += (compression_level >= 200);
+ c->num_optim_passes += (compression_level >= 300);
+
+ /* max_search_depth must be at least 1. */
+ c->max_search_depth = max(c->max_search_depth, 1);
}
- /* max_search_depth == 0 is invalid. */
- if (c->max_search_depth < 1)
- c->max_search_depth = 1;
-
- if (c->nice_match_length > LZX_MAX_MATCH_LEN)
- c->nice_match_length = LZX_MAX_MATCH_LEN;
-
+ /* Prepare the offset => offset slot mapping. */
lzx_init_offset_slot_tabs(c);
+
*c_ret = c;
return 0;
return WIMLIB_ERR_NOMEM;
}
+/* Compress a buffer of data. */
static size_t
lzx_compress(const void *restrict in, size_t in_nbytes,
void *restrict out, size_t out_nbytes_avail, void *restrict _c)
struct lzx_output_bitstream os;
size_t result;
- /* Don't bother trying to compress very small inputs. */
- if (in_nbytes < 100)
+ /* Don't bother trying to compress very small inputs. */
+ if (in_nbytes < 64)
return 0;
- /* Copy the input data into the internal buffer and preprocess it. */
- if (c->destructive)
- c->in_buffer = (void *)in;
- else
+ /* If the compressor is in "destructive" mode, then we can directly
+ * preprocess the input data. Otherwise, we need to copy it into an
+ * internal buffer first. */
+ if (!c->destructive) {
memcpy(c->in_buffer, in, in_nbytes);
- c->in_nbytes = in_nbytes;
- lzx_preprocess(c->in_buffer, in_nbytes);
+ in = c->in_buffer;
+ }
+
+ /* Preprocess the input data. */
+ lzx_preprocess((void *)in, in_nbytes);
- /* Initially, the previous Huffman codeword lengths are all zeroes. */
+ /* Initially, the previous Huffman codeword lengths are all zeroes. */
c->codes_index = 0;
memset(&c->codes[1].lens, 0, sizeof(struct lzx_lens));
- /* Initialize the output bitstream. */
+ /* Initialize the output bitstream. */
lzx_init_output(&os, out, out_nbytes_avail);
- /* Call the compression level-specific compress() function. */
- (*c->impl)(c, &os);
+ /* Call the compression level-specific compress() function. */
+ (*c->impl)(c, in, in_nbytes, &os);
- /* Flush the output bitstream and return the compressed size or 0. */
+ /* Flush the output bitstream. */
result = lzx_flush_output(&os);
- if (!result && c->destructive)
- lzx_postprocess(c->in_buffer, c->in_nbytes);
+
+ /* If the data did not compress to less than its original size and we
+ * preprocessed the original buffer, then postprocess it to restore it
+ * to its original state. */
+ if (result == 0 && c->destructive)
+ lzx_postprocess((void *)in, in_nbytes);
+
+ /* Return the number of compressed bytes, or 0 if the input did not
+ * compress to less than its original size. */
return result;
}
+/* Free an LZX compressor. */
static void
lzx_free_compressor(void *_c)
{
git://wimlib.net / wimlib / blobdiff