/* * lzx_compress.c * * A compressor for the LZX compression format, as used in WIM archives. */ /* * 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 * Software Foundation; either version 3 of the License, or (at your option) any * later version. * * This file is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS * FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more * details. * * You should have received a copy of the GNU Lesser General Public License * along with this file; if not, see http://www.gnu.org/licenses/. */ /* * This file contains a compressor for the LZX ("Lempel-Ziv eXtended") * compression format, as used in the WIM (Windows IMaging) file format. * * 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. * * 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. * * - LZX uses a "main" alphabet which combines literals and matches, with the * match symbols containing a "length header" (giving all or part of the match * length) and an "offset slot" (giving, roughly speaking, the order of * magnitude of the match offset). * * - LZX does not have static Huffman blocks (that is, the kind with preset * Huffman codes); however it does have two types of dynamic Huffman blocks * ("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 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. */ /******************************************************************************/ /* 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 /* * 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: * * - The near-optimal compressor may choose a match of up to LZX_MAX_MATCH_LEN * bytes starting at position 'SOFT_MAX_BLOCK_SIZE - 1'. * * - 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 SOFT_MAX_BLOCK_SIZE 100000 /* * 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 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 /* * 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 CACHE_LENGTH (SOFT_MAX_BLOCK_SIZE * 5) /* * 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 MAX_MATCHES_PER_POS LZX_NUM_LENS /* * 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. Ultimately, each token in LZX requires a whole number of bits to * output. */ #define BIT_COST 64 /* * Should the compressor take into account the costs of aligned offset symbols * instead of assuming that all are equally likely? */ #define CONSIDER_ALIGNED_COSTS 1 /* * 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 CONSIDER_GAP_MATCHES 1 /******************************************************************************/ /* Includes */ /*----------------------------------------------------------------------------*/ #ifdef HAVE_CONFIG_H # include "config.h" #endif #include "wimlib/compress_common.h" #include "wimlib/compressor_ops.h" #include "wimlib/error.h" #include "wimlib/lz_extend.h" #include "wimlib/lzx_common.h" #include "wimlib/unaligned.h" #include "wimlib/util.h" /* 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 */ #undef mf_pos_t #undef MF_SUFFIX #define mf_pos_t u32 #define MF_SUFFIX _32 #include "wimlib/bt_matchfinder.h" #include "wimlib/hc_matchfinder.h" /******************************************************************************/ /* Compressor structure */ /*----------------------------------------------------------------------------*/ /* 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 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]; }; /* Codewords and lengths for the Huffman codes */ struct lzx_codes { struct lzx_codewords codewords; struct lzx_lens lens; }; /* 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 * 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 { /* The number of literals in the run. This may be 0. The literals are * not stored explicitly in this structure; instead, they are read * directly from the uncompressed data. */ u16 litrunlen; /* If the next field doesn't indicate end-of-block, then this is the * match length minus LZX_MIN_MATCH_LEN. */ u16 adjusted_length; /* If bit 31 is clear, then this field contains the match header in bits * 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; }; /* * This structure represents a byte position in the input buffer and a node in * the graph of possible match/literal choices. * * Logically, each incoming edge to this node is labeled with a literal or a * match that can be taken to reach this position from an earlier position; and * each outgoing edge from this node is labeled with a literal or a match that * can be taken to advance from this position to a later position. */ struct lzx_optimum_node { /* The cost, in bits, of the lowest-cost path that has been found to * reach this position. This can change as progressively lower cost * paths are found to reach this position. */ u32 cost; /* * 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. * * 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 * 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) #if CONSIDER_GAP_MATCHES # define OPTIMUM_GAP_MATCH 0x80000000 #endif } _aligned_attribute(8); /* The cost model for near-optimal parsing */ struct lzx_costs { /* * '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]; /* Cost of each symbol in the main code */ u32 main[LZX_MAINCODE_MAX_NUM_SYMBOLS]; /* Cost of each symbol in the length code */ u32 len[LZX_LENCODE_NUM_SYMBOLS]; #if CONSIDER_ALIGNED_COSTS /* Cost of each symbol in the aligned offset code */ u32 aligned[LZX_ALIGNEDCODE_NUM_SYMBOLS]; #endif }; struct lzx_output_bitstream; /* The main LZX compressor structure */ struct lzx_compressor { /* The buffer for preprocessed input data, if not using destructive * compression */ void *in_buffer; /* If true, then the compressor need not preserve the input buffer if it * compresses the data successfully */ bool destructive; /* Pointer to the compress() implementation chosen at allocation time */ void (*impl)(struct lzx_compressor *, const u8 *, size_t, struct lzx_output_bitstream *); /* 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 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 it * is chosen immediately without further consideration. */ unsigned nice_match_length; /* The maximum search depth: at most this many potential matches are * considered at each position. */ unsigned max_search_depth; /* The number of optimization passes per block */ unsigned num_optim_passes; /* 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. */ struct lzx_codes codes[2]; unsigned codes_index; /* 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(SOFT_MAX_BLOCK_SIZE, LZX_MIN_MATCH_LEN) + 1]; /* 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 lzx_compress_lazy() */ struct { /* Hash chains matchfinder (MUST BE LAST!!!) */ union { struct hc_matchfinder_16 hc_mf_16; struct hc_matchfinder_32 hc_mf_32; }; }; /* Data for lzx_compress_near_optimal() */ struct { /* * Array of nodes, one per position, for running the * minimum-cost path algorithm. * * 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[ SOFT_MAX_BLOCK_SIZE - 1 + LZX_MAX_MATCH_LEN + 1]; /* The cost model for the current optimization pass */ struct lzx_costs costs; /* * Cached matches for the current block. This array * contains the matches that were found at each position * in the block. Specifically, for each position, there * is a special 'struct lz_match' whose 'length' field * contains the number of matches that were found at * that position; this is followed by the matches * themselves, if any, sorted by strictly increasing * length. * * 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. 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[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[CACHE_LENGTH + MAX_MATCHES_PER_POS + LZX_MAX_MATCH_LEN - 1]; /* 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 * also want to optimize out the use of offset_slot_tab_2 in the 16-bit case. * This requires that the limit be no more than the length of offset_slot_tab_1 * (currently 32768). */ static inline bool lzx_is_16_bit(size_t max_bufsize) { STATIC_ASSERT(ARRAY_LEN(((struct lzx_compressor *)0)->offset_slot_tab_1) == 32768); 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 * at compilation time. */ #define CALL_HC_MF(is_16_bit, c, funcname, ...) \ ((is_16_bit) ? CONCAT(funcname, _16)(&(c)->hc_mf_16, ##__VA_ARGS__) : \ CONCAT(funcname, _32)(&(c)->hc_mf_32, ##__VA_ARGS__)); #define CALL_BT_MF(is_16_bit, c, funcname, ...) \ ((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. */ struct lzx_output_bitstream { /* Bits that haven't yet been written to the output buffer */ machine_word_t bitbuf; /* Number of bits currently held in @bitbuf */ machine_word_t bitcount; /* 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 */ u8 *next; /* 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 all * pending 16-bit coding units have been flushed? */ #define CAN_BUFFER(n) ((n) <= WORDBITS - 15) /* 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 = 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. */ static inline void lzx_add_bits(struct lzx_output_bitstream *os, u32 bits, unsigned num_bits) { os->bitbuf = (os->bitbuf << num_bits) | bits; os->bitcount += 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. */ 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_le16(os->bitbuf >> ((os->bitcount - 16) & shift_mask), os->next + 0); if (max_num_bits > 16) put_unaligned_le16(os->bitbuf >> ((os->bitcount - 32) & shift_mask), os->next + 2); if (max_num_bits > 32) put_unaligned_le16(os->bitbuf >> ((os->bitcount - 48) & shift_mask), os->next + 4); os->next += (os->bitcount >> 4) << 1; os->bitcount &= 15; } /* Add at most 16 bits to the bitbuffer and flush it. */ static inline void lzx_write_bits(struct lzx_output_bitstream *os, u32 bits, unsigned num_bits) { lzx_add_bits(os, bits, num_bits); lzx_flush_bits(os, 16); } /* * 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 size_t lzx_flush_output(struct lzx_output_bitstream *os) { if (os->end - os->next < 6) return 0; if (os->bitcount != 0) { put_unaligned_le16(os->bitbuf << (16 - os->bitcount), os->next); os->next += 2; } return os->next - os->start; } /******************************************************************************/ /* 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_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); 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->lens.aligned, codes->codewords.aligned); } /* Reset the symbol frequencies for the current block. */ static void lzx_reset_symbol_frequencies(struct lzx_compressor *c) { memset(&c->freqs, 0, sizeof(c->freqs)); } static unsigned lzx_compute_precode_items(const u8 lens[restrict], const u8 prev_lens[restrict], u32 precode_freqs[restrict], unsigned precode_items[restrict]) { unsigned *itemptr; unsigned run_start; unsigned run_end; unsigned extra_bits; int delta; u8 len; itemptr = precode_items; run_start = 0; while (!((len = lens[run_start]) & 0x80)) { /* len = the length being repeated */ /* Find the next run of codeword lengths. */ run_end = run_start + 1; /* Fast case for a single length. */ if (likely(len != lens[run_end])) { delta = prev_lens[run_start] - len; if (delta < 0) delta += 17; precode_freqs[delta]++; *itemptr++ = delta; run_start++; continue; } /* Extend the run. */ do { run_end++; } while (len == lens[run_end]); if (len == 0) { /* Run of zeroes. */ /* 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); 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); precode_freqs[17]++; *itemptr++ = 17 | (extra_bits << 5); run_start += 4 + extra_bits; } } else { /* A run of nonzero lengths. */ /* Symbol 19: RLE 4 to 5 of any length at a time. */ while ((run_end - run_start) >= 4) { extra_bits = (run_end - run_start) > 4; delta = prev_lens[run_start] - len; if (delta < 0) delta += 17; precode_freqs[19]++; precode_freqs[delta]++; *itemptr++ = 19 | (extra_bits << 5) | (delta << 6); run_start += 4 + extra_bits; } } /* Output any remaining lengths without RLE. */ while (run_start != run_end) { delta = prev_lens[run_start] - len; if (delta < 0) delta += 17; precode_freqs[delta]++; *itemptr++ = delta; run_start++; } } return itemptr - precode_items; } /******************************************************************************/ /* Outputting compressed data */ /*----------------------------------------------------------------------------*/ /* * Output a Huffman code in the compressed form used in LZX. * * The Huffman code is represented in the output as a logical series of codeword * lengths from which the Huffman code, which must be in canonical form, can be * reconstructed. * * The codeword lengths are themselves compressed using a separate Huffman code, * the "precode", which contains a symbol for each possible codeword length in * the larger code as well as several special symbols to represent repeated * codeword lengths (a form of run-length encoding). The precode is itself * constructed in canonical form, and its codeword lengths are represented * literally in 20 4-bit fields that immediately precede the compressed codeword * lengths of the larger code. * * Furthermore, the codeword lengths of the larger code are actually represented * as deltas from the codeword lengths of the corresponding code in the previous * block. * * @os: * Bitstream to which to write the compressed Huffman code. * @lens: * The codeword lengths, indexed by symbol, in the Huffman code. * @prev_lens: * The codeword lengths, indexed by symbol, in the corresponding Huffman * code in the previous block, or all zeroes if this is the first block. * @num_lens: * The number of symbols in the Huffman code. */ static void lzx_write_compressed_code(struct lzx_output_bitstream *os, const u8 lens[restrict], const u8 prev_lens[restrict], unsigned num_lens) { u32 precode_freqs[LZX_PRECODE_NUM_SYMBOLS]; u8 precode_lens[LZX_PRECODE_NUM_SYMBOLS]; u32 precode_codewords[LZX_PRECODE_NUM_SYMBOLS]; unsigned precode_items[num_lens]; unsigned num_precode_items; unsigned precode_item; unsigned precode_sym; unsigned i; u8 saved = lens[num_lens]; *(u8 *)(lens + num_lens) = 0x80; for (i = 0; i < LZX_PRECODE_NUM_SYMBOLS; i++) precode_freqs[i] = 0; /* Compute the "items" (RLE / literal tokens and extra bits) with which * the codeword lengths in the larger code will be output. */ num_precode_items = lzx_compute_precode_items(lens, prev_lens, precode_freqs, precode_items); /* 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, precode_freqs, precode_lens, precode_codewords); /* Output the lengths of the codewords in the precode. */ for (i = 0; i < LZX_PRECODE_NUM_SYMBOLS; i++) lzx_write_bits(os, precode_lens[i], LZX_PRECODE_ELEMENT_SIZE); /* Output the encoded lengths of the codewords in the larger code. */ for (i = 0; i < num_precode_items; i++) { precode_item = precode_items[i]; precode_sym = precode_item & 0x1F; lzx_add_bits(os, precode_codewords[precode_sym], precode_lens[precode_sym]); if (precode_sym >= 17) { if (precode_sym == 17) { lzx_add_bits(os, precode_item >> 5, 4); } else if (precode_sym == 18) { lzx_add_bits(os, precode_item >> 5, 5); } else { lzx_add_bits(os, (precode_item >> 5) & 1, 1); precode_sym = precode_item >> 6; lzx_add_bits(os, precode_codewords[precode_sym], precode_lens[precode_sym]); } } STATIC_ASSERT(CAN_BUFFER(2 * PRE_CODEWORD_LIMIT + 1)); lzx_flush_bits(os, 2 * PRE_CODEWORD_LIMIT + 1); } *(u8 *)(lens + num_lens) = saved; } /* * Write all matches and literal bytes (which were precomputed) in an LZX * compressed block to the output bitstream in the final compressed * representation. * * @os * The output bitstream. * @block_type * The chosen type of the LZX compressed block (LZX_BLOCKTYPE_ALIGNED or * LZX_BLOCKTYPE_VERBATIM). * @block_data * The uncompressed data of the block. * @sequences * The matches and literals to output, given as a series of sequences. * @codes * The main, length, and aligned offset Huffman codes for the block. */ static void lzx_write_sequences(struct lzx_output_bitstream *os, int block_type, const u8 *block_data, const struct lzx_sequence sequences[], const struct lzx_codes *codes) { const struct lzx_sequence *seq = sequences; u32 ones_if_aligned = 0 - (block_type == LZX_BLOCKTYPE_ALIGNED); for (;;) { /* Output the next sequence. */ unsigned litrunlen = seq->litrunlen; unsigned match_hdr; unsigned main_symbol; unsigned adjusted_length; u32 adjusted_offset; unsigned offset_slot; unsigned num_extra_bits; u32 extra_bits; /* Output the literal run of the sequence. */ if (litrunlen) { /* Is the literal run nonempty? */ /* Verify optimization is enabled on 64-bit */ STATIC_ASSERT(WORDBITS < 64 || CAN_BUFFER(3 * MAIN_CODEWORD_LIMIT)); if (CAN_BUFFER(3 * MAIN_CODEWORD_LIMIT)) { /* 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]; 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]); if (litrunlen--) { unsigned lit = *block_data++; 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); } } } else { /* 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_flush_bits(os, MAIN_CODEWORD_LIMIT); } while (--litrunlen); } } /* Was this the last literal run? */ if (seq->adjusted_offset_and_match_hdr & 0x80000000) return; /* Nope; output the match. */ match_hdr = seq->adjusted_offset_and_match_hdr & 0x1FF; main_symbol = LZX_NUM_CHARS + match_hdr; adjusted_length = seq->adjusted_length; block_data += adjusted_length + LZX_MIN_MATCH_LEN; offset_slot = match_hdr / LZX_NUM_LEN_HEADERS; 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] + 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(WORDBITS < 64 || CAN_BUFFER(MAX_MATCH_BITS)); /* Output the main symbol for the match. */ lzx_add_bits(os, codes->codewords.main[main_symbol], codes->lens.main[main_symbol]); if (!CAN_BUFFER(MAX_MATCH_BITS)) lzx_flush_bits(os, MAIN_CODEWORD_LIMIT); /* 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]); if (!CAN_BUFFER(MAX_MATCH_BITS)) lzx_flush_bits(os, LENGTH_CODEWORD_LIMIT); } /* Output the extra offset bits for the match. In aligned * offset blocks, the lowest 3 bits of the adjusted offset are * Huffman-encoded using the aligned offset code, provided that * there are at least extra 3 offset bits required. All other * extra offset bits are output verbatim. */ if ((adjusted_offset & ones_if_aligned) >= 16) { lzx_add_bits(os, extra_bits >> LZX_NUM_ALIGNED_OFFSET_BITS, num_extra_bits - LZX_NUM_ALIGNED_OFFSET_BITS); 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]); if (!CAN_BUFFER(MAX_MATCH_BITS)) lzx_flush_bits(os, ALIGNED_CODEWORD_LIMIT); } else { STATIC_ASSERT(CAN_BUFFER(17)); lzx_add_bits(os, extra_bits, num_extra_bits); if (!CAN_BUFFER(MAX_MATCH_BITS)) lzx_flush_bits(os, 17); } if (CAN_BUFFER(MAX_MATCH_BITS)) lzx_flush_bits(os, MAX_MATCH_BITS); /* Advance to the next sequence. */ seq++; } } static void lzx_write_compressed_block(const u8 *block_begin, int block_type, u32 block_size, unsigned window_order, unsigned num_main_syms, const struct lzx_sequence sequences[], const struct lzx_codes * codes, const struct lzx_lens * prev_lens, struct lzx_output_bitstream * os) { /* The first three bits indicate the type of block and are one of the * LZX_BLOCKTYPE_* constants. */ lzx_write_bits(os, block_type, 3); /* * Output the block size. * * 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. * * 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_bits(os, 0, 1); if (window_order >= 16) lzx_write_bits(os, block_size >> 16, 8); lzx_write_bits(os, block_size & 0xFFFF, 16); } /* If it's an aligned offset block, output the aligned offset code. */ if (block_type == LZX_BLOCKTYPE_ALIGNED) { for (int i = 0; i < LZX_ALIGNEDCODE_NUM_SYMBOLS; i++) { lzx_write_bits(os, codes->lens.aligned[i], LZX_ALIGNEDCODE_ELEMENT_SIZE); } } /* Output the main code (two parts). */ lzx_write_compressed_code(os, codes->lens.main, prev_lens->main, LZX_NUM_CHARS); lzx_write_compressed_code(os, codes->lens.main + LZX_NUM_CHARS, prev_lens->main + LZX_NUM_CHARS, num_main_syms - LZX_NUM_CHARS); /* Output the length code. */ lzx_write_compressed_code(os, codes->lens.len, prev_lens->len, LZX_LENCODE_NUM_SYMBOLS); /* Output the compressed matches and literals. */ lzx_write_sequences(os, block_type, block_begin, sequences, codes); } /* * Given the frequencies of symbols in an LZX-compressed block and the * corresponding Huffman codes, return LZX_BLOCKTYPE_ALIGNED or * LZX_BLOCKTYPE_VERBATIM if an aligned offset or verbatim block, respectively, * will take fewer bits to output. */ static int lzx_choose_verbatim_or_aligned(const struct lzx_freqs * freqs, const struct lzx_codes * codes) { u32 verbatim_cost = 0; u32 aligned_cost = 0; /* 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 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; else return LZX_BLOCKTYPE_VERBATIM; } /* * Flush an LZX block: * * 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_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_build_huffman_codes(c); block_type = lzx_choose_verbatim_or_aligned(&c->freqs, &c->codes[c->codes_index]); lzx_write_compressed_block(block_begin, block_type, block_size, c->window_order, c->num_main_syms, &c->chosen_sequences[seq_idx], &c->codes[c->codes_index], &c->codes[c->codes_index ^ 1].lens, os); c->codes_index ^= 1; } /******************************************************************************/ /* 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. */ /* Initialize the block split statistics when starting a new block. */ static void lzx_init_block_split_stats(struct lzx_block_split_stats *stats) { memset(stats, 0, sizeof(*stats)); } /* 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_observe_literal(struct lzx_block_split_stats *stats, u8 lit) { stats->new_observations[((lit >> 5) & 0x6) | (lit & 1)]++; stats->num_new_observations++; } /* 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++; } 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; } /* Ready to end the block? */ if (total_delta >= stats->num_new_observations * 7 / 8 * stats->num_observations) return true; } 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; } /******************************************************************************/ /* Slower ("near-optimal") compression algorithm */ /*----------------------------------------------------------------------------*/ /* * 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_QUEUE_OFFSET_SHIFT 21 #define LZX_QUEUE_OFFSET_MASK (((u64)1 << LZX_QUEUE_OFFSET_SHIFT) - 1) #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) #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) #define LZX_QUEUE_INITIALIZER { \ ((u64)1 << LZX_QUEUE_R0_SHIFT) | \ ((u64)1 << LZX_QUEUE_R1_SHIFT) | \ ((u64)1 << LZX_QUEUE_R2_SHIFT) } static inline u64 lzx_lru_queue_R0(struct lzx_lru_queue queue) { return (queue.R >> LZX_QUEUE_R0_SHIFT) & LZX_QUEUE_OFFSET_MASK; } static inline u64 lzx_lru_queue_R1(struct lzx_lru_queue queue) { return (queue.R >> LZX_QUEUE_R1_SHIFT) & LZX_QUEUE_OFFSET_MASK; } static inline u64 lzx_lru_queue_R2(struct lzx_lru_queue queue) { return (queue.R >> LZX_QUEUE_R2_SHIFT) & LZX_QUEUE_OFFSET_MASK; } /* 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, }; } /* 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; return (struct lzx_lru_queue) { (queue.R & ~(mask | mask_high)) | ((queue.R & mask_high) >> shift) | ((queue.R & mask) << shift) }; } static inline u32 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; if (record) { /* Special value to mark last sequence */ c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr = 0x80000000; lit_start_node = node_idx; } for (;;) { u32 item; u32 len; u32 adjusted_offset; unsigned v; unsigned offset_slot; /* 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 (;;) { item = c->optimum_nodes[node_idx].item; if (item & OPTIMUM_LEN_MASK) break; c->freqs.main[item >> OPTIMUM_OFFSET_SHIFT]++; node_idx--; } #if CONSIDER_GAP_MATCHES if (item & OPTIMUM_GAP_MATCH) { if (node_idx == 0) break; /* 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; } /* 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; } /* Record a match. */ /* Tally the aligned offset symbol if needed. */ if (adjusted_offset >= 16) c->freqs.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK]++; /* Record the adjusted length. */ v = len - LZX_MIN_MATCH_LEN; if (record) c->chosen_sequences[seq_idx].adjusted_length = v; /* 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_get_offset_slot(c, adjusted_offset, is_16_bit); v += offset_slot * LZX_NUM_LEN_HEADERS; c->freqs.main[LZX_NUM_CHARS + v]++; /* Record the adjusted offset and match header. */ if (record) { c->chosen_sequences[seq_idx].adjusted_offset_and_match_hdr = (adjusted_offset << 9) | v; } } /* 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 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]', 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 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 * true minimum cost, or "optimal", path over the graph of all valid LZX * representations of this block. * * Also, note that because of the presence of the recent offsets queue (which is * a type of adaptive state), the algorithm cannot work backwards and compute * "cost to end" instead of "cost to beginning". Furthermore, the way the * algorithm handles this adaptive state in the "minimum cost" parse is actually * only an approximation. It's possible for the globally optimal, minimum cost * 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 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, const u8 * const restrict block_begin, const u32 block_size, const struct lzx_lru_queue initial_queue, bool is_16_bit) { struct lzx_optimum_node *cur_node = c->optimum_nodes; 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 * '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 * (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); 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". * * 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])); /* Initialize the recent offsets queue for the first node. */ QUEUE(cur_node) = initial_queue; do { /* For each node in the block in position order... */ unsigned num_matches; unsigned literal; u32 cost; /* * A selection of matches for the block was already saved in * memory so that we don't have to run the uncompressed data * through the matchfinder on every optimization pass. However, * we still search for repeat offset matches during each * optimization pass because we cannot predict the state of the * recent offsets queue. But as a heuristic, we don't bother * searching for repeat offset matches if the general-purpose * matchfinder failed to find any matches. * * Note that a match of length n at some offset implies there is * also a match of length l for LZX_MIN_MATCH_LEN <= l <= n at * that same offset. In other words, we don't necessarily need * to use the full length of a match. The key heuristic that * saves a significicant amount of time is that for each * distinct length, we only consider the smallest offset for * which that length is available. This heuristic also applies * to repeat offsets, which we order specially: R0 < R1 < R2 < * any explicit offset. Of course, this heuristic may be * produce suboptimal results because offset slots in LZX are * subject to entropy encoding, but in practice this is a useful * heuristic. */ num_matches = cache_ptr->length; cache_ptr++; if (num_matches) { struct lz_match *end_matches = cache_ptr + num_matches; unsigned next_len = LZX_MIN_MATCH_LEN; unsigned max_len = min(block_end - in_next, LZX_MAX_MATCH_LEN); const u8 *matchptr; /* Consider rep0 matches. */ matchptr = in_next - lzx_lru_queue_R0(QUEUE(cur_node)); if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next)) goto rep0_done; STATIC_ASSERT(LZX_MIN_MATCH_LEN == 2); do { u32 cost = cur_node->cost + c->costs.match_cost[0][ next_len - LZX_MIN_MATCH_LEN]; if (cost <= (cur_node + next_len)->cost) { (cur_node + next_len)->cost = cost; (cur_node + next_len)->item = (0 << OPTIMUM_OFFSET_SHIFT) | next_len; } if (unlikely(++next_len > max_len)) { cache_ptr = end_matches; goto done_matches; } } while (in_next[next_len - 1] == matchptr[next_len - 1]); rep0_done: /* Consider rep1 matches. */ matchptr = in_next - lzx_lru_queue_R1(QUEUE(cur_node)); if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next)) goto rep1_done; if (matchptr[next_len - 1] != in_next[next_len - 1]) goto rep1_done; for (unsigned len = 2; len < next_len - 1; len++) if (matchptr[len] != in_next[len]) goto rep1_done; do { u32 cost = cur_node->cost + c->costs.match_cost[1][ next_len - LZX_MIN_MATCH_LEN]; if (cost <= (cur_node + next_len)->cost) { (cur_node + next_len)->cost = cost; (cur_node + next_len)->item = (1 << OPTIMUM_OFFSET_SHIFT) | next_len; } if (unlikely(++next_len > max_len)) { cache_ptr = end_matches; goto done_matches; } } while (in_next[next_len - 1] == matchptr[next_len - 1]); rep1_done: /* Consider rep2 matches. */ matchptr = in_next - lzx_lru_queue_R2(QUEUE(cur_node)); if (load_u16_unaligned(matchptr) != load_u16_unaligned(in_next)) goto rep2_done; if (matchptr[next_len - 1] != in_next[next_len - 1]) goto rep2_done; for (unsigned len = 2; len < next_len - 1; len++) if (matchptr[len] != in_next[len]) goto rep2_done; do { u32 cost = cur_node->cost + c->costs.match_cost[2][ next_len - LZX_MIN_MATCH_LEN]; if (cost <= (cur_node + next_len)->cost) { (cur_node + next_len)->cost = cost; (cur_node + next_len)->item = (2 << OPTIMUM_OFFSET_SHIFT) | next_len; } if (unlikely(++next_len > max_len)) { cache_ptr = end_matches; goto done_matches; } } while (in_next[next_len - 1] == matchptr[next_len - 1]); rep2_done: while (next_len > cache_ptr->length) if (++cache_ptr == end_matches) goto done_matches; /* Consider explicit offset matches. */ for (;;) { u32 offset = cache_ptr->offset; 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 CONSIDER_ALIGNED_COSTS if (offset >= 16 - LZX_OFFSET_ADJUSTMENT) base_cost += c->costs.aligned[adjusted_offset & LZX_ALIGNED_OFFSET_BITMASK]; #endif do { 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 = (adjusted_offset << OPTIMUM_OFFSET_SHIFT) | next_len; } } while (++next_len <= cache_ptr->length); 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: /* Consider coding a literal. * To avoid an extra branch, actually checking the preferability * of coding the literal is integrated into the queue update * code below. */ literal = *in_next++; cost = cur_node->cost + c->costs.main[literal]; /* 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. */ if (cost <= cur_node->cost) { /* Literal: queue remains unchanged. */ cur_node->cost = cost; cur_node->item = (u32)literal << OPTIMUM_OFFSET_SHIFT; QUEUE(cur_node) = QUEUE(cur_node - 1); } else { /* Match: queue update is needed. */ unsigned len = cur_node->item & OPTIMUM_LEN_MASK; #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 recent offsets queue at the end of the path. */ return QUEUE(cur_node); } /* * 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; 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 = lzx_extra_offset_bits[offset_slot] * BIT_COST; unsigned i; #if CONSIDER_ALIGNED_COSTS if (offset_slot >= 8) extra_cost -= LZX_NUM_ALIGNED_OFFSET_BITS * BIT_COST; #endif 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++]; for (; i < LZX_NUM_LENS; i++) { costs->match_cost[offset_slot][i] = costs->len[i - LZX_NUM_PRIMARY_LENS] + extra_cost; } } } /* * 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) { 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; /* 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); /* * 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). * * 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) * * Use this to get the cost in fractional bits. Then multiply by our * scaling factor of BIT_COST and truncate to a u32. * * 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); } /* * 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 }; /* * 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, }; /* 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); } /* 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] = 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] = lzx_default_len_costs[i]; #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 } /* Update the current cost model to reflect the computed Huffman codes. */ static void 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) * BIT_COST; } for (i = 0; i < LZX_LENCODE_NUM_SYMBOLS; i++) { c->costs.len[i] = (lens->len[i] ? lens->len[i] : LENGTH_CODEWORD_LIMIT) * BIT_COST; } #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) * BIT_COST; } #endif } /* * 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_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, const struct lzx_lru_queue initial_queue, bool is_16_bit) { unsigned num_passes_remaining = c->num_optim_passes; struct lzx_lru_queue new_queue; u32 seq_idx; lzx_set_default_costs(c); 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 == 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_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 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 * certainly not "optimal". The goal is not to produce the optimal compression * ratio, which for LZX is probably impossible within any practical amount of * time, but rather to produce a compression ratio significantly better than a * simpler "greedy" or "lazy" parse while still being relatively fast. */ static inline void 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 * in_next = in_begin; 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] = {0, 0}; struct lzx_lru_queue queue = LZX_QUEUE_INITIALIZER; /* Initialize the matchfinder. */ CALL_BT_MF(is_16_bit, c, bt_matchfinder_init); do { /* Starting a new block */ 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; 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); if (in_next >= next_pause_point) goto pause; /* * 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; } } /* * 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++; in_next++; } } } /* 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, const u8 *in, size_t in_nbytes, struct lzx_output_bitstream *os) { lzx_compress_near_optimal(c, in, in_nbytes, os, true); } static void 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, in, in_nbytes, os, false); } /******************************************************************************/ /* Faster ("lazy") compression algorithm */ /*----------------------------------------------------------------------------*/ /* * 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. * * 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 recent_offsets[], const unsigned max_len, unsigned *best_rep_idx_ret) { STATIC_ASSERT(LZX_NUM_RECENT_OFFSETS == 3); /* loop is unrolled */ const u32 seq3 = load_u24_unaligned(in_next); const u8 *matchptr; 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_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_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_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; } } *best_rep_idx_ret = best_rep_idx; return best_rep_len; } /* * 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) { unsigned score = len; if (adjusted_offset < 4096) score++; if (adjusted_offset < 256) score++; return score; } static inline unsigned lzx_repeat_offset_match_score(unsigned rep_len, unsigned rep_idx) { return rep_len + 3; } /* * 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 * restrict c, const u8 * const restrict in_begin, size_t in_nbytes, struct lzx_output_bitstream * restrict os, bool is_16_bit) { const u8 * in_next = in_begin; 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[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 */ 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 lzx_sequence *next_seq = c->chosen_sequences; u32 litrunlen = 0; unsigned cur_len; u32 cur_offset; u32 cur_adjusted_offset; unsigned cur_score; unsigned next_len; u32 next_offset; u32 next_adjusted_offset; unsigned next_score; unsigned best_rep_len; unsigned best_rep_idx; unsigned rep_score; unsigned skip_len; 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 (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, max_len, nice_len, 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[0] && cur_offset != recent_offsets[1] && cur_offset != recent_offsets[2])) { 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++; skip_len = cur_len - 1; cur_adjusted_offset = 0; goto choose_cur_match; } /* 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 (best_rep_len != 0 && (rep_score = lzx_repeat_offset_match_score(best_rep_len, best_rep_idx)) >= cur_score) { 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. */ if (cur_len >= nice_len) { skip_len = cur_len - 1; goto choose_cur_match; } 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, in_begin, in_next - in_begin, cur_len - 2, max_len, nice_len, c->max_search_depth / 2, next_hashes, &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_adjusted_offset = next_offset + LZX_OFFSET_ADJUSTMENT; next_score = lzx_explicit_offset_match_score(next_len, next_adjusted_offset); best_rep_len = lzx_find_longest_repeat_offset_match(in_next, recent_offsets, max_len, &best_rep_idx); in_next++; 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_choose_literal(c, *(in_next - 2), &litrunlen); 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_choose_literal(c, *(in_next - 2), &litrunlen); cur_len = next_len; cur_adjusted_offset = next_adjusted_offset; cur_score = next_score; goto have_cur_match; } } /* The original match was better; choose it. */ skip_len = cur_len - 2; choose_cur_match: /* 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_begin, in_next - in_begin, in_end - in_begin, skip_len, next_hashes); /* 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))); /* 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, const u8 *in, size_t in_nbytes, struct lzx_output_bitstream *os) { lzx_compress_lazy(c, in, in_nbytes, os, true); } static void lzx_compress_lazy_32(struct lzx_compressor *c, const u8 *in, size_t in_nbytes, struct lzx_output_bitstream *os) { lzx_compress_lazy(c, in, in_nbytes, os, false); } /******************************************************************************/ /* 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] */ for (; adjusted_offset < ARRAY_LEN(c->offset_slot_tab_1); adjusted_offset++) { if (adjusted_offset >= lzx_offset_slot_base[slot + 1] + LZX_OFFSET_ADJUSTMENT) slot++; c->offset_slot_tab_1[adjusted_offset] = slot; } /* slots [30, 49] */ for (; adjusted_offset < LZX_MAX_WINDOW_SIZE; adjusted_offset += (u32)1 << 14) { 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 <= MAX_FAST_LEVEL) { if (lzx_is_16_bit(max_bufsize)) return offsetof(struct lzx_compressor, hc_mf_16) + hc_matchfinder_size_16(max_bufsize); else return offsetof(struct lzx_compressor, hc_mf_32) + hc_matchfinder_size_32(max_bufsize); } else { if (lzx_is_16_bit(max_bufsize)) return offsetof(struct lzx_compressor, bt_mf_16) + bt_matchfinder_size_16(max_bufsize); else return offsetof(struct lzx_compressor, bt_mf_32) + bt_matchfinder_size_32(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) { u64 size = 0; if (max_bufsize > LZX_MAX_WINDOW_SIZE) return 0; size += lzx_get_compressor_size(max_bufsize, compression_level); if (!destructive) 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->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 <= 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 must be at least 1. */ c->max_search_depth = max(c->max_search_depth, 2); } else { /* 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 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; /* 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; 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); } /* Prepare the offset => offset slot mapping. */ lzx_init_offset_slot_tabs(c); *c_ret = c; return 0; oom1: FREE(c); oom0: 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_compressor *c = _c; struct lzx_output_bitstream os; size_t result; /* Don't bother trying to compress very small inputs. */ if (in_nbytes < 64) return 0; /* 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); in = c->in_buffer; } /* Preprocess the input data. */ lzx_preprocess((void *)in, in_nbytes); /* 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. */ lzx_init_output(&os, out, out_nbytes_avail); /* Call the compression level-specific compress() function. */ (*c->impl)(c, in, in_nbytes, &os); /* Flush the output bitstream. */ result = lzx_flush_output(&os); /* 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) { struct lzx_compressor *c = _c; if (!c->destructive) FREE(c->in_buffer); FREE(c); } const struct compressor_ops lzx_compressor_ops = { .get_needed_memory = lzx_get_needed_memory, .create_compressor = lzx_create_compressor, .compress = lzx_compress, .free_compressor = lzx_free_compressor, };