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libdeflate/lib/deflate_compress.c
Eric Biggers 5e9226fff8 deflate_compress: optimize level 1 
Introduce deflate_compress_fastest(), and use it for level 1.  It uses
the ht_matchfinder instead of the hc_matchfinder, and it skips the block
splitting algorithm.  This speeds up level 1, without changing the
compression ratio too much (relative to what is expected for level 1).
2022-01-01 20:15:27 -06:00

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/*
* deflate_compress.c - a compressor for DEFLATE
*
* Copyright 2016 Eric Biggers
*
* Permission is hereby granted, free of charge, to any person
* obtaining a copy of this software and associated documentation
* files (the "Software"), to deal in the Software without
* restriction, including without limitation the rights to use,
* copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the
* Software is furnished to do so, subject to the following
* conditions:
*
* The above copyright notice and this permission notice shall be
* included in all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
* OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
* NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
* HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
* WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
* FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
* OTHER DEALINGS IN THE SOFTWARE.
*/
#include "deflate_compress.h"
#include "deflate_constants.h"
#include "unaligned.h"
#include "libdeflate.h"
/******************************************************************************/
/*
* The following parameters can be changed at build time to customize the
* compression algorithms slightly:
*
* (Note, not all customizable parameters are here. Some others can be found in
* libdeflate_alloc_compressor() and in *_matchfinder.h.)
*/
/*
* If this parameter is defined to 1, then the near-optimal parsing algorithm
* will be included, and compression levels 10-12 will use it. This algorithm
* usually produces a compression ratio significantly better than the other
* algorithms. However, it is slow. If this parameter is defined to 0, then
* levels 10-12 will be the same as level 9 and will use the lazy2 algorithm.
*/
#define SUPPORT_NEAR_OPTIMAL_PARSING 1
/*
* If this parameter is defined to 1, then the compressor will maintain a full
* map from match offsets to offset slots, rather than a condensed map. This
* will usually improve performance, especially for the near-optimal parsing
* algorithm. However, it will use an additional 32257 bytes of memory.
*/
#define USE_FULL_OFFSET_SLOT_FAST SUPPORT_NEAR_OPTIMAL_PARSING
/*
* This is the minimum block length, in uncompressed bytes, which the compressor
* will use. This should be a value below which using shorter blocks is very
* unlikely to be worthwhile, due to the per-block overhead. This parameter
* doesn't apply to the final block, which can be arbitrarily short.
*
* Defining a fixed minimum block length is needed in order to guarantee a
* reasonable upper bound on the compressed size. It's also needed because our
* block splitting algorithm doesn't work well on very short blocks.
*/
#define MIN_BLOCK_LENGTH 10000
/*
* This is the soft maximum block length, in uncompressed bytes, which the
* compressor will use. This is a "soft" maximum, meaning that the compressor
* will try to end blocks at this length, but it may go slightly past it if
* there is a match that straddles this limit. This parameter doesn't apply to
* uncompressed blocks, which the DEFLATE format limits to 65535 bytes.
*
* This should be a value above which it is very likely that splitting the block
* would produce a better compression ratio. Increasing/decreasing this
* parameter will increase/decrease per-compressor memory usage linearly.
*/
#define SOFT_MAX_BLOCK_LENGTH 300000
/*
* Block length, in uncompressed bytes, used by deflate_compress_fastest().
* deflate_compress_fastest() doesn't use the other block length settings.
*/
#define FAST_BLOCK_LENGTH MIN(32768, SOFT_MAX_BLOCK_LENGTH)
/*
* These are the maximum codeword lengths, in bits, the compressor will use for
* each Huffman code. The DEFLATE format defines limits for these. However,
* further limiting litlen codewords to 14 bits is beneficial, since it has
* negligible effect on compression ratio but allows some optimizations when
* outputting bits. (It allows 4 literals to be written at once rather than 3.)
*/
#define MAX_LITLEN_CODEWORD_LEN 14
#define MAX_OFFSET_CODEWORD_LEN DEFLATE_MAX_OFFSET_CODEWORD_LEN
#define MAX_PRE_CODEWORD_LEN DEFLATE_MAX_PRE_CODEWORD_LEN
#if SUPPORT_NEAR_OPTIMAL_PARSING
/* Parameters specific to the near-optimal parsing algorithm */
/*
* BIT_COST is a scaling factor that allows the compressor to consider
* fractional bit costs when deciding which literal/match sequence to use. This
* is useful when the true symbol costs are unknown. For example, if the
* compressor thinks that a symbol has 6.5 bits of entropy, it can set its cost
* to 6.5 bits rather than have to use 6 or 7 bits. Although in the end each
* symbol will use a whole number of bits due to the Huffman coding, considering
* fractional bits can be helpful due to the limited information.
*
* BIT_COST should be a power of 2. A value of 8 or 16 works well. A higher
* value isn't very useful since the calculations are approximate anyway.
*/
#define BIT_COST 16
/*
* The NOSTAT_BITS value for a given alphabet is the number of bits assumed to
* be needed to output a symbol that was unused in the previous optimization
* pass. Assigning a default cost allows the symbol to be used in the next
* optimization pass. However, the cost should be relatively high because the
* symbol probably won't be used very many times (if at all).
*/
#define LITERAL_NOSTAT_BITS 13
#define LENGTH_NOSTAT_BITS 13
#define OFFSET_NOSTAT_BITS 10
/*
* This is (approximately) the maximum number of matches that the compressor
* will cache per block. If the match cache becomes full, then the compressor
* will be forced to end the block early. This value should be large enough so
* that this rarely happens, due to the block being ended normally before the
* cache fills up. Increasing/decreasing this parameter will increase/decrease
* per-compressor memory usage linearly.
*/
#define MATCH_CACHE_LENGTH (SOFT_MAX_BLOCK_LENGTH * 5)
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
/******************************************************************************/
/* Include the needed matchfinders. */
#define MATCHFINDER_WINDOW_ORDER DEFLATE_WINDOW_ORDER
#include "hc_matchfinder.h"
#include "ht_matchfinder.h"
#if SUPPORT_NEAR_OPTIMAL_PARSING
# include "bt_matchfinder.h"
/*
* This is the maximum number of matches the binary trees matchfinder can find
* at a single position. Since the matchfinder never finds more than one match
* for the same length, presuming one of each possible length is sufficient for
* an upper bound. (This says nothing about whether it is worthwhile to
* consider so many matches; this is just defining the worst case.)
*/
#define MAX_MATCHES_PER_POS \
(DEFLATE_MAX_MATCH_LEN - DEFLATE_MIN_MATCH_LEN + 1)
#endif
/* Table: length slot => length slot base value */
static const unsigned deflate_length_slot_base[] = {
3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ,
11 , 13 , 15 , 17 , 19 , 23 , 27 , 31 ,
35 , 43 , 51 , 59 , 67 , 83 , 99 , 115 ,
131 , 163 , 195 , 227 , 258 ,
};
/* Table: length slot => number of extra length bits */
static const u8 deflate_extra_length_bits[] = {
0 , 0 , 0 , 0 , 0 , 0 , 0 , 0 ,
1 , 1 , 1 , 1 , 2 , 2 , 2 , 2 ,
3 , 3 , 3 , 3 , 4 , 4 , 4 , 4 ,
5 , 5 , 5 , 5 , 0 ,
};
/* Table: offset slot => offset slot base value */
static const unsigned deflate_offset_slot_base[] = {
1 , 2 , 3 , 4 , 5 , 7 , 9 , 13 ,
17 , 25 , 33 , 49 , 65 , 97 , 129 , 193 ,
257 , 385 , 513 , 769 , 1025 , 1537 , 2049 , 3073 ,
4097 , 6145 , 8193 , 12289 , 16385 , 24577 ,
};
/* Table: offset slot => number of extra offset bits */
static const u8 deflate_extra_offset_bits[] = {
0 , 0 , 0 , 0 , 1 , 1 , 2 , 2 ,
3 , 3 , 4 , 4 , 5 , 5 , 6 , 6 ,
7 , 7 , 8 , 8 , 9 , 9 , 10 , 10 ,
11 , 11 , 12 , 12 , 13 , 13 ,
};
/* Table: length => length slot */
static const u8 deflate_length_slot[DEFLATE_MAX_MATCH_LEN + 1] = {
0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 8, 9, 9, 10, 10, 11, 11, 12, 12, 12,
12, 13, 13, 13, 13, 14, 14, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 16,
16, 16, 16, 17, 17, 17, 17, 17, 17, 17, 17, 18, 18, 18, 18, 18, 18, 18,
18, 19, 19, 19, 19, 19, 19, 19, 19, 20, 20, 20, 20, 20, 20, 20, 20, 20,
20, 20, 20, 20, 20, 20, 20, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
21, 21, 21, 21, 21, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
22, 22, 22, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
23, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 25, 25, 25,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25,
25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 25, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26, 26,
26, 26, 26, 26, 26, 26, 26, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27, 27,
27, 27, 28,
};
/* The order in which precode codeword lengths are stored */
static const u8 deflate_precode_lens_permutation[DEFLATE_NUM_PRECODE_SYMS] = {
16, 17, 18, 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15
};
/* Codewords for the DEFLATE Huffman codes. */
struct deflate_codewords {
u32 litlen[DEFLATE_NUM_LITLEN_SYMS];
u32 offset[DEFLATE_NUM_OFFSET_SYMS];
};
/* Codeword lengths (in bits) for the DEFLATE Huffman codes.
* A zero length means the corresponding symbol had zero frequency. */
struct deflate_lens {
u8 litlen[DEFLATE_NUM_LITLEN_SYMS];
u8 offset[DEFLATE_NUM_OFFSET_SYMS];
};
/* Codewords and lengths for the DEFLATE Huffman codes. */
struct deflate_codes {
struct deflate_codewords codewords;
struct deflate_lens lens;
};
/* Symbol frequency counters for the DEFLATE Huffman codes. */
struct deflate_freqs {
u32 litlen[DEFLATE_NUM_LITLEN_SYMS];
u32 offset[DEFLATE_NUM_OFFSET_SYMS];
};
/*
* Represents a run of literals followed by a match or end-of-block. This
* struct is needed to temporarily store items chosen by the parser, since items
* cannot be written until all items for the block have been chosen and the
* block's Huffman codes have been computed.
*/
struct deflate_sequence {
/* Bits 0..22: the number of literals in this run. This may be 0 and
* can be at most about SOFT_MAX_BLOCK_LENGTH. The literals are not
* stored explicitly in this structure; instead, they are read directly
* from the uncompressed data.
*
* Bits 23..31: the length of the match which follows the literals, or 0
* if this literal run was the last in the block, so there is no match
* which follows it. */
u32 litrunlen_and_length;
/* If 'length' doesn't indicate end-of-block, then this is the offset of
* the match which follows the literals. */
u16 offset;
/* If 'length' doesn't indicate end-of-block, then this is the offset
* symbol of the match which follows the literals. */
u8 offset_symbol;
/* If 'length' doesn't indicate end-of-block, then this is the length
* slot of the match which follows the literals. */
u8 length_slot;
};
#if SUPPORT_NEAR_OPTIMAL_PARSING
/* Costs for the near-optimal parsing algorithm. */
struct deflate_costs {
/* The cost to output each possible literal. */
u32 literal[DEFLATE_NUM_LITERALS];
/* The cost to output each possible match length. */
u32 length[DEFLATE_MAX_MATCH_LEN + 1];
/* The cost to output a match offset of each possible offset slot. */
u32 offset_slot[DEFLATE_NUM_OFFSET_SYMS];
};
/*
* This structure represents a byte position in the input data and a node in the
* graph of possible match/literal choices for the current block.
*
* 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.
*
* But these "edges" are actually stored elsewhere (in 'match_cache'). Here we
* associate with each node just two pieces of information:
*
* 'cost_to_end' is the minimum cost to reach the end of the block from
* this position.
*
* 'item' represents the literal or match that must be chosen from here to
* reach the end of the block with the minimum cost. Equivalently, this
* can be interpreted as the label of the outgoing edge on the minimum-cost
* path to the "end of block" node from this node.
*/
struct deflate_optimum_node {
u32 cost_to_end;
/*
* Notes on the match/literal representation used here:
*
* The low bits of 'item' are the length: 1 if this is a literal,
* or the match length if this is a match.
*
* The high bits of 'item' are the actual literal byte if this is a
* literal, or the match offset if this is a match.
*/
#define OPTIMUM_OFFSET_SHIFT 9
#define OPTIMUM_LEN_MASK (((u32)1 << OPTIMUM_OFFSET_SHIFT) - 1)
u32 item;
};
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
/* Block split statistics. See "Block splitting algorithm" below. */
#define NUM_LITERAL_OBSERVATION_TYPES 8
#define NUM_MATCH_OBSERVATION_TYPES 2
#define NUM_OBSERVATION_TYPES (NUM_LITERAL_OBSERVATION_TYPES + NUM_MATCH_OBSERVATION_TYPES)
#define NUM_OBSERVATIONS_PER_BLOCK_CHECK 512
struct block_split_stats {
u32 new_observations[NUM_OBSERVATION_TYPES];
u32 observations[NUM_OBSERVATION_TYPES];
u32 num_new_observations;
u32 num_observations;
};
/* The main DEFLATE compressor structure */
struct libdeflate_compressor {
/* Pointer to the compress() implementation chosen at allocation time */
size_t (*impl)(struct libdeflate_compressor *,
const u8 *, size_t, u8 *, size_t);
/* Frequency counters for the current block */
struct deflate_freqs freqs;
/* Dynamic Huffman codes for the current block */
struct deflate_codes codes;
/* Static Huffman codes */
struct deflate_codes static_codes;
/* Block split statistics for the currently pending block */
struct block_split_stats split_stats;
/* A table for fast lookups of offset slot by match offset.
*
* If the full table is being used, it is a direct mapping from offset
* to offset slot.
*
* If the condensed table is being used, the first 256 entries map
* directly to the offset slots of offsets 1 through 256. The next 256
* entries map to the offset slots for the remaining offsets, stepping
* through the offsets with a stride of 128. This relies on the fact
* that each of the remaining offset slots contains at least 128 offsets
* and has an offset base that is a multiple of 128. */
#if USE_FULL_OFFSET_SLOT_FAST
u8 offset_slot_fast[DEFLATE_MAX_MATCH_OFFSET + 1];
#else
u8 offset_slot_fast[512];
#endif
/* The "nice" match length: if a match of this length is found, choose
* it immediately without further consideration. */
unsigned nice_match_length;
/* The maximum search depth: consider at most this many potential
* matches at each position. */
unsigned max_search_depth;
/* The compression level with which this compressor was created. */
unsigned compression_level;
/* Anything smaller than this we won't bother trying to compress. */
unsigned min_size_to_compress;
/* Temporary space for Huffman code output */
u32 precode_freqs[DEFLATE_NUM_PRECODE_SYMS];
u8 precode_lens[DEFLATE_NUM_PRECODE_SYMS];
u32 precode_codewords[DEFLATE_NUM_PRECODE_SYMS];
unsigned precode_items[DEFLATE_NUM_LITLEN_SYMS + DEFLATE_NUM_OFFSET_SYMS];
unsigned num_litlen_syms;
unsigned num_offset_syms;
unsigned num_explicit_lens;
unsigned num_precode_items;
union {
/* Data for greedy or lazy parsing */
struct {
/* Hash chains matchfinder */
struct hc_matchfinder hc_mf;
/* The matches and literals that the parser has chosen
* for the current block. The required length of this
* array is limited by the maximum number of matches
* that can ever be chosen for a single block, plus one
* for the special entry at the end. */
struct deflate_sequence sequences[
DIV_ROUND_UP(SOFT_MAX_BLOCK_LENGTH,
DEFLATE_MIN_MATCH_LEN) + 1];
} g; /* (g)reedy */
/* Data for fastest parsing */
struct {
/* Hash table matchfinder */
struct ht_matchfinder ht_mf;
struct deflate_sequence sequences[
DIV_ROUND_UP(FAST_BLOCK_LENGTH,
HT_MATCHFINDER_MIN_MATCH_LEN) + 1];
} f; /* (f)astest */
#if SUPPORT_NEAR_OPTIMAL_PARSING
/* Data for near-optimal parsing */
struct {
/* Binary tree matchfinder */
struct bt_matchfinder bt_mf;
/*
* 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 list of matches found at that position, if any,
* sorted by strictly increasing length. In addition,
* following the matches for each position, there is a
* special 'struct lz_match' whose 'length' member
* contains the number of matches found at that
* position, and whose 'offset' member contains the
* literal at that position.
*
* 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. MATCH_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[MATCH_CACHE_LENGTH - 1], we write
* MAX_MATCHES_PER_POS matches and a match count header,
* then skip searching for matches at
* 'DEFLATE_MAX_MATCH_LEN - 1' positions and write the
* match count header for each.
*/
struct lz_match match_cache[MATCH_CACHE_LENGTH +
MAX_MATCHES_PER_POS +
DEFLATE_MAX_MATCH_LEN - 1];
/*
* 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 we find a
* match of length DEFLATE_MAX_MATCH_LEN at position
* SOFT_MAX_BLOCK_LENGTH - 1, producing a block of
* length SOFT_MAX_BLOCK_LENGTH - 1 +
* DEFLATE_MAX_MATCH_LEN. Add one for the end-of-block
* node.
*/
struct deflate_optimum_node optimum_nodes[SOFT_MAX_BLOCK_LENGTH - 1 +
DEFLATE_MAX_MATCH_LEN + 1];
/* The current cost model being used. */
struct deflate_costs costs;
/* Literal/match statistics saved from previous block */
u32 prev_observations[NUM_OBSERVATION_TYPES];
u32 prev_num_observations;
/*
* Approximate match length frequencies based on a
* greedy parse, gathered during matchfinding. This is
* used for setting the initial symbol costs.
*/
u32 match_len_freqs[DEFLATE_MAX_MATCH_LEN + 1];
unsigned num_optim_passes;
} n; /* (n)ear-optimal */
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
} p; /* (p)arser */
};
/*
* The type for the bitbuffer variable, which temporarily holds bits that are
* being packed into bytes and written to the output buffer. For best
* performance, this should have size equal to a machine word.
*/
typedef machine_word_t bitbuf_t;
#define BITBUF_NBITS (8 * sizeof(bitbuf_t))
/* Can the specified number of bits always be added to 'bitbuf' after any
* pending bytes have been flushed? */
#define CAN_BUFFER(n) ((n) <= BITBUF_NBITS - 7)
/*
* Structure to keep track of the current state of sending bits to the
* compressed output buffer.
*/
struct deflate_output_bitstream {
/* Bits that haven't yet been written to the output buffer. */
bitbuf_t bitbuf;
/* Number of bits currently held in @bitbuf. */
unsigned bitcount;
/* Pointer to the beginning of the output buffer. */
u8 *begin;
/* Pointer to the position in the output buffer at which the next byte
* should be written. */
u8 *next;
/* Pointer just past the end of the output buffer. */
u8 *end;
};
/*
* OUTPUT_END_PADDING is the size, in bytes, of the extra space that must be
* present following os->end, in order to not overrun the buffer when generating
* output. When UNALIGNED_ACCESS_IS_FAST, we need at least sizeof(bitbuf_t)
* bytes for put_unaligned_leword(). Otherwise we need only 1 byte. However,
* to make the compression algorithm produce the same result on all CPU
* architectures (which is sometimes desirable), we have to unconditionally use
* the maximum for any CPU, which is sizeof(bitbuf_t) == 8.
*/
#define OUTPUT_END_PADDING 8
/* Initialize the output bitstream. 'size' is assumed to be at least
* OUTPUT_END_PADDING. */
static void
deflate_init_output(struct deflate_output_bitstream *os,
void *buffer, size_t size)
{
os->bitbuf = 0;
os->bitcount = 0;
os->begin = buffer;
os->next = os->begin;
os->end = os->begin + size - OUTPUT_END_PADDING;
}
/* Add some bits to the bitbuffer variable of the output bitstream. The caller
* must make sure there is enough room. */
static forceinline void
deflate_add_bits(struct deflate_output_bitstream *os,
const bitbuf_t bits, const unsigned num_bits)
{
os->bitbuf |= bits << os->bitcount;
os->bitcount += num_bits;
}
/* Flush bits from the bitbuffer variable to the output buffer. */
static forceinline void
deflate_flush_bits(struct deflate_output_bitstream *os)
{
if (UNALIGNED_ACCESS_IS_FAST) {
/* Flush a whole word (branchlessly). */
put_unaligned_leword(os->bitbuf, os->next);
os->bitbuf >>= os->bitcount & ~7;
os->next += MIN(os->end - os->next, os->bitcount >> 3);
os->bitcount &= 7;
} else {
/* Flush a byte at a time. */
while (os->bitcount >= 8) {
*os->next = os->bitbuf;
if (os->next != os->end)
os->next++;
os->bitcount -= 8;
os->bitbuf >>= 8;
}
}
}
/* Align the bitstream on a byte boundary. */
static forceinline void
deflate_align_bitstream(struct deflate_output_bitstream *os)
{
os->bitcount += -os->bitcount & 7;
deflate_flush_bits(os);
}
/*
* Flush any remaining bits 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
deflate_flush_output(struct deflate_output_bitstream *os)
{
if (os->next == os->end) /* overflow? */
return 0;
while ((int)os->bitcount > 0) {
*os->next++ = os->bitbuf;
os->bitcount -= 8;
os->bitbuf >>= 8;
}
return os->next - os->begin;
}
/* Given the binary tree node A[subtree_idx] whose children already
* satisfy the maxheap property, swap the node with its greater child
* until it is greater than both its children, so that the maxheap
* property is satisfied in the subtree rooted at A[subtree_idx]. */
static void
heapify_subtree(u32 A[], unsigned length, unsigned subtree_idx)
{
unsigned parent_idx;
unsigned child_idx;
u32 v;
v = A[subtree_idx];
parent_idx = subtree_idx;
while ((child_idx = parent_idx * 2) <= length) {
if (child_idx < length && A[child_idx + 1] > A[child_idx])
child_idx++;
if (v >= A[child_idx])
break;
A[parent_idx] = A[child_idx];
parent_idx = child_idx;
}
A[parent_idx] = v;
}
/* Rearrange the array 'A' so that it satisfies the maxheap property.
* 'A' uses 1-based indices, so the children of A[i] are A[i*2] and A[i*2 + 1].
*/
static void
heapify_array(u32 A[], unsigned length)
{
unsigned subtree_idx;
for (subtree_idx = length / 2; subtree_idx >= 1; subtree_idx--)
heapify_subtree(A, length, subtree_idx);
}
/*
* Sort the array 'A', which contains 'length' unsigned 32-bit integers.
*
* Note: name this function heap_sort() instead of heapsort() to avoid colliding
* with heapsort() from stdlib.h on BSD-derived systems --- though this isn't
* necessary when compiling with -D_ANSI_SOURCE, which is the better solution.
*/
static void
heap_sort(u32 A[], unsigned length)
{
A--; /* Use 1-based indices */
heapify_array(A, length);
while (length >= 2) {
u32 tmp = A[length];
A[length] = A[1];
A[1] = tmp;
length--;
heapify_subtree(A, length, 1);
}
}
#define NUM_SYMBOL_BITS 10
#define SYMBOL_MASK ((1 << NUM_SYMBOL_BITS) - 1)
#define GET_NUM_COUNTERS(num_syms) ((((num_syms) + 3 / 4) + 3) & ~3)
/*
* Sort the symbols primarily by frequency and secondarily by symbol
* value. Discard symbols with zero frequency and fill in an array with
* the remaining symbols, along with their frequencies. The low
* NUM_SYMBOL_BITS bits of each array entry will contain the symbol
* value, and the remaining bits will contain the frequency.
*
* @num_syms
* Number of symbols in the alphabet.
* Can't be greater than (1 << NUM_SYMBOL_BITS).
*
* @freqs[num_syms]
* The frequency of each symbol.
*
* @lens[num_syms]
* An array that eventually will hold the length of each codeword.
* This function only fills in the codeword lengths for symbols that
* have zero frequency, which are not well defined per se but will
* be set to 0.
*
* @symout[num_syms]
* The output array, described above.
*
* Returns the number of entries in 'symout' that were filled. This is
* the number of symbols that have nonzero frequency.
*/
static unsigned
sort_symbols(unsigned num_syms, const u32 freqs[restrict],
u8 lens[restrict], u32 symout[restrict])
{
unsigned sym;
unsigned i;
unsigned num_used_syms;
unsigned num_counters;
unsigned counters[GET_NUM_COUNTERS(DEFLATE_MAX_NUM_SYMS)];
/* We rely on heapsort, but with an added optimization. Since
* it's common for most symbol frequencies to be low, we first do
* a count sort using a limited number of counters. High
* frequencies will be counted in the last counter, and only they
* will be sorted with heapsort.
*
* Note: with more symbols, it is generally beneficial to have more
* counters. About 1 counter per 4 symbols seems fast.
*
* Note: I also tested radix sort, but even for large symbol
* counts (> 255) and frequencies bounded at 16 bits (enabling
* radix sort by just two base-256 digits), it didn't seem any
* faster than the method implemented here.
*
* Note: I tested the optimized quicksort implementation from
* glibc (with indirection overhead removed), but it was only
* marginally faster than the simple heapsort implemented here.
*
* Tests were done with building the codes for LZX. Results may
* vary for different compression algorithms...! */
num_counters = GET_NUM_COUNTERS(num_syms);
memset(counters, 0, num_counters * sizeof(counters[0]));
/* Count the frequencies. */
for (sym = 0; sym < num_syms; sym++)
counters[MIN(freqs[sym], num_counters - 1)]++;
/* Make the counters cumulative, ignoring the zero-th, which
* counted symbols with zero frequency. As a side effect, this
* calculates the number of symbols with nonzero frequency. */
num_used_syms = 0;
for (i = 1; i < num_counters; i++) {
unsigned count = counters[i];
counters[i] = num_used_syms;
num_used_syms += count;
}
/* Sort nonzero-frequency symbols using the counters. At the
* same time, set the codeword lengths of zero-frequency symbols
* to 0. */
for (sym = 0; sym < num_syms; sym++) {
u32 freq = freqs[sym];
if (freq != 0) {
symout[counters[MIN(freq, num_counters - 1)]++] =
sym | (freq << NUM_SYMBOL_BITS);
} else {
lens[sym] = 0;
}
}
/* Sort the symbols counted in the last counter. */
heap_sort(symout + counters[num_counters - 2],
counters[num_counters - 1] - counters[num_counters - 2]);
return num_used_syms;
}
/*
* Build the Huffman tree.
*
* This is an optimized implementation that
* (a) takes advantage of the frequencies being already sorted;
* (b) only generates non-leaf nodes, since the non-leaf nodes of a
* Huffman tree are sufficient to generate a canonical code;
* (c) Only stores parent pointers, not child pointers;
* (d) Produces the nodes in the same memory used for input
* frequency information.
*
* Array 'A', which contains 'sym_count' entries, is used for both input
* and output. For this function, 'sym_count' must be at least 2.
*
* For input, the array must contain the frequencies of the symbols,
* sorted in increasing order. Specifically, each entry must contain a
* frequency left shifted by NUM_SYMBOL_BITS bits. Any data in the low
* NUM_SYMBOL_BITS bits of the entries will be ignored by this function.
* Although these bits will, in fact, contain the symbols that correspond
* to the frequencies, this function is concerned with frequencies only
* and keeps the symbols as-is.
*
* For output, this function will produce the non-leaf nodes of the
* Huffman tree. These nodes will be stored in the first (sym_count - 1)
* entries of the array. Entry A[sym_count - 2] will represent the root
* node. Each other node will contain the zero-based index of its parent
* node in 'A', left shifted by NUM_SYMBOL_BITS bits. The low
* NUM_SYMBOL_BITS bits of each entry in A will be kept as-is. Again,
* note that although these low bits will, in fact, contain a symbol
* value, this symbol will have *no relationship* with the Huffman tree
* node that happens to occupy the same slot. This is because this
* implementation only generates the non-leaf nodes of the tree.
*/
static void
build_tree(u32 A[], unsigned sym_count)
{
/* Index, in 'A', of next lowest frequency symbol that has not
* yet been processed. */
unsigned i = 0;
/* Index, in 'A', of next lowest frequency parentless non-leaf
* node; or, if equal to 'e', then no such node exists yet. */
unsigned b = 0;
/* Index, in 'A', of next node to allocate as a non-leaf. */
unsigned e = 0;
do {
unsigned m, n;
u32 freq_shifted;
/* Choose the two next lowest frequency entries. */
if (i != sym_count &&
(b == e || (A[i] >> NUM_SYMBOL_BITS) <= (A[b] >> NUM_SYMBOL_BITS)))
m = i++;
else
m = b++;
if (i != sym_count &&
(b == e || (A[i] >> NUM_SYMBOL_BITS) <= (A[b] >> NUM_SYMBOL_BITS)))
n = i++;
else
n = b++;
/* Allocate a non-leaf node and link the entries to it.
*
* If we link an entry that we're visiting for the first
* time (via index 'i'), then we're actually linking a
* leaf node and it will have no effect, since the leaf
* will be overwritten with a non-leaf when index 'e'
* catches up to it. But it's not any slower to
* unconditionally set the parent index.
*
* We also compute the frequency of the non-leaf node as
* the sum of its two children's frequencies. */
freq_shifted = (A[m] & ~SYMBOL_MASK) + (A[n] & ~SYMBOL_MASK);
A[m] = (A[m] & SYMBOL_MASK) | (e << NUM_SYMBOL_BITS);
A[n] = (A[n] & SYMBOL_MASK) | (e << NUM_SYMBOL_BITS);
A[e] = (A[e] & SYMBOL_MASK) | freq_shifted;
e++;
} while (sym_count - e > 1);
/* When just one entry remains, it is a "leaf" that was
* linked to some other node. We ignore it, since the
* rest of the array contains the non-leaves which we
* need. (Note that we're assuming the cases with 0 or 1
* symbols were handled separately.) */
}
/*
* Given the stripped-down Huffman tree constructed by build_tree(),
* determine the number of codewords that should be assigned each
* possible length, taking into account the length-limited constraint.
*
* @A
* The array produced by build_tree(), containing parent index
* information for the non-leaf nodes of the Huffman tree. Each
* entry in this array is a node; a node's parent always has a
* greater index than that node itself. This function will
* overwrite the parent index information in this array, so
* essentially it will destroy the tree. However, the data in the
* low NUM_SYMBOL_BITS of each entry will be preserved.
*
* @root_idx
* The 0-based index of the root node in 'A', and consequently one
* less than the number of tree node entries in 'A'. (Or, really 2
* less than the actual length of 'A'.)
*
* @len_counts
* An array of length ('max_codeword_len' + 1) in which the number of
* codewords having each length <= max_codeword_len will be
* returned.
*
* @max_codeword_len
* The maximum permissible codeword length.
*/
static void
compute_length_counts(u32 A[restrict], unsigned root_idx,
unsigned len_counts[restrict], unsigned max_codeword_len)
{
unsigned len;
int node;
/* The key observations are:
*
* (1) We can traverse the non-leaf nodes of the tree, always
* visiting a parent before its children, by simply iterating
* through the array in reverse order. Consequently, we can
* compute the depth of each node in one pass, overwriting the
* parent indices with depths.
*
* (2) We can initially assume that in the real Huffman tree,
* both children of the root are leaves. This corresponds to two
* codewords of length 1. Then, whenever we visit a (non-leaf)
* node during the traversal, we modify this assumption to
* account for the current node *not* being a leaf, but rather
* its two children being leaves. This causes the loss of one
* codeword for the current depth and the addition of two
* codewords for the current depth plus one.
*
* (3) We can handle the length-limited constraint fairly easily
* by simply using the largest length available when a depth
* exceeds max_codeword_len.
*/
for (len = 0; len <= max_codeword_len; len++)
len_counts[len] = 0;
len_counts[1] = 2;
/* Set the root node's depth to 0. */
A[root_idx] &= SYMBOL_MASK;
for (node = root_idx - 1; node >= 0; node--) {
/* Calculate the depth of this node. */
unsigned parent = A[node] >> NUM_SYMBOL_BITS;
unsigned parent_depth = A[parent] >> NUM_SYMBOL_BITS;
unsigned depth = parent_depth + 1;
unsigned len = depth;
/* Set the depth of this node so that it is available
* when its children (if any) are processed. */
A[node] = (A[node] & SYMBOL_MASK) | (depth << NUM_SYMBOL_BITS);
/* If needed, decrease the length to meet the
* length-limited constraint. This is not the optimal
* method for generating length-limited Huffman codes!
* But it should be good enough. */
if (len >= max_codeword_len) {
len = max_codeword_len;
do {
len--;
} while (len_counts[len] == 0);
}
/* Account for the fact that we have a non-leaf node at
* the current depth. */
len_counts[len]--;
len_counts[len + 1] += 2;
}
}
/*
* Generate the codewords for a canonical Huffman code.
*
* @A
* The output array for codewords. In addition, initially this
* array must contain the symbols, sorted primarily by frequency and
* secondarily by symbol value, in the low NUM_SYMBOL_BITS bits of
* each entry.
*
* @len
* Output array for codeword lengths.
*
* @len_counts
* An array that provides the number of codewords that will have
* each possible length <= max_codeword_len.
*
* @max_codeword_len
* Maximum length, in bits, of each codeword.
*
* @num_syms
* Number of symbols in the alphabet, including symbols with zero
* frequency. This is the length of the 'A' and 'len' arrays.
*/
static void
gen_codewords(u32 A[restrict], u8 lens[restrict],
const unsigned len_counts[restrict],
unsigned max_codeword_len, unsigned num_syms)
{
u32 next_codewords[DEFLATE_MAX_CODEWORD_LEN + 1];
unsigned i;
unsigned len;
unsigned sym;
/* Given the number of codewords that will have each length,
* assign codeword lengths to symbols. We do this by assigning
* the lengths in decreasing order to the symbols sorted
* primarily by increasing frequency and secondarily by
* increasing symbol value. */
for (i = 0, len = max_codeword_len; len >= 1; len--) {
unsigned count = len_counts[len];
while (count--)
lens[A[i++] & SYMBOL_MASK] = len;
}
/* Generate the codewords themselves. We initialize the
* 'next_codewords' array to provide the lexicographically first
* codeword of each length, then assign codewords in symbol
* order. This produces a canonical code. */
next_codewords[0] = 0;
next_codewords[1] = 0;
for (len = 2; len <= max_codeword_len; len++)
next_codewords[len] =
(next_codewords[len - 1] + len_counts[len - 1]) << 1;
for (sym = 0; sym < num_syms; sym++)
A[sym] = next_codewords[lens[sym]]++;
}
/*
* ---------------------------------------------------------------------
* make_canonical_huffman_code()
* ---------------------------------------------------------------------
*
* Given an alphabet and the frequency of each symbol in it, construct a
* length-limited canonical Huffman code.
*
* @num_syms
* The number of symbols in the alphabet. The symbols are the
* integers in the range [0, num_syms - 1]. This parameter must be
* at least 2 and can't be greater than (1 << NUM_SYMBOL_BITS).
*
* @max_codeword_len
* The maximum permissible codeword length.
*
* @freqs
* An array of @num_syms entries, each of which specifies the
* frequency of the corresponding symbol. It is valid for some,
* none, or all of the frequencies to be 0.
*
* @lens
* An array of @num_syms entries in which this function will return
* the length, in bits, of the codeword assigned to each symbol.
* Symbols with 0 frequency will not have codewords per se, but
* their entries in this array will be set to 0. No lengths greater
* than @max_codeword_len will be assigned.
*
* @codewords
* An array of @num_syms entries in which this function will return
* the codeword for each symbol, right-justified and padded on the
* left with zeroes. Codewords for symbols with 0 frequency will be
* undefined.
*
* ---------------------------------------------------------------------
*
* This function builds a length-limited canonical Huffman code.
*
* A length-limited Huffman code contains no codewords longer than some
* specified length, and has exactly (with some algorithms) or
* approximately (with the algorithm used here) the minimum weighted path
* length from the root, given this constraint.
*
* A canonical Huffman code satisfies the properties that a longer
* codeword never lexicographically precedes a shorter codeword, and the
* lexicographic ordering of codewords of the same length is the same as
* the lexicographic ordering of the corresponding symbols. A canonical
* Huffman code, or more generally a canonical prefix code, can be
* reconstructed from only a list containing the codeword length of each
* symbol.
*
* The classic algorithm to generate a Huffman code creates a node for
* each symbol, then inserts these nodes into a min-heap keyed by symbol
* frequency. Then, repeatedly, the two lowest-frequency nodes are
* removed from the min-heap and added as the children of a new node
* having frequency equal to the sum of its two children, which is then
* inserted into the min-heap. When only a single node remains in the
* min-heap, it is the root of the Huffman tree. The codeword for each
* symbol is determined by the path needed to reach the corresponding
* node from the root. Descending to the left child appends a 0 bit,
* whereas descending to the right child appends a 1 bit.
*
* The classic algorithm is relatively easy to understand, but it is
* subject to a number of inefficiencies. In practice, it is fastest to
* first sort the symbols by frequency. (This itself can be subject to
* an optimization based on the fact that most frequencies tend to be
* low.) At the same time, we sort secondarily by symbol value, which
* aids the process of generating a canonical code. Then, during tree
* construction, no heap is necessary because both the leaf nodes and the
* unparented non-leaf nodes can be easily maintained in sorted order.
* Consequently, there can never be more than two possibilities for the
* next-lowest-frequency node.
*
* In addition, because we're generating a canonical code, we actually
* don't need the leaf nodes of the tree at all, only the non-leaf nodes.
* This is because for canonical code generation we don't need to know
* where the symbols are in the tree. Rather, we only need to know how
* many leaf nodes have each depth (codeword length). And this
* information can, in fact, be quickly generated from the tree of
* non-leaves only.
*
* Furthermore, we can build this stripped-down Huffman tree directly in
* the array in which the codewords are to be generated, provided that
* these array slots are large enough to hold a symbol and frequency
* value.
*
* Still furthermore, we don't even need to maintain explicit child
* pointers. We only need the parent pointers, and even those can be
* overwritten in-place with depth information as part of the process of
* extracting codeword lengths from the tree. So in summary, we do NOT
* need a big structure like:
*
* struct huffman_tree_node {
* unsigned int symbol;
* unsigned int frequency;
* unsigned int depth;
* struct huffman_tree_node *left_child;
* struct huffman_tree_node *right_child;
* };
*
*
* ... which often gets used in "naive" implementations of Huffman code
* generation.
*
* Many of these optimizations are based on the implementation in 7-Zip
* (source file: C/HuffEnc.c), which has been placed in the public domain
* by Igor Pavlov.
*/
static void
make_canonical_huffman_code(unsigned num_syms, unsigned max_codeword_len,
const u32 freqs[restrict],
u8 lens[restrict], u32 codewords[restrict])
{
u32 *A = codewords;
unsigned num_used_syms;
STATIC_ASSERT(DEFLATE_MAX_NUM_SYMS <= 1 << NUM_SYMBOL_BITS);
/* We begin by sorting the symbols primarily by frequency and
* secondarily by symbol value. As an optimization, the array
* used for this purpose ('A') shares storage with the space in
* which we will eventually return the codewords. */
num_used_syms = sort_symbols(num_syms, freqs, lens, A);
/* 'num_used_syms' is the number of symbols with nonzero
* frequency. This may be less than @num_syms. 'num_used_syms'
* is also the number of entries in 'A' that are valid. Each
* entry consists of a distinct symbol and a nonzero frequency
* packed into a 32-bit integer. */
/* Handle special cases where only 0 or 1 symbols were used (had
* nonzero frequency). */
if (unlikely(num_used_syms == 0)) {
/* Code is empty. sort_symbols() already set all lengths
* to 0, so there is nothing more to do. */
return;
}
if (unlikely(num_used_syms == 1)) {
/* Only one symbol was used, so we only need one
* codeword. But two codewords are needed to form the
* smallest complete Huffman code, which uses codewords 0
* and 1. Therefore, we choose another symbol to which
* to assign a codeword. We use 0 (if the used symbol is
* not 0) or 1 (if the used symbol is 0). In either
* case, the lesser-valued symbol must be assigned
* codeword 0 so that the resulting code is canonical. */
unsigned sym = A[0] & SYMBOL_MASK;
unsigned nonzero_idx = sym ? sym : 1;
codewords[0] = 0;
lens[0] = 1;
codewords[nonzero_idx] = 1;
lens[nonzero_idx] = 1;
return;
}
/* Build a stripped-down version of the Huffman tree, sharing the
* array 'A' with the symbol values. Then extract length counts
* from the tree and use them to generate the final codewords. */
build_tree(A, num_used_syms);
{
unsigned len_counts[DEFLATE_MAX_CODEWORD_LEN + 1];
compute_length_counts(A, num_used_syms - 2,
len_counts, max_codeword_len);
gen_codewords(A, lens, len_counts, max_codeword_len, num_syms);
}
}
/*
* Clear the Huffman symbol frequency counters.
* This must be called when starting a new DEFLATE block.
*/
static void
deflate_reset_symbol_frequencies(struct libdeflate_compressor *c)
{
memset(&c->freqs, 0, sizeof(c->freqs));
}
/* Reverse the Huffman codeword 'codeword', which is 'len' bits in length. */
static u32
deflate_reverse_codeword(u32 codeword, u8 len)
{
/* The following branchless algorithm is faster than going bit by bit.
* Note: since no codewords are longer than 16 bits, we only need to
* reverse the low 16 bits of the 'u32'. */
STATIC_ASSERT(DEFLATE_MAX_CODEWORD_LEN <= 16);
/* Flip adjacent 1-bit fields */
codeword = ((codeword & 0x5555) << 1) | ((codeword & 0xAAAA) >> 1);
/* Flip adjacent 2-bit fields */
codeword = ((codeword & 0x3333) << 2) | ((codeword & 0xCCCC) >> 2);
/* Flip adjacent 4-bit fields */
codeword = ((codeword & 0x0F0F) << 4) | ((codeword & 0xF0F0) >> 4);
/* Flip adjacent 8-bit fields */
codeword = ((codeword & 0x00FF) << 8) | ((codeword & 0xFF00) >> 8);
/* Return the high 'len' bits of the bit-reversed 16 bit value. */
return codeword >> (16 - len);
}
/* Make a canonical Huffman code with bit-reversed codewords. */
static void
deflate_make_huffman_code(unsigned num_syms, unsigned max_codeword_len,
const u32 freqs[], u8 lens[], u32 codewords[])
{
unsigned sym;
make_canonical_huffman_code(num_syms, max_codeword_len,
freqs, lens, codewords);
for (sym = 0; sym < num_syms; sym++)
codewords[sym] = deflate_reverse_codeword(codewords[sym], lens[sym]);
}
/*
* Build the literal/length and offset Huffman codes for a DEFLATE block.
*
* 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
deflate_make_huffman_codes(const struct deflate_freqs *freqs,
struct deflate_codes *codes)
{
STATIC_ASSERT(MAX_LITLEN_CODEWORD_LEN <= DEFLATE_MAX_LITLEN_CODEWORD_LEN);
STATIC_ASSERT(MAX_OFFSET_CODEWORD_LEN <= DEFLATE_MAX_OFFSET_CODEWORD_LEN);
deflate_make_huffman_code(DEFLATE_NUM_LITLEN_SYMS,
MAX_LITLEN_CODEWORD_LEN,
freqs->litlen,
codes->lens.litlen,
codes->codewords.litlen);
deflate_make_huffman_code(DEFLATE_NUM_OFFSET_SYMS,
MAX_OFFSET_CODEWORD_LEN,
freqs->offset,
codes->lens.offset,
codes->codewords.offset);
}
/* Initialize c->static_codes. */
static void
deflate_init_static_codes(struct libdeflate_compressor *c)
{
unsigned i;
for (i = 0; i < 144; i++)
c->freqs.litlen[i] = 1 << (9 - 8);
for (; i < 256; i++)
c->freqs.litlen[i] = 1 << (9 - 9);
for (; i < 280; i++)
c->freqs.litlen[i] = 1 << (9 - 7);
for (; i < 288; i++)
c->freqs.litlen[i] = 1 << (9 - 8);
for (i = 0; i < 32; i++)
c->freqs.offset[i] = 1 << (5 - 5);
deflate_make_huffman_codes(&c->freqs, &c->static_codes);
}
/* Return the offset slot for the specified match offset. */
static forceinline unsigned
deflate_get_offset_slot(struct libdeflate_compressor *c, unsigned offset)
{
#if USE_FULL_OFFSET_SLOT_FAST
return c->offset_slot_fast[offset];
#else
if (offset <= 256)
return c->offset_slot_fast[offset - 1];
else
return c->offset_slot_fast[256 + ((offset - 1) >> 7)];
#endif
}
/* Write the header fields common to all DEFLATE block types. */
static void
deflate_write_block_header(struct deflate_output_bitstream *os,
bool is_final_block, unsigned block_type)
{
deflate_add_bits(os, is_final_block, 1);
deflate_add_bits(os, block_type, 2);
deflate_flush_bits(os);
}
static unsigned
deflate_compute_precode_items(const u8 lens[restrict],
const unsigned num_lens,
u32 precode_freqs[restrict],
unsigned precode_items[restrict])
{
unsigned *itemptr;
unsigned run_start;
unsigned run_end;
unsigned extra_bits;
u8 len;
memset(precode_freqs, 0,
DEFLATE_NUM_PRECODE_SYMS * sizeof(precode_freqs[0]));
itemptr = precode_items;
run_start = 0;
do {
/* Find the next run of codeword lengths. */
/* len = the length being repeated */
len = lens[run_start];
/* Extend the run. */
run_end = run_start;
do {
run_end++;
} while (run_end != num_lens && len == lens[run_end]);
if (len == 0) {
/* Run of zeroes. */
/* Symbol 18: RLE 11 to 138 zeroes at a time. */
while ((run_end - run_start) >= 11) {
extra_bits = MIN((run_end - run_start) - 11, 0x7F);
precode_freqs[18]++;
*itemptr++ = 18 | (extra_bits << 5);
run_start += 11 + extra_bits;
}
/* Symbol 17: RLE 3 to 10 zeroes at a time. */
if ((run_end - run_start) >= 3) {
extra_bits = MIN((run_end - run_start) - 3, 0x7);
precode_freqs[17]++;
*itemptr++ = 17 | (extra_bits << 5);
run_start += 3 + extra_bits;
}
} else {
/* A run of nonzero lengths. */
/* Symbol 16: RLE 3 to 6 of the previous length. */
if ((run_end - run_start) >= 4) {
precode_freqs[len]++;
*itemptr++ = len;
run_start++;
do {
extra_bits = MIN((run_end - run_start) - 3, 0x3);
precode_freqs[16]++;
*itemptr++ = 16 | (extra_bits << 5);
run_start += 3 + extra_bits;
} while ((run_end - run_start) >= 3);
}
}
/* Output any remaining lengths without RLE. */
while (run_start != run_end) {
precode_freqs[len]++;
*itemptr++ = len;
run_start++;
}
} while (run_start != num_lens);
return itemptr - precode_items;
}
/*
* Huffman codeword lengths for dynamic Huffman blocks are 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 19 3-bit fields that
* immediately precede the compressed codeword lengths of the larger code.
*/
/* Precompute the information needed to output Huffman codes. */
static void
deflate_precompute_huffman_header(struct libdeflate_compressor *c)
{
/* Compute how many litlen and offset symbols are needed. */
for (c->num_litlen_syms = DEFLATE_NUM_LITLEN_SYMS;
c->num_litlen_syms > 257;
c->num_litlen_syms--)
if (c->codes.lens.litlen[c->num_litlen_syms - 1] != 0)
break;
for (c->num_offset_syms = DEFLATE_NUM_OFFSET_SYMS;
c->num_offset_syms > 1;
c->num_offset_syms--)
if (c->codes.lens.offset[c->num_offset_syms - 1] != 0)
break;
/* If we're not using the full set of literal/length codeword lengths,
* then temporarily move the offset codeword lengths over so that the
* literal/length and offset codeword lengths are contiguous. */
STATIC_ASSERT(offsetof(struct deflate_lens, offset) ==
DEFLATE_NUM_LITLEN_SYMS);
if (c->num_litlen_syms != DEFLATE_NUM_LITLEN_SYMS) {
memmove((u8 *)&c->codes.lens + c->num_litlen_syms,
(u8 *)&c->codes.lens + DEFLATE_NUM_LITLEN_SYMS,
c->num_offset_syms);
}
/* Compute the "items" (RLE / literal tokens and extra bits) with which
* the codeword lengths in the larger code will be output. */
c->num_precode_items =
deflate_compute_precode_items((u8 *)&c->codes.lens,
c->num_litlen_syms +
c->num_offset_syms,
c->precode_freqs,
c->precode_items);
/* Build the precode. */
STATIC_ASSERT(MAX_PRE_CODEWORD_LEN <= DEFLATE_MAX_PRE_CODEWORD_LEN);
deflate_make_huffman_code(DEFLATE_NUM_PRECODE_SYMS,
MAX_PRE_CODEWORD_LEN,
c->precode_freqs, c->precode_lens,
c->precode_codewords);
/* Count how many precode lengths we actually need to output. */
for (c->num_explicit_lens = DEFLATE_NUM_PRECODE_SYMS;
c->num_explicit_lens > 4;
c->num_explicit_lens--)
if (c->precode_lens[deflate_precode_lens_permutation[
c->num_explicit_lens - 1]] != 0)
break;
/* Restore the offset codeword lengths if needed. */
if (c->num_litlen_syms != DEFLATE_NUM_LITLEN_SYMS) {
memmove((u8 *)&c->codes.lens + DEFLATE_NUM_LITLEN_SYMS,
(u8 *)&c->codes.lens + c->num_litlen_syms,
c->num_offset_syms);
}
}
/* Output the Huffman codes. */
static void
deflate_write_huffman_header(struct libdeflate_compressor *c,
struct deflate_output_bitstream *os)
{
unsigned i;
deflate_add_bits(os, c->num_litlen_syms - 257, 5);
deflate_add_bits(os, c->num_offset_syms - 1, 5);
deflate_add_bits(os, c->num_explicit_lens - 4, 4);
deflate_flush_bits(os);
/* Output the lengths of the codewords in the precode. */
for (i = 0; i < c->num_explicit_lens; i++) {
deflate_add_bits(os, c->precode_lens[
deflate_precode_lens_permutation[i]], 3);
deflate_flush_bits(os);
}
/* Output the encoded lengths of the codewords in the larger code. */
for (i = 0; i < c->num_precode_items; i++) {
unsigned precode_item = c->precode_items[i];
unsigned precode_sym = precode_item & 0x1F;
deflate_add_bits(os, c->precode_codewords[precode_sym],
c->precode_lens[precode_sym]);
if (precode_sym >= 16) {
if (precode_sym == 16)
deflate_add_bits(os, precode_item >> 5, 2);
else if (precode_sym == 17)
deflate_add_bits(os, precode_item >> 5, 3);
else
deflate_add_bits(os, precode_item >> 5, 7);
}
STATIC_ASSERT(CAN_BUFFER(MAX_PRE_CODEWORD_LEN + 7));
deflate_flush_bits(os);
}
}
static void
deflate_write_sequences(struct deflate_output_bitstream * restrict os,
const struct deflate_codes * restrict codes,
const struct deflate_sequence sequences[restrict],
const u8 * restrict in_next)
{
const struct deflate_sequence *seq = sequences;
for (;;) {
u32 litrunlen = seq->litrunlen_and_length & 0x7FFFFF;
unsigned length = seq->litrunlen_and_length >> 23;
unsigned length_slot;
unsigned litlen_symbol;
unsigned offset_symbol;
if (litrunlen) {
#if 1
while (litrunlen >= 4) {
unsigned lit0 = in_next[0];
unsigned lit1 = in_next[1];
unsigned lit2 = in_next[2];
unsigned lit3 = in_next[3];
deflate_add_bits(os, codes->codewords.litlen[lit0],
codes->lens.litlen[lit0]);
if (!CAN_BUFFER(2 * MAX_LITLEN_CODEWORD_LEN))
deflate_flush_bits(os);
deflate_add_bits(os, codes->codewords.litlen[lit1],
codes->lens.litlen[lit1]);
if (!CAN_BUFFER(4 * MAX_LITLEN_CODEWORD_LEN))
deflate_flush_bits(os);
deflate_add_bits(os, codes->codewords.litlen[lit2],
codes->lens.litlen[lit2]);
if (!CAN_BUFFER(2 * MAX_LITLEN_CODEWORD_LEN))
deflate_flush_bits(os);
deflate_add_bits(os, codes->codewords.litlen[lit3],
codes->lens.litlen[lit3]);
deflate_flush_bits(os);
in_next += 4;
litrunlen -= 4;
}
if (litrunlen-- != 0) {
deflate_add_bits(os, codes->codewords.litlen[*in_next],
codes->lens.litlen[*in_next]);
if (!CAN_BUFFER(3 * MAX_LITLEN_CODEWORD_LEN))
deflate_flush_bits(os);
in_next++;
if (litrunlen-- != 0) {
deflate_add_bits(os, codes->codewords.litlen[*in_next],
codes->lens.litlen[*in_next]);
if (!CAN_BUFFER(3 * MAX_LITLEN_CODEWORD_LEN))
deflate_flush_bits(os);
in_next++;
if (litrunlen-- != 0) {
deflate_add_bits(os, codes->codewords.litlen[*in_next],
codes->lens.litlen[*in_next]);
if (!CAN_BUFFER(3 * MAX_LITLEN_CODEWORD_LEN))
deflate_flush_bits(os);
in_next++;
}
}
if (CAN_BUFFER(3 * MAX_LITLEN_CODEWORD_LEN))
deflate_flush_bits(os);
}
#else
do {
unsigned lit = *in_next++;
deflate_add_bits(os, codes->codewords.litlen[lit],
codes->lens.litlen[lit]);
deflate_flush_bits(os);
} while (--litrunlen);
#endif
}
if (length == 0)
return;
in_next += length;
length_slot = seq->length_slot;
litlen_symbol = DEFLATE_FIRST_LEN_SYM + length_slot;
/* Litlen symbol */
deflate_add_bits(os, codes->codewords.litlen[litlen_symbol],
codes->lens.litlen[litlen_symbol]);
/* Extra length bits */
STATIC_ASSERT(CAN_BUFFER(MAX_LITLEN_CODEWORD_LEN +
DEFLATE_MAX_EXTRA_LENGTH_BITS));
deflate_add_bits(os, length - deflate_length_slot_base[length_slot],
deflate_extra_length_bits[length_slot]);
if (!CAN_BUFFER(MAX_LITLEN_CODEWORD_LEN +
DEFLATE_MAX_EXTRA_LENGTH_BITS +
MAX_OFFSET_CODEWORD_LEN +
DEFLATE_MAX_EXTRA_OFFSET_BITS))
deflate_flush_bits(os);
/* Offset symbol */
offset_symbol = seq->offset_symbol;
deflate_add_bits(os, codes->codewords.offset[offset_symbol],
codes->lens.offset[offset_symbol]);
if (!CAN_BUFFER(MAX_OFFSET_CODEWORD_LEN +
DEFLATE_MAX_EXTRA_OFFSET_BITS))
deflate_flush_bits(os);
/* Extra offset bits */
deflate_add_bits(os, seq->offset - deflate_offset_slot_base[offset_symbol],
deflate_extra_offset_bits[offset_symbol]);
deflate_flush_bits(os);
seq++;
}
}
#if SUPPORT_NEAR_OPTIMAL_PARSING
/*
* Follow the minimum-cost path in the graph of possible match/literal choices
* for the current block and write out the matches/literals using the specified
* Huffman codes.
*
* Note: this is slightly duplicated with deflate_write_sequences(), the reason
* being that we don't want to waste time translating between intermediate
* match/literal representations.
*/
static void
deflate_write_item_list(struct deflate_output_bitstream *os,
const struct deflate_codes *codes,
struct libdeflate_compressor *c,
u32 block_length)
{
struct deflate_optimum_node *cur_node = &c->p.n.optimum_nodes[0];
struct deflate_optimum_node * const end_node = &c->p.n.optimum_nodes[block_length];
do {
unsigned length = cur_node->item & OPTIMUM_LEN_MASK;
unsigned offset = cur_node->item >> OPTIMUM_OFFSET_SHIFT;
unsigned litlen_symbol;
unsigned length_slot;
unsigned offset_slot;
if (length == 1) {
/* Literal */
litlen_symbol = offset;
deflate_add_bits(os, codes->codewords.litlen[litlen_symbol],
codes->lens.litlen[litlen_symbol]);
deflate_flush_bits(os);
} else {
/* Match length */
length_slot = deflate_length_slot[length];
litlen_symbol = DEFLATE_FIRST_LEN_SYM + length_slot;
deflate_add_bits(os, codes->codewords.litlen[litlen_symbol],
codes->lens.litlen[litlen_symbol]);
deflate_add_bits(os, length - deflate_length_slot_base[length_slot],
deflate_extra_length_bits[length_slot]);
if (!CAN_BUFFER(MAX_LITLEN_CODEWORD_LEN +
DEFLATE_MAX_EXTRA_LENGTH_BITS +
MAX_OFFSET_CODEWORD_LEN +
DEFLATE_MAX_EXTRA_OFFSET_BITS))
deflate_flush_bits(os);
/* Match offset */
offset_slot = deflate_get_offset_slot(c, offset);
deflate_add_bits(os, codes->codewords.offset[offset_slot],
codes->lens.offset[offset_slot]);
if (!CAN_BUFFER(MAX_OFFSET_CODEWORD_LEN +
DEFLATE_MAX_EXTRA_OFFSET_BITS))
deflate_flush_bits(os);
deflate_add_bits(os, offset - deflate_offset_slot_base[offset_slot],
deflate_extra_offset_bits[offset_slot]);
deflate_flush_bits(os);
}
cur_node += length;
} while (cur_node != end_node);
}
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
/* Output the end-of-block symbol. */
static void
deflate_write_end_of_block(struct deflate_output_bitstream *os,
const struct deflate_codes *codes)
{
deflate_add_bits(os, codes->codewords.litlen[DEFLATE_END_OF_BLOCK],
codes->lens.litlen[DEFLATE_END_OF_BLOCK]);
deflate_flush_bits(os);
}
static void
deflate_write_uncompressed_block(struct deflate_output_bitstream *os,
const u8 *data, u16 len,
bool is_final_block)
{
deflate_write_block_header(os, is_final_block,
DEFLATE_BLOCKTYPE_UNCOMPRESSED);
deflate_align_bitstream(os);
if (4 + (u32)len >= os->end - os->next) {
os->next = os->end;
return;
}
put_unaligned_le16(len, os->next);
os->next += 2;
put_unaligned_le16(~len, os->next);
os->next += 2;
memcpy(os->next, data, len);
os->next += len;
}
static void
deflate_write_uncompressed_blocks(struct deflate_output_bitstream *os,
const u8 *data, size_t data_length,
bool is_final_block)
{
do {
u16 len = MIN(data_length, UINT16_MAX);
deflate_write_uncompressed_block(os, data, len,
is_final_block && len == data_length);
data += len;
data_length -= len;
} while (data_length != 0);
}
/*
* Choose the best type of block to use (dynamic Huffman, static Huffman, or
* uncompressed), then output it.
*/
static void
deflate_flush_block(struct libdeflate_compressor * restrict c,
struct deflate_output_bitstream * restrict os,
const u8 * restrict block_begin, u32 block_length,
const struct deflate_sequence *sequences,
bool is_final_block)
{
static const u8 deflate_extra_precode_bits[DEFLATE_NUM_PRECODE_SYMS] = {
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 3, 7,
};
/* Costs are measured in bits */
u32 dynamic_cost = 0;
u32 static_cost = 0;
u32 uncompressed_cost = 0;
struct deflate_codes *codes;
int block_type;
unsigned sym;
if (sequences != NULL /* !near_optimal */ ||
!SUPPORT_NEAR_OPTIMAL_PARSING) {
/* Tally the end-of-block symbol. */
c->freqs.litlen[DEFLATE_END_OF_BLOCK]++;
/* Build dynamic Huffman codes. */
deflate_make_huffman_codes(&c->freqs, &c->codes);
} /* Else, this was already done */
/* Account for the cost of sending dynamic Huffman codes. */
deflate_precompute_huffman_header(c);
dynamic_cost += 5 + 5 + 4 + (3 * c->num_explicit_lens);
for (sym = 0; sym < DEFLATE_NUM_PRECODE_SYMS; sym++) {
u32 extra = deflate_extra_precode_bits[sym];
dynamic_cost += c->precode_freqs[sym] *
(extra + c->precode_lens[sym]);
}
/* Account for the cost of encoding literals. */
for (sym = 0; sym < 256; sym++) {
dynamic_cost += c->freqs.litlen[sym] *
c->codes.lens.litlen[sym];
}
for (sym = 0; sym < 144; sym++)
static_cost += c->freqs.litlen[sym] * 8;
for (; sym < 256; sym++)
static_cost += c->freqs.litlen[sym] * 9;
/* Account for the cost of encoding the end-of-block symbol. */
dynamic_cost += c->codes.lens.litlen[DEFLATE_END_OF_BLOCK];
static_cost += 7;
/* Account for the cost of encoding lengths. */
for (sym = DEFLATE_FIRST_LEN_SYM;
sym < DEFLATE_FIRST_LEN_SYM + ARRAY_LEN(deflate_extra_length_bits);
sym++) {
u32 extra = deflate_extra_length_bits[
sym - DEFLATE_FIRST_LEN_SYM];
dynamic_cost += c->freqs.litlen[sym] *
(extra + c->codes.lens.litlen[sym]);
static_cost += c->freqs.litlen[sym] *
(extra + c->static_codes.lens.litlen[sym]);
}
/* Account for the cost of encoding offsets. */
for (sym = 0; sym < ARRAY_LEN(deflate_extra_offset_bits); sym++) {
u32 extra = deflate_extra_offset_bits[sym];
dynamic_cost += c->freqs.offset[sym] *
(extra + c->codes.lens.offset[sym]);
static_cost += c->freqs.offset[sym] * (extra + 5);
}
/* Compute the cost of using uncompressed blocks. */
uncompressed_cost += (-(os->bitcount + 3) & 7) + 32 +
(40 * (DIV_ROUND_UP(block_length,
UINT16_MAX) - 1)) +
(8 * block_length);
/* Choose the cheapest block type. */
if (dynamic_cost < MIN(static_cost, uncompressed_cost)) {
block_type = DEFLATE_BLOCKTYPE_DYNAMIC_HUFFMAN;
codes = &c->codes;
} else if (static_cost < uncompressed_cost) {
block_type = DEFLATE_BLOCKTYPE_STATIC_HUFFMAN;
codes = &c->static_codes;
} else {
block_type = DEFLATE_BLOCKTYPE_UNCOMPRESSED;
}
/* Now actually output the block. */
if (block_type == DEFLATE_BLOCKTYPE_UNCOMPRESSED) {
/* Note: the length being flushed may exceed the maximum length
* of an uncompressed block (65535 bytes). Therefore, more than
* one uncompressed block might be needed. */
deflate_write_uncompressed_blocks(os, block_begin, block_length,
is_final_block);
} else {
/* Output the block header. */
deflate_write_block_header(os, is_final_block, block_type);
/* Output the Huffman codes (dynamic Huffman blocks only). */
if (block_type == DEFLATE_BLOCKTYPE_DYNAMIC_HUFFMAN)
deflate_write_huffman_header(c, os);
/* Output the literals, matches, and end-of-block symbol. */
#if SUPPORT_NEAR_OPTIMAL_PARSING
if (sequences == NULL)
deflate_write_item_list(os, codes, c, block_length);
else
#endif
deflate_write_sequences(os, codes, sequences,
block_begin);
deflate_write_end_of_block(os, codes);
}
}
static void
deflate_begin_sequences(struct libdeflate_compressor *c,
struct deflate_sequence *first_seq)
{
deflate_reset_symbol_frequencies(c);
first_seq->litrunlen_and_length = 0;
}
static forceinline void
deflate_choose_literal(struct libdeflate_compressor *c, unsigned literal,
struct deflate_sequence *seq)
{
c->freqs.litlen[literal]++;
seq->litrunlen_and_length++;
}
static forceinline void
deflate_choose_match(struct libdeflate_compressor *c,
unsigned length, unsigned offset,
struct deflate_sequence **seq_p)
{
struct deflate_sequence *seq = *seq_p;
unsigned length_slot = deflate_length_slot[length];
unsigned offset_slot = deflate_get_offset_slot(c, offset);
c->freqs.litlen[DEFLATE_FIRST_LEN_SYM + length_slot]++;
c->freqs.offset[offset_slot]++;
seq->litrunlen_and_length |= (u32)length << 23;
seq->offset = offset;
seq->length_slot = length_slot;
seq->offset_symbol = offset_slot;
seq++;
seq->litrunlen_and_length = 0;
*seq_p = seq;
}
/******************************************************************************/
/*
* Block splitting algorithm. The problem is to decide when it is worthwhile to
* start a new block with new Huffman 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 noticeable
* 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. We also add a number
* proportional to the block length so that the algorithm is more likely to end
* long blocks than short blocks. This reflects the general expectation that it
* will become increasingly beneficial to start a new block as the current
* block grows longer.
*
* 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
init_block_split_stats(struct block_split_stats *stats)
{
int i;
for (i = 0; i < NUM_OBSERVATION_TYPES; i++) {
stats->new_observations[i] = 0;
stats->observations[i] = 0;
}
stats->num_new_observations = 0;
stats->num_observations = 0;
}
/* Literal observation. Heuristic: use the top 2 bits and low 1 bits of the
* literal, for 8 possible literal observation types. */
static forceinline void
observe_literal(struct 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 forceinline void
observe_match(struct block_split_stats *stats, unsigned length)
{
stats->new_observations[NUM_LITERAL_OBSERVATION_TYPES + (length >= 9)]++;
stats->num_new_observations++;
}
static void
merge_new_observations(struct block_split_stats *stats)
{
int i;
for (i = 0; i < NUM_OBSERVATION_TYPES; i++) {
stats->observations[i] += stats->new_observations[i];
stats->new_observations[i] = 0;
}
stats->num_observations += stats->num_new_observations;
stats->num_new_observations = 0;
}
static bool
do_end_block_check(struct block_split_stats *stats, u32 block_length)
{
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;
int i;
for (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 + (block_length / 4096) * stats->num_observations >=
NUM_OBSERVATIONS_PER_BLOCK_CHECK * 200 / 512 * stats->num_observations)
return true;
}
merge_new_observations(stats);
return false;
}
static forceinline bool
should_end_block(struct block_split_stats *stats,
const u8 *in_block_begin, const u8 *in_next, const u8 *in_end)
{
/* Ready to check block split statistics? */
if (stats->num_new_observations < NUM_OBSERVATIONS_PER_BLOCK_CHECK ||
in_next - in_block_begin < MIN_BLOCK_LENGTH ||
in_end - in_next < MIN_BLOCK_LENGTH)
return false;
return do_end_block_check(stats, in_next - in_block_begin);
}
/******************************************************************************/
/*
* Decrease the maximum and nice match lengths if we're approaching the end of
* the input buffer.
*/
static forceinline void
adjust_max_and_nice_len(unsigned *max_len, unsigned *nice_len, size_t remaining)
{
if (unlikely(remaining < DEFLATE_MAX_MATCH_LEN)) {
*max_len = remaining;
*nice_len = MIN(*nice_len, *max_len);
}
}
/*
* Choose the minimum match length for the greedy and lazy parsers.
*
* By default the minimum match length is 3, which is the smallest length the
* DEFLATE format allows. However, with greedy and lazy parsing, some data
* (e.g. DNA sequencing data) benefits greatly from a longer minimum length.
* Typically, this is because literals are very cheap. In general, the
* near-optimal parser handles this case naturally, but the greedy and lazy
* parsers need a heuristic to decide when to use short matches.
*
* The heuristic we use is to make the minimum match length depend on the number
* of different literals that exist in the data. If there are many different
* literals, then literals will probably be expensive, so short matches will
* probably be worthwhile. Conversely, if not many literals are used, then
* probably literals will be cheap and short matches won't be worthwhile.
*/
static unsigned
choose_min_match_len(unsigned num_used_literals, unsigned max_search_depth)
{
/* map from num_used_literals to min_len */
static const u8 min_lens[] = {
9, 9, 9, 9, 9, 9, 8, 8, 7, 7, 6, 6, 6, 6, 6, 6,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 4, 4, 4,
4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
/* the rest is implicitly 3 */
};
unsigned min_len;
STATIC_ASSERT(DEFLATE_MIN_MATCH_LEN <= 3);
STATIC_ASSERT(ARRAY_LEN(min_lens) <= DEFLATE_NUM_LITERALS + 1);
if (num_used_literals >= ARRAY_LEN(min_lens))
return 3;
min_len = min_lens[num_used_literals];
/*
* With a low max_search_depth, it may be too hard to find long matches.
*/
if (max_search_depth < 16) {
if (max_search_depth < 5)
min_len = MIN(min_len, 4);
else if (max_search_depth < 10)
min_len = MIN(min_len, 5);
else
min_len = MIN(min_len, 7);
}
return min_len;
}
static unsigned
calculate_min_match_len(const u8 *data, size_t data_len,
unsigned max_search_depth)
{
u8 used[256] = { 0 };
unsigned num_used_literals = 0;
int i;
/*
* For an initial approximation, scan the first 4 KiB of data.
* recalculate_min_match_len() will update the min_len later.
*/
data_len = MIN(data_len, 4096);
for (i = 0; i < data_len; i++)
used[data[i]] = 1;
for (i = 0; i < 256; i++)
num_used_literals += used[i];
return choose_min_match_len(num_used_literals, max_search_depth);
}
/*
* Recalculate the minimum match length for a block, now that we know the
* distribution of literals that are actually being used (freqs->litlen).
*/
static unsigned
recalculate_min_match_len(const struct deflate_freqs *freqs,
unsigned max_search_depth)
{
u32 literal_freq = 0;
u32 cutoff;
unsigned num_used_literals = 0;
int i;
for (i = 0; i < DEFLATE_NUM_LITERALS; i++)
literal_freq += freqs->litlen[i];
cutoff = literal_freq >> 10; /* Ignore literals used very rarely */
for (i = 0; i < DEFLATE_NUM_LITERALS; i++) {
if (freqs->litlen[i] > cutoff)
num_used_literals++;
}
return choose_min_match_len(num_used_literals, max_search_depth);
}
/*
* This is the level 0 "compressor". It always outputs uncompressed blocks.
*/
static size_t
deflate_compress_none(struct libdeflate_compressor * restrict c,
const u8 * restrict in, size_t in_nbytes,
u8 * restrict out, size_t out_nbytes_avail)
{
struct deflate_output_bitstream os;
deflate_init_output(&os, out, out_nbytes_avail);
deflate_write_uncompressed_blocks(&os, in, in_nbytes, true);
return deflate_flush_output(&os);
}
/*
* This is a faster variant of deflate_compress_greedy(). It uses the
* ht_matchfinder rather than the hc_matchfinder. It also skips the block
* splitting algorithm and just uses fixed length blocks. c->max_search_depth
* has no effect with this algorithm, as it is hardcoded in ht_matchfinder.h.
*/
static size_t
deflate_compress_fastest(struct libdeflate_compressor * restrict c,
const u8 * restrict in, size_t in_nbytes,
u8 * restrict out, size_t out_nbytes_avail)
{
const u8 *in_next = in;
const u8 *in_end = in_next + in_nbytes;
struct deflate_output_bitstream os;
const u8 *in_cur_base = in_next;
unsigned max_len = DEFLATE_MAX_MATCH_LEN;
unsigned nice_len = MIN(c->nice_match_length, max_len);
u32 next_hash = 0;
deflate_init_output(&os, out, out_nbytes_avail);
ht_matchfinder_init(&c->p.f.ht_mf);
do {
/* Starting a new DEFLATE block. */
const u8 * const in_block_begin = in_next;
const u8 * const in_max_block_end =
in_next + MIN(in_end - in_next, FAST_BLOCK_LENGTH);
struct deflate_sequence *seq = c->p.f.sequences;
deflate_begin_sequences(c, seq);
do {
u32 length;
u32 offset;
size_t remaining = in_end - in_next;
if (unlikely(remaining < DEFLATE_MAX_MATCH_LEN)) {
max_len = remaining;
if (max_len < HT_MATCHFINDER_REQUIRED_NBYTES) {
do {
deflate_choose_literal(
c, *in_next++, seq);
} while (--max_len);
break;
}
nice_len = MIN(nice_len, max_len);
}
length = ht_matchfinder_longest_match(&c->p.f.ht_mf,
&in_cur_base,
in_next,
max_len,
nice_len,
&next_hash,
&offset);
if (length) {
/* Match found. */
deflate_choose_match(c, length, offset, &seq);
ht_matchfinder_skip_bytes(&c->p.f.ht_mf,
&in_cur_base,
in_next + 1,
in_end,
length - 1,
&next_hash);
in_next += length;
} else {
/* No match found. */
deflate_choose_literal(c, *in_next++, seq);
}
/* Check if it's time to output another block. */
} while (in_next < in_max_block_end);
deflate_flush_block(c, &os, in_block_begin,
in_next - in_block_begin,
c->p.f.sequences, in_next == in_end);
} while (in_next != in_end);
return deflate_flush_output(&os);
}
/*
* This is the "greedy" DEFLATE compressor. It always chooses the longest match.
*/
static size_t
deflate_compress_greedy(struct libdeflate_compressor * restrict c,
const u8 * restrict in, size_t in_nbytes,
u8 * restrict out, size_t out_nbytes_avail)
{
const u8 *in_next = in;
const u8 *in_end = in_next + in_nbytes;
struct deflate_output_bitstream os;
const u8 *in_cur_base = in_next;
unsigned max_len = DEFLATE_MAX_MATCH_LEN;
unsigned nice_len = MIN(c->nice_match_length, max_len);
u32 next_hashes[2] = {0, 0};
deflate_init_output(&os, out, out_nbytes_avail);
hc_matchfinder_init(&c->p.g.hc_mf);
do {
/* Starting a new DEFLATE block. */
const u8 * const in_block_begin = in_next;
const u8 * const in_max_block_end =
in_next + MIN(in_end - in_next, SOFT_MAX_BLOCK_LENGTH);
unsigned min_len;
struct deflate_sequence *seq = c->p.g.sequences;
init_block_split_stats(&c->split_stats);
deflate_begin_sequences(c, seq);
min_len = calculate_min_match_len(in_next,
in_max_block_end - in_next,
c->max_search_depth);
do {
u32 length;
u32 offset;
adjust_max_and_nice_len(&max_len, &nice_len,
in_end - in_next);
length = hc_matchfinder_longest_match(
&c->p.g.hc_mf,
&in_cur_base,
in_next,
min_len - 1,
max_len,
nice_len,
c->max_search_depth,
next_hashes,
&offset);
if (length >= min_len &&
(length > DEFLATE_MIN_MATCH_LEN ||
offset <= 4096)) {
/* Match found. */
deflate_choose_match(c, length, offset, &seq);
observe_match(&c->split_stats, length);
hc_matchfinder_skip_bytes(&c->p.g.hc_mf,
&in_cur_base,
in_next + 1,
in_end,
length - 1,
next_hashes);
in_next += length;
} else {
/* No match found. */
deflate_choose_literal(c, *in_next, seq);
observe_literal(&c->split_stats, *in_next);
in_next++;
}
/* Check if it's time to output another block. */
} while (in_next < in_max_block_end &&
!should_end_block(&c->split_stats,
in_block_begin, in_next, in_end));
deflate_flush_block(c, &os, in_block_begin,
in_next - in_block_begin,
c->p.g.sequences, in_next == in_end);
} while (in_next != in_end);
return deflate_flush_output(&os);
}
static forceinline size_t
deflate_compress_lazy_generic(struct libdeflate_compressor * restrict c,
const u8 * restrict in, size_t in_nbytes,
u8 * restrict out, size_t out_nbytes_avail,
bool lazy2)
{
const u8 *in_next = in;
const u8 *in_end = in_next + in_nbytes;
struct deflate_output_bitstream os;
const u8 *in_cur_base = in_next;
unsigned max_len = DEFLATE_MAX_MATCH_LEN;
unsigned nice_len = MIN(c->nice_match_length, max_len);
u32 next_hashes[2] = {0, 0};
deflate_init_output(&os, out, out_nbytes_avail);
hc_matchfinder_init(&c->p.g.hc_mf);
do {
/* Starting a new DEFLATE block. */
const u8 * const in_block_begin = in_next;
const u8 * const in_max_block_end =
in_next + MIN(in_end - in_next, SOFT_MAX_BLOCK_LENGTH);
const u8 *next_recalc_min_len =
in_next + MIN(in_end - in_next, 10000);
unsigned min_len = DEFLATE_MIN_MATCH_LEN;
struct deflate_sequence *seq = c->p.g.sequences;
init_block_split_stats(&c->split_stats);
deflate_begin_sequences(c, seq);
min_len = calculate_min_match_len(in_next,
in_max_block_end - in_next,
c->max_search_depth);
do {
unsigned cur_len;
unsigned cur_offset;
unsigned next_len;
unsigned next_offset;
/*
* Recalculate the minimum match length if it hasn't
* been done recently.
*/
if (in_next >= next_recalc_min_len) {
min_len = recalculate_min_match_len(
&c->freqs,
c->max_search_depth);
next_recalc_min_len +=
MIN(in_end - next_recalc_min_len,
in_next - in_block_begin);
}
/* Find the longest match at the current position. */
adjust_max_and_nice_len(&max_len, &nice_len,
in_end - in_next);
cur_len = hc_matchfinder_longest_match(
&c->p.g.hc_mf,
&in_cur_base,
in_next,
min_len - 1,
max_len,
nice_len,
c->max_search_depth,
next_hashes,
&cur_offset);
if (cur_len < min_len ||
(cur_len == DEFLATE_MIN_MATCH_LEN &&
cur_offset > 8192)) {
/* No match found. Choose a literal. */
deflate_choose_literal(c, *in_next, seq);
observe_literal(&c->split_stats, *in_next);
in_next++;
continue;
}
in_next++;
have_cur_match:
observe_match(&c->split_stats, cur_len);
/*
* We have a match at the current position.
* If it's very long, choose it immediately.
*/
if (cur_len >= nice_len) {
deflate_choose_match(c, cur_len, cur_offset,
&seq);
hc_matchfinder_skip_bytes(&c->p.g.hc_mf,
&in_cur_base,
in_next,
in_end,
cur_len - 1,
next_hashes);
in_next += cur_len - 1;
continue;
}
/*
* Try to find a better match at the next position.
*
* Note: since we already have a match at the *current*
* position, we use only half the 'max_search_depth'
* when checking the *next* position. This is a useful
* trade-off because it's more worthwhile to use a
* greater search depth on the initial match.
*
* Note: it's possible to structure the code such that
* there's only one call to longest_match(), which
* handles both the "find the initial match" and "try to
* find a better match" cases. However, it is faster to
* have two call sites, with longest_match() inlined at
* each.
*/
adjust_max_and_nice_len(&max_len, &nice_len,
in_end - in_next);
next_len = hc_matchfinder_longest_match(
&c->p.g.hc_mf,
&in_cur_base,
in_next++,
cur_len - 1,
max_len,
nice_len,
c->max_search_depth >> 1,
next_hashes,
&next_offset);
if (next_len >= cur_len &&
4 * (int)(next_len - cur_len) +
((int)bsr32(cur_offset) -
(int)bsr32(next_offset)) > 2) {
/*
* Found a better match at the next position.
* Output a literal. Then the next match
* becomes the current match.
*/
deflate_choose_literal(c, *(in_next - 2), seq);
cur_len = next_len;
cur_offset = next_offset;
goto have_cur_match;
}
if (lazy2) {
/* In lazy2 mode, look ahead another position */
adjust_max_and_nice_len(&max_len, &nice_len,
in_end - in_next);
next_len = hc_matchfinder_longest_match(
&c->p.g.hc_mf,
&in_cur_base,
in_next++,
cur_len - 1,
max_len,
nice_len,
c->max_search_depth >> 2,
next_hashes,
&next_offset);
if (next_len >= cur_len &&
4 * (int)(next_len - cur_len) +
((int)bsr32(cur_offset) -
(int)bsr32(next_offset)) > 6) {
/*
* There's a much better match two
* positions ahead, so use two literals.
*/
deflate_choose_literal(
c, *(in_next - 3), seq);
deflate_choose_literal(
c, *(in_next - 2), seq);
cur_len = next_len;
cur_offset = next_offset;
goto have_cur_match;
}
/*
* No better match at either of the next 2
* positions. Output the current match.
*/
deflate_choose_match(c, cur_len, cur_offset,
&seq);
if (cur_len > 3) {
hc_matchfinder_skip_bytes(&c->p.g.hc_mf,
&in_cur_base,
in_next,
in_end,
cur_len - 3,
next_hashes);
in_next += cur_len - 3;
}
} else { /* !lazy2 */
/*
* No better match at the next position. Output
* the current match.
*/
deflate_choose_match(c, cur_len, cur_offset,
&seq);
hc_matchfinder_skip_bytes(&c->p.g.hc_mf,
&in_cur_base,
in_next,
in_end,
cur_len - 2,
next_hashes);
in_next += cur_len - 2;
}
/* Check if it's time to output another block. */
} while (in_next < in_max_block_end &&
!should_end_block(&c->split_stats,
in_block_begin, in_next, in_end));
deflate_flush_block(c, &os, in_block_begin,
in_next - in_block_begin,
c->p.g.sequences, in_next == in_end);
} while (in_next != in_end);
return deflate_flush_output(&os);
}
/*
* This is the "lazy" DEFLATE compressor. Before choosing a match, it checks to
* see if there's a better match at the next position. If yes, it outputs a
* literal and continues to the next position. If no, it outputs the match.
*/
static size_t
deflate_compress_lazy(struct libdeflate_compressor * restrict c,
const u8 * restrict in, size_t in_nbytes,
u8 * restrict out, size_t out_nbytes_avail)
{
return deflate_compress_lazy_generic(c, in, in_nbytes, out,
out_nbytes_avail, false);
}
/*
* The lazy2 compressor. This is similar to the regular lazy one, but it looks
* for a better match at the next 2 positions rather than the next 1. This
* makes it take slightly more time, but compress some inputs slightly more.
*/
static size_t
deflate_compress_lazy2(struct libdeflate_compressor * restrict c,
const u8 * restrict in, size_t in_nbytes,
u8 * restrict out, size_t out_nbytes_avail)
{
return deflate_compress_lazy_generic(c, in, in_nbytes, out,
out_nbytes_avail, true);
}
#if SUPPORT_NEAR_OPTIMAL_PARSING
/*
* Follow the minimum-cost path in the graph of possible match/literal choices
* for the current block and compute the frequencies of the Huffman symbols that
* would be needed to output those matches and literals.
*/
static void
deflate_tally_item_list(struct libdeflate_compressor *c, u32 block_length)
{
struct deflate_optimum_node *cur_node = &c->p.n.optimum_nodes[0];
struct deflate_optimum_node *end_node = &c->p.n.optimum_nodes[block_length];
do {
unsigned length = cur_node->item & OPTIMUM_LEN_MASK;
unsigned offset = cur_node->item >> OPTIMUM_OFFSET_SHIFT;
if (length == 1) {
/* Literal */
c->freqs.litlen[offset]++;
} else {
/* Match */
c->freqs.litlen[DEFLATE_FIRST_LEN_SYM +
deflate_length_slot[length]]++;
c->freqs.offset[deflate_get_offset_slot(c, offset)]++;
}
cur_node += length;
} while (cur_node != end_node);
/* Tally the end-of-block symbol. */
c->freqs.litlen[DEFLATE_END_OF_BLOCK]++;
}
/* Set the current cost model from the codeword lengths specified in @lens. */
static void
deflate_set_costs_from_codes(struct libdeflate_compressor *c,
const struct deflate_lens *lens)
{
unsigned i;
/* Literals */
for (i = 0; i < DEFLATE_NUM_LITERALS; i++) {
u32 bits = (lens->litlen[i] ? lens->litlen[i] : LITERAL_NOSTAT_BITS);
c->p.n.costs.literal[i] = bits * BIT_COST;
}
/* Lengths */
for (i = DEFLATE_MIN_MATCH_LEN; i <= DEFLATE_MAX_MATCH_LEN; i++) {
unsigned length_slot = deflate_length_slot[i];
unsigned litlen_sym = DEFLATE_FIRST_LEN_SYM + length_slot;
u32 bits = (lens->litlen[litlen_sym] ? lens->litlen[litlen_sym] : LENGTH_NOSTAT_BITS);
bits += deflate_extra_length_bits[length_slot];
c->p.n.costs.length[i] = bits * BIT_COST;
}
/* Offset slots */
for (i = 0; i < ARRAY_LEN(deflate_offset_slot_base); i++) {
u32 bits = (lens->offset[i] ? lens->offset[i] : OFFSET_NOSTAT_BITS);
bits += deflate_extra_offset_bits[i];
c->p.n.costs.offset_slot[i] = bits * BIT_COST;
}
}
/*
* This lookup table gives the default cost of a literal symbol and of a length
* symbol, depending on the characteristics of the input data. It was generated
* by scripts/gen_default_litlen_costs.py.
*
* This table is indexed first by the estimated match probability:
*
* i=0: data doesn't contain many matches [match_prob=0.25]
* i=1: neutral [match_prob=0.50]
* i=2: data contains lots of matches [match_prob=0.75]
*
* This lookup produces a subtable which maps the number of distinct used
* literals to the default cost of a literal symbol, i.e.:
*
* int(-log2((1 - match_prob) / num_used_literals) * BIT_COST)
*
* ... for num_used_literals in [1, 256] (and 0, which is copied from 1). This
* accounts for literals usually getting cheaper as the number of distinct
* literals decreases, and as the proportion of literals to matches increases.
*
* The lookup also produces the cost of a length symbol, which is:
*
* int(-log2(match_prob/NUM_LEN_SLOTS) * BIT_COST)
*
* Note: we don't currently assign different costs to different literal symbols,
* or to different length symbols, as this is hard to do.
*/
static const struct {
u8 used_lits_to_lit_cost[257];
u8 len_sym_cost;
} default_litlen_costs[] = {
{ /* match_prob = 0.25 */
.used_lits_to_lit_cost = {
6, 6, 22, 32, 38, 43, 48, 51,
54, 57, 59, 61, 64, 65, 67, 69,
70, 72, 73, 74, 75, 76, 77, 79,
80, 80, 81, 82, 83, 84, 85, 85,
86, 87, 88, 88, 89, 89, 90, 91,
91, 92, 92, 93, 93, 94, 95, 95,
96, 96, 96, 97, 97, 98, 98, 99,
99, 99, 100, 100, 101, 101, 101, 102,
102, 102, 103, 103, 104, 104, 104, 105,
105, 105, 105, 106, 106, 106, 107, 107,
107, 108, 108, 108, 108, 109, 109, 109,
109, 110, 110, 110, 111, 111, 111, 111,
112, 112, 112, 112, 112, 113, 113, 113,
113, 114, 114, 114, 114, 114, 115, 115,
115, 115, 115, 116, 116, 116, 116, 116,
117, 117, 117, 117, 117, 118, 118, 118,
118, 118, 118, 119, 119, 119, 119, 119,
120, 120, 120, 120, 120, 120, 121, 121,
121, 121, 121, 121, 121, 122, 122, 122,
122, 122, 122, 123, 123, 123, 123, 123,
123, 123, 124, 124, 124, 124, 124, 124,
124, 125, 125, 125, 125, 125, 125, 125,
125, 126, 126, 126, 126, 126, 126, 126,
127, 127, 127, 127, 127, 127, 127, 127,
128, 128, 128, 128, 128, 128, 128, 128,
128, 129, 129, 129, 129, 129, 129, 129,
129, 129, 130, 130, 130, 130, 130, 130,
130, 130, 130, 131, 131, 131, 131, 131,
131, 131, 131, 131, 131, 132, 132, 132,
132, 132, 132, 132, 132, 132, 132, 133,
133, 133, 133, 133, 133, 133, 133, 133,
133, 134, 134, 134, 134, 134, 134, 134,
134,
},
.len_sym_cost = 109,
}, { /* match_prob = 0.5 */
.used_lits_to_lit_cost = {
16, 16, 32, 41, 48, 53, 57, 60,
64, 66, 69, 71, 73, 75, 76, 78,
80, 81, 82, 83, 85, 86, 87, 88,
89, 90, 91, 92, 92, 93, 94, 95,
96, 96, 97, 98, 98, 99, 99, 100,
101, 101, 102, 102, 103, 103, 104, 104,
105, 105, 106, 106, 107, 107, 108, 108,
108, 109, 109, 110, 110, 110, 111, 111,
112, 112, 112, 113, 113, 113, 114, 114,
114, 115, 115, 115, 115, 116, 116, 116,
117, 117, 117, 118, 118, 118, 118, 119,
119, 119, 119, 120, 120, 120, 120, 121,
121, 121, 121, 122, 122, 122, 122, 122,
123, 123, 123, 123, 124, 124, 124, 124,
124, 125, 125, 125, 125, 125, 126, 126,
126, 126, 126, 127, 127, 127, 127, 127,
128, 128, 128, 128, 128, 128, 129, 129,
129, 129, 129, 129, 130, 130, 130, 130,
130, 130, 131, 131, 131, 131, 131, 131,
131, 132, 132, 132, 132, 132, 132, 133,
133, 133, 133, 133, 133, 133, 134, 134,
134, 134, 134, 134, 134, 134, 135, 135,
135, 135, 135, 135, 135, 135, 136, 136,
136, 136, 136, 136, 136, 136, 137, 137,
137, 137, 137, 137, 137, 137, 138, 138,
138, 138, 138, 138, 138, 138, 138, 139,
139, 139, 139, 139, 139, 139, 139, 139,
140, 140, 140, 140, 140, 140, 140, 140,
140, 141, 141, 141, 141, 141, 141, 141,
141, 141, 141, 142, 142, 142, 142, 142,
142, 142, 142, 142, 142, 142, 143, 143,
143, 143, 143, 143, 143, 143, 143, 143,
144,
},
.len_sym_cost = 93,
}, { /* match_prob = 0.75 */
.used_lits_to_lit_cost = {
32, 32, 48, 57, 64, 69, 73, 76,
80, 82, 85, 87, 89, 91, 92, 94,
96, 97, 98, 99, 101, 102, 103, 104,
105, 106, 107, 108, 108, 109, 110, 111,
112, 112, 113, 114, 114, 115, 115, 116,
117, 117, 118, 118, 119, 119, 120, 120,
121, 121, 122, 122, 123, 123, 124, 124,
124, 125, 125, 126, 126, 126, 127, 127,
128, 128, 128, 129, 129, 129, 130, 130,
130, 131, 131, 131, 131, 132, 132, 132,
133, 133, 133, 134, 134, 134, 134, 135,
135, 135, 135, 136, 136, 136, 136, 137,
137, 137, 137, 138, 138, 138, 138, 138,
139, 139, 139, 139, 140, 140, 140, 140,
140, 141, 141, 141, 141, 141, 142, 142,
142, 142, 142, 143, 143, 143, 143, 143,
144, 144, 144, 144, 144, 144, 145, 145,
145, 145, 145, 145, 146, 146, 146, 146,
146, 146, 147, 147, 147, 147, 147, 147,
147, 148, 148, 148, 148, 148, 148, 149,
149, 149, 149, 149, 149, 149, 150, 150,
150, 150, 150, 150, 150, 150, 151, 151,
151, 151, 151, 151, 151, 151, 152, 152,
152, 152, 152, 152, 152, 152, 153, 153,
153, 153, 153, 153, 153, 153, 154, 154,
154, 154, 154, 154, 154, 154, 154, 155,
155, 155, 155, 155, 155, 155, 155, 155,
156, 156, 156, 156, 156, 156, 156, 156,
156, 157, 157, 157, 157, 157, 157, 157,
157, 157, 157, 158, 158, 158, 158, 158,
158, 158, 158, 158, 158, 158, 159, 159,
159, 159, 159, 159, 159, 159, 159, 159,
160,
},
.len_sym_cost = 84,
},
};
/*
* Choose the default costs for literal and length symbols. These symbols are
* both part of the litlen alphabet.
*/
static void
deflate_choose_default_litlen_costs(struct libdeflate_compressor *c,
u32 block_length,
u32 *lit_cost, u32 *len_sym_cost)
{
unsigned num_used_literals = 0;
u32 literal_freq = block_length;
u32 match_freq = 0;
u32 cutoff;
int i;
/* Calculate the number of distinct literals that exist in the data. */
cutoff = literal_freq >> 11; /* Ignore literals used very rarely */
for (i = 0; i < DEFLATE_NUM_LITERALS; i++) {
if (c->freqs.litlen[i] > cutoff)
num_used_literals++;
}
if (num_used_literals == 0)
num_used_literals = 1;
/*
* Estimate the relative frequency of literals and matches in the
* optimal parsing solution. We don't know the optimal solution, so
* this can only be a very rough estimate. Therefore, we basically use
* the match frequency from a greedy parse. We also apply the min_len
* heuristic used by the greedy and lazy parsers, to avoid counting too
* many matches when literals are cheaper than short matches.
*/
match_freq = 0;
i = choose_min_match_len(num_used_literals, c->max_search_depth);
for (; i < ARRAY_LEN(c->p.n.match_len_freqs); i++) {
match_freq += c->p.n.match_len_freqs[i];
literal_freq -= i * c->p.n.match_len_freqs[i];
}
if ((s32)literal_freq < 0) /* shouldn't happen */
literal_freq = 0;
if (match_freq > literal_freq)
i = 2; /* many matches */
else if (match_freq * 4 > literal_freq)
i = 1; /* neutral */
else
i = 0; /* few matches */
*lit_cost = default_litlen_costs[i].used_lits_to_lit_cost[
num_used_literals];
*len_sym_cost = default_litlen_costs[i].len_sym_cost;
}
static forceinline u32
deflate_default_length_cost(unsigned len, u32 len_sym_cost)
{
unsigned slot = deflate_length_slot[len];
u32 num_extra_bits = deflate_extra_length_bits[slot];
return len_sym_cost + (num_extra_bits * BIT_COST);
}
static forceinline u32
deflate_default_offset_slot_cost(unsigned slot)
{
u32 num_extra_bits = deflate_extra_offset_bits[slot];
/*
* Assume that all offset symbols are equally probable.
* The resulting cost is 'int(-log2(1/30) * BIT_COST)',
* where 30 is the number of potentially-used offset symbols.
*/
u32 offset_sym_cost = 4*BIT_COST + (907*BIT_COST)/1000;
return offset_sym_cost + (num_extra_bits * BIT_COST);
}
/* Set default symbol costs for the first block's first optimization pass. */
static void
deflate_set_default_costs(struct libdeflate_compressor *c,
u32 lit_cost, u32 len_sym_cost)
{
unsigned i;
/* Literals */
for (i = 0; i < DEFLATE_NUM_LITERALS; i++)
c->p.n.costs.literal[i] = lit_cost;
/* Lengths */
for (i = DEFLATE_MIN_MATCH_LEN; i <= DEFLATE_MAX_MATCH_LEN; i++)
c->p.n.costs.length[i] =
deflate_default_length_cost(i, len_sym_cost);
/* Offset slots */
for (i = 0; i < ARRAY_LEN(deflate_offset_slot_base); i++)
c->p.n.costs.offset_slot[i] =
deflate_default_offset_slot_cost(i);
}
static forceinline void
deflate_adjust_cost(u32 *cost_p, u32 default_cost, int change_amount)
{
if (change_amount == 0)
/* Block is very similar to previous; prefer previous costs. */
*cost_p = (default_cost + 3 * *cost_p) / 4;
else if (change_amount == 1)
*cost_p = (default_cost + *cost_p) / 2;
else if (change_amount == 2)
*cost_p = (5 * default_cost + 3 * *cost_p) / 8;
else
/* Block differs greatly from previous; prefer default costs. */
*cost_p = (3 * default_cost + *cost_p) / 4;
}
static forceinline void
deflate_adjust_costs_impl(struct libdeflate_compressor *c,
u32 lit_cost, u32 len_sym_cost, int change_amount)
{
unsigned i;
/* Literals */
for (i = 0; i < DEFLATE_NUM_LITERALS; i++)
deflate_adjust_cost(&c->p.n.costs.literal[i], lit_cost,
change_amount);
/* Lengths */
for (i = DEFLATE_MIN_MATCH_LEN; i <= DEFLATE_MAX_MATCH_LEN; i++)
deflate_adjust_cost(&c->p.n.costs.length[i],
deflate_default_length_cost(i,
len_sym_cost),
change_amount);
/* Offset slots */
for (i = 0; i < ARRAY_LEN(deflate_offset_slot_base); i++)
deflate_adjust_cost(&c->p.n.costs.offset_slot[i],
deflate_default_offset_slot_cost(i),
change_amount);
}
/*
* Adjust the costs when beginning a new block.
*
* Since the current costs have been optimized for the data, it's undesirable to
* throw them away and start over with the default costs. At the same time, we
* don't want to bias the parse by assuming that the next block will be similar
* to the current block. As a compromise, make the costs closer to the
* defaults, but don't simply set them to the defaults.
*/
static void
deflate_adjust_costs(struct libdeflate_compressor *c,
u32 lit_cost, u32 len_sym_cost)
{
u64 total_delta = 0;
u64 cutoff;
int i;
/*
* Decide how different the current block is from the previous block,
* using the block splitting statistics from the current and previous
* blocks. The more different the current block is, the more we prefer
* the default costs rather than the previous block's costs.
*
* The algorithm here is similar to the end-of-block check one, but here
* we compare two entire blocks rather than a partial block with a small
* extra part, and therefore we need 64-bit numbers in some places.
*/
for (i = 0; i < NUM_OBSERVATION_TYPES; i++) {
u64 prev = (u64)c->p.n.prev_observations[i] *
c->split_stats.num_observations;
u64 cur = (u64)c->split_stats.observations[i] *
c->p.n.prev_num_observations;
total_delta += prev > cur ? prev - cur : cur - prev;
}
cutoff = ((u64)c->p.n.prev_num_observations *
c->split_stats.num_observations * 200) / 512;
if (4 * total_delta > 9 * cutoff)
deflate_adjust_costs_impl(c, lit_cost, len_sym_cost, 3);
else if (2 * total_delta > 3 * cutoff)
deflate_adjust_costs_impl(c, lit_cost, len_sym_cost, 2);
else if (2 * total_delta > cutoff)
deflate_adjust_costs_impl(c, lit_cost, len_sym_cost, 1);
else
deflate_adjust_costs_impl(c, lit_cost, len_sym_cost, 0);
}
/*
* Find the minimum-cost path through the graph of possible match/literal
* choices for this block.
*
* We find the minimum cost path from 'c->p.n.optimum_nodes[0]', which
* represents the node at the beginning of the block, to
* 'c->p.n.optimum_nodes[block_length]', which represents the node at the end of
* the block. Edge costs are evaluated using the cost model 'c->p.n.costs'.
*
* The algorithm works backwards, starting at the end node and proceeding
* backwards one node at a time. At each node, the minimum cost to reach the
* end node is computed and the match/literal choice that begins that path is
* saved.
*/
static void
deflate_find_min_cost_path(struct libdeflate_compressor *c,
const u32 block_length,
const struct lz_match *cache_ptr)
{
struct deflate_optimum_node *end_node = &c->p.n.optimum_nodes[block_length];
struct deflate_optimum_node *cur_node = end_node;
cur_node->cost_to_end = 0;
do {
unsigned num_matches;
unsigned literal;
u32 best_cost_to_end;
cur_node--;
cache_ptr--;
num_matches = cache_ptr->length;
literal = cache_ptr->offset;
/* It's always possible to choose a literal. */
best_cost_to_end = c->p.n.costs.literal[literal] +
(cur_node + 1)->cost_to_end;
cur_node->item = ((u32)literal << OPTIMUM_OFFSET_SHIFT) | 1;
/* Also consider matches if there are any. */
if (num_matches) {
const struct lz_match *match;
unsigned len;
unsigned offset;
unsigned offset_slot;
u32 offset_cost;
u32 cost_to_end;
/*
* Consider each length from the minimum
* (DEFLATE_MIN_MATCH_LEN) to the length of the longest
* match found at this position. For each length, we
* consider only the smallest offset for which that
* length is available. Although this is not guaranteed
* to be optimal due to the possibility of a larger
* offset costing less than a smaller offset to code,
* this is a very useful heuristic.
*/
match = cache_ptr - num_matches;
len = DEFLATE_MIN_MATCH_LEN;
do {
offset = match->offset;
offset_slot = deflate_get_offset_slot(c, offset);
offset_cost = c->p.n.costs.offset_slot[offset_slot];
do {
cost_to_end = offset_cost +
c->p.n.costs.length[len] +
(cur_node + len)->cost_to_end;
if (cost_to_end < best_cost_to_end) {
best_cost_to_end = cost_to_end;
cur_node->item = ((u32)offset << OPTIMUM_OFFSET_SHIFT) | len;
}
} while (++len <= match->length);
} while (++match != cache_ptr);
cache_ptr -= num_matches;
}
cur_node->cost_to_end = best_cost_to_end;
} while (cur_node != &c->p.n.optimum_nodes[0]);
}
/*
* Choose the literal/match sequence to use for the current block. The basic
* algorithm finds a minimum-cost path through the block's graph of
* literal/match choices, given a cost model. However, the cost of each symbol
* is unknown until the Huffman codes have been built, but at the same time the
* Huffman codes depend on the frequencies of chosen symbols. Consequently,
* multiple passes must be used to try to approximate an optimal solution. The
* first pass uses default costs, mixed with the costs from the previous block
* if any. Later passes use the Huffman codeword lengths from the previous pass
* as the costs.
*/
static void
deflate_optimize_block(struct libdeflate_compressor *c, u32 block_length,
const struct lz_match *cache_ptr, bool is_first_block,
bool is_final_block)
{
unsigned num_passes_remaining = c->p.n.num_optim_passes;
u32 lit_cost, len_sym_cost;
u32 i;
/* Force the block to really end at the desired length, even if some
* matches extend beyond it. */
for (i = block_length; i <= MIN(block_length - 1 + DEFLATE_MAX_MATCH_LEN,
ARRAY_LEN(c->p.n.optimum_nodes) - 1); i++)
c->p.n.optimum_nodes[i].cost_to_end = 0x80000000;
/* Make sure the literal/match statistics are up to date. */
merge_new_observations(&c->split_stats);
/* Set the initial costs. */
deflate_choose_default_litlen_costs(c, block_length,
&lit_cost, &len_sym_cost);
if (is_first_block)
deflate_set_default_costs(c, lit_cost, len_sym_cost);
else
deflate_adjust_costs(c, lit_cost, len_sym_cost);
do {
/* Find the minimum cost path for this pass. */
deflate_find_min_cost_path(c, block_length, cache_ptr);
/* Compute frequencies of the chosen symbols. */
deflate_reset_symbol_frequencies(c);
deflate_tally_item_list(c, block_length);
/* Make the Huffman codes. */
deflate_make_huffman_codes(&c->freqs, &c->codes);
/*
* Update the costs. After the last optimization pass, the
* final costs won't be needed for this block, but they will be
* used in determining the initial costs for the next block.
*/
if (--num_passes_remaining || !is_final_block)
deflate_set_costs_from_codes(c, &c->codes.lens);
} while (num_passes_remaining);
}
static void deflate_near_optimal_begin_block(struct libdeflate_compressor *c,
bool is_first_block)
{
int i;
if (!is_first_block) {
/*
* Save some literal/match statistics from the previous block so
* that deflate_adjust_costs() will be able to decide how much
* the current block differs from the previous one.
*/
for (i = 0; i < NUM_OBSERVATION_TYPES; i++) {
c->p.n.prev_observations[i] =
c->split_stats.observations[i];
}
c->p.n.prev_num_observations = c->split_stats.num_observations;
}
init_block_split_stats(&c->split_stats);
/*
* During matchfinding, we keep track of appropximate literal and match
* length frequencies for the purpose of setting the initial costs.
*/
memset(c->freqs.litlen, 0,
DEFLATE_NUM_LITERALS * sizeof(c->freqs.litlen[0]));
memset(c->p.n.match_len_freqs, 0, sizeof(c->p.n.match_len_freqs));
}
/*
* This is the "near-optimal" DEFLATE compressor. It computes the optimal
* representation of each DEFLATE block using a minimum-cost path search over
* the graph of possible match/literal choices for that block, assuming a
* certain cost for each Huffman symbol.
*
* For several reasons, the end result is not guaranteed to be optimal:
*
* - Nonoptimal choice of blocks
* - Heuristic limitations on which matches are actually considered
* - Symbol costs are unknown until the symbols have already been chosen
* (so iterative optimization must be used)
*/
static size_t
deflate_compress_near_optimal(struct libdeflate_compressor * restrict c,
const u8 * restrict in, size_t in_nbytes,
u8 * restrict out, size_t out_nbytes_avail)
{
const u8 *in_next = in;
const u8 *in_end = in_next + in_nbytes;
struct deflate_output_bitstream os;
const u8 *in_cur_base = in_next;
const u8 *in_next_slide =
in_next + MIN(in_end - in_next, MATCHFINDER_WINDOW_SIZE);
unsigned max_len = DEFLATE_MAX_MATCH_LEN;
unsigned nice_len = MIN(c->nice_match_length, max_len);
u32 next_hashes[2] = {0, 0};
deflate_init_output(&os, out, out_nbytes_avail);
bt_matchfinder_init(&c->p.n.bt_mf);
do {
/* Starting a new DEFLATE block. */
struct lz_match *cache_ptr = c->p.n.match_cache;
const u8 * const in_block_begin = in_next;
const u8 * const in_max_block_end =
in_next + MIN(in_end - in_next, SOFT_MAX_BLOCK_LENGTH);
const u8 *next_observation = in_next;
deflate_near_optimal_begin_block(c, in_block_begin == in);
/*
* Find matches until we decide to end the block. We end the
* block if any of the following is true:
*
* (1) Maximum block length has been reached
* (2) Match catch may overflow.
* (3) Block split heuristic says to split now.
*/
do {
struct lz_match *matches;
unsigned best_len;
size_t remaining = in_end - in_next;
/* Slide the window forward if needed. */
if (in_next == in_next_slide) {
bt_matchfinder_slide_window(&c->p.n.bt_mf);
in_cur_base = in_next;
in_next_slide = in_next +
MIN(remaining, MATCHFINDER_WINDOW_SIZE);
}
/*
* Find matches with the current position using the
* binary tree matchfinder and save them in
* 'match_cache'.
*
* Note: the binary tree matchfinder is more suited for
* optimal parsing than the hash chain matchfinder. The
* reasons for this include:
*
* - The binary tree matchfinder can find more matches
* in the same number of steps.
* - One of the major advantages of hash chains is that
* skipping positions (not searching for matches at
* them) is faster; however, with optimal parsing we
* search for matches at almost all positions, so this
* advantage of hash chains is negated.
*/
matches = cache_ptr;
best_len = 0;
adjust_max_and_nice_len(&max_len, &nice_len, remaining);
if (likely(max_len >= BT_MATCHFINDER_REQUIRED_NBYTES)) {
cache_ptr = bt_matchfinder_get_matches(
&c->p.n.bt_mf,
in_cur_base,
in_next - in_cur_base,
max_len,
nice_len,
c->max_search_depth,
next_hashes,
&best_len,
matches);
}
c->freqs.litlen[*in_next]++;
if (in_next >= next_observation) {
if (best_len >= 4) {
observe_match(&c->split_stats,
best_len);
next_observation = in_next + best_len;
c->p.n.match_len_freqs[best_len]++;
} else {
observe_literal(&c->split_stats,
*in_next);
next_observation = in_next + 1;
}
}
cache_ptr->length = cache_ptr - matches;
cache_ptr->offset = *in_next;
in_next++;
cache_ptr++;
/*
* If there was a very long match found, don't cache any
* matches for the bytes covered by that match. This
* avoids degenerate behavior when compressing highly
* redundant data, where the number of matches can be
* very large.
*
* This heuristic doesn't actually hurt the compression
* ratio very much. If there's a long match, then the
* data must be highly compressible, so it doesn't
* matter much what we do.
*/
if (best_len >= DEFLATE_MIN_MATCH_LEN &&
best_len >= nice_len) {
--best_len;
do {
remaining = in_end - in_next;
if (in_next == in_next_slide) {
bt_matchfinder_slide_window(
&c->p.n.bt_mf);
in_cur_base = in_next;
in_next_slide = in_next +
MIN(remaining,
MATCHFINDER_WINDOW_SIZE);
}
adjust_max_and_nice_len(&max_len,
&nice_len,
remaining);
if (max_len >=
BT_MATCHFINDER_REQUIRED_NBYTES) {
bt_matchfinder_skip_byte(
&c->p.n.bt_mf,
in_cur_base,
in_next - in_cur_base,
nice_len,
c->max_search_depth,
next_hashes);
}
cache_ptr->length = 0;
cache_ptr->offset = *in_next;
c->freqs.litlen[*in_next]++;
in_next++;
cache_ptr++;
} while (--best_len);
}
} while (in_next < in_max_block_end &&
cache_ptr < &c->p.n.match_cache[MATCH_CACHE_LENGTH] &&
!should_end_block(&c->split_stats,
in_block_begin, in_next, in_end));
/*
* All the matches for this block have been cached. Now choose
* the sequence of items to output and flush the block.
*/
deflate_optimize_block(c, in_next - in_block_begin, cache_ptr,
in_block_begin == in, in_next == in_end);
deflate_flush_block(c, &os, in_block_begin,
in_next - in_block_begin,
NULL, in_next == in_end);
} while (in_next != in_end);
return deflate_flush_output(&os);
}
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
/* Initialize c->offset_slot_fast. */
static void
deflate_init_offset_slot_fast(struct libdeflate_compressor *c)
{
unsigned offset_slot;
unsigned offset;
unsigned offset_end;
for (offset_slot = 0;
offset_slot < ARRAY_LEN(deflate_offset_slot_base);
offset_slot++)
{
offset = deflate_offset_slot_base[offset_slot];
#if USE_FULL_OFFSET_SLOT_FAST
offset_end = offset + (1 << deflate_extra_offset_bits[offset_slot]);
do {
c->offset_slot_fast[offset] = offset_slot;
} while (++offset != offset_end);
#else
if (offset <= 256) {
offset_end = offset + (1 << deflate_extra_offset_bits[offset_slot]);
do {
c->offset_slot_fast[offset - 1] = offset_slot;
} while (++offset != offset_end);
} else {
offset_end = offset + (1 << deflate_extra_offset_bits[offset_slot]);
do {
c->offset_slot_fast[256 + ((offset - 1) >> 7)] = offset_slot;
} while ((offset += (1 << 7)) != offset_end);
}
#endif
}
}
LIBDEFLATEEXPORT struct libdeflate_compressor * LIBDEFLATEAPI
libdeflate_alloc_compressor(int compression_level)
{
struct libdeflate_compressor *c;
size_t size = offsetof(struct libdeflate_compressor, p);
if (compression_level < 0 || compression_level > 12)
return NULL;
#if SUPPORT_NEAR_OPTIMAL_PARSING
if (compression_level >= 10)
size += sizeof(c->p.n);
else
#endif
{
if (compression_level >= 2)
size += sizeof(c->p.g);
else if (compression_level == 1)
size += sizeof(c->p.f);
}
c = libdeflate_aligned_malloc(MATCHFINDER_MEM_ALIGNMENT, size);
if (!c)
return NULL;
c->compression_level = compression_level;
/*
* The higher the compression level, the more we should bother trying to
* compress very small inputs.
*/
c->min_size_to_compress = 56 - (compression_level * 4);
switch (compression_level) {
case 0:
c->impl = deflate_compress_none;
break;
case 1:
c->impl = deflate_compress_fastest;
/* max_search_depth is unused */
c->nice_match_length = 32;
break;
case 2:
c->impl = deflate_compress_greedy;
c->max_search_depth = 6;
c->nice_match_length = 10;
break;
case 3:
c->impl = deflate_compress_greedy;
c->max_search_depth = 12;
c->nice_match_length = 14;
break;
case 4:
c->impl = deflate_compress_greedy;
c->max_search_depth = 16;
c->nice_match_length = 30;
break;
case 5:
c->impl = deflate_compress_lazy;
c->max_search_depth = 16;
c->nice_match_length = 30;
break;
case 6:
c->impl = deflate_compress_lazy;
c->max_search_depth = 35;
c->nice_match_length = 65;
break;
case 7:
c->impl = deflate_compress_lazy;
c->max_search_depth = 100;
c->nice_match_length = 130;
break;
case 8:
c->impl = deflate_compress_lazy2;
c->max_search_depth = 300;
c->nice_match_length = DEFLATE_MAX_MATCH_LEN;
break;
case 9:
#if !SUPPORT_NEAR_OPTIMAL_PARSING
default:
#endif
c->impl = deflate_compress_lazy2;
c->max_search_depth = 600;
c->nice_match_length = DEFLATE_MAX_MATCH_LEN;
break;
#if SUPPORT_NEAR_OPTIMAL_PARSING
case 10:
c->impl = deflate_compress_near_optimal;
c->max_search_depth = 35;
c->nice_match_length = 75;
c->p.n.num_optim_passes = 2;
break;
case 11:
c->impl = deflate_compress_near_optimal;
c->max_search_depth = 70;
c->nice_match_length = 150;
c->p.n.num_optim_passes = 3;
break;
case 12:
default:
c->impl = deflate_compress_near_optimal;
c->max_search_depth = 150;
c->nice_match_length = DEFLATE_MAX_MATCH_LEN;
c->p.n.num_optim_passes = 4;
break;
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
}
deflate_init_offset_slot_fast(c);
deflate_init_static_codes(c);
return c;
}
LIBDEFLATEEXPORT size_t LIBDEFLATEAPI
libdeflate_deflate_compress(struct libdeflate_compressor *c,
const void *in, size_t in_nbytes,
void *out, size_t out_nbytes_avail)
{
if (unlikely(out_nbytes_avail < OUTPUT_END_PADDING))
return 0;
/* For extremely small inputs just use a single uncompressed block. */
if (unlikely(in_nbytes < c->min_size_to_compress)) {
struct deflate_output_bitstream os;
deflate_init_output(&os, out, out_nbytes_avail);
if (in_nbytes == 0)
in = &os; /* Avoid passing NULL to memcpy() */
deflate_write_uncompressed_block(&os, in, in_nbytes, true);
return deflate_flush_output(&os);
}
return (*c->impl)(c, in, in_nbytes, out, out_nbytes_avail);
}
LIBDEFLATEEXPORT void LIBDEFLATEAPI
libdeflate_free_compressor(struct libdeflate_compressor *c)
{
libdeflate_aligned_free(c);
}
unsigned int
deflate_get_compression_level(struct libdeflate_compressor *c)
{
return c->compression_level;
}
LIBDEFLATEEXPORT size_t LIBDEFLATEAPI
libdeflate_deflate_compress_bound(struct libdeflate_compressor *c,
size_t in_nbytes)
{
/*
* The worst case is all uncompressed blocks where one block has length
* <= MIN_BLOCK_LENGTH and the others have length MIN_BLOCK_LENGTH.
* Each uncompressed block has 5 bytes of overhead: 1 for BFINAL, BTYPE,
* and alignment to a byte boundary; 2 for LEN; and 2 for NLEN.
*/
size_t max_num_blocks = MAX(DIV_ROUND_UP(in_nbytes, MIN_BLOCK_LENGTH), 1);
return (5 * max_num_blocks) + in_nbytes + 1 + OUTPUT_END_PADDING;
}
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