/src/libdeflate/lib/deflate_compress.c

Source
/*
 * 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"

/******************************************************************************/

/*
 * 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

/*
 * This is the minimum block length that the compressor will use, in
 * uncompressed bytes.  This should be a value below which using shorter blocks
 * is unlikely to be worthwhile, due to the per-block overhead.  This value does
 * not apply to the final block, which may be shorter than this (if the input is
 * shorter, it will have to be), or to the final uncompressed block in a series
 * of uncompressed blocks that cover more than UINT16_MAX bytes.
 *
 * This value is also approximately the amount by which what would otherwise be
 * the second-to-last block is allowed to grow past the soft maximum length in
 * order to avoid having to use a very short final block.
 *
 * 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  5000

/*
 * For the greedy, lazy, lazy2, and near-optimal compressors: This is the soft
 * maximum block length, in uncompressed bytes.  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 or if the input data ends soon after 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.  For the near-optimal compressor,
 * increasing/decreasing this parameter will increase/decrease per-compressor
 * memory usage linearly.
 */
#define SOFT_MAX_BLOCK_LENGTH 300000

/*
 * For the greedy, lazy, and lazy2 compressors: this is the length of the
 * sequence store, which is an array where the compressor temporarily stores
 * matches that it's going to use in the current block.  This value is the
 * maximum number of matches that can be used in a block.  If the sequence store
 * fills up, 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 then.  Increasing/decreasing this value will
 * increase/decrease per-compressor memory usage linearly.
 */
#define SEQ_STORE_LENGTH  50000

/*
 * For deflate_compress_fastest(): This is the soft maximum block length.
 * deflate_compress_fastest() doesn't use the regular block splitting algorithm;
 * it only ends blocks when they reach FAST_SOFT_MAX_BLOCK_LENGTH bytes or
 * FAST_SEQ_STORE_LENGTH matches.  Therefore, this value should be lower than
 * the regular SOFT_MAX_BLOCK_LENGTH.
 */
#define FAST_SOFT_MAX_BLOCK_LENGTH  65535

/*
 * For deflate_compress_fastest(): this is the length of the sequence store.
 * This is like SEQ_STORE_LENGTH, but this should be a lower value.
 */
#define FAST_SEQ_STORE_LENGTH 8192

/*
 * 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 near-optimal 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.
 *
 * BIT_COST doesn't apply to deflate_flush_block() and
 * deflate_compute_true_cost(), which consider whole bits.
 */
#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 (slightly less than) the maximum number of matches that the
 * near-optimal compressor will cache per block.  This behaves similarly to
 * SEQ_STORE_LENGTH for the other compressors.
 */
#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

/*
 * The largest block length we will ever use is when the final block is of
 * length SOFT_MAX_BLOCK_LENGTH + MIN_BLOCK_LENGTH - 1, or when any block is of
 * length SOFT_MAX_BLOCK_LENGTH + 1 + DEFLATE_MAX_MATCH_LEN.  The latter case
 * occurs when the lazy2 compressor chooses two literals and a maximum-length
 * match, starting at SOFT_MAX_BLOCK_LENGTH - 1.
 */
#define MAX_BLOCK_LENGTH  \
  MAX(SOFT_MAX_BLOCK_LENGTH + MIN_BLOCK_LENGTH - 1, \
      SOFT_MAX_BLOCK_LENGTH + 1 + DEFLATE_MAX_MATCH_LEN)

static forceinline void
check_buildtime_parameters(void)
{
  /*
   * Verify that MIN_BLOCK_LENGTH is being honored, as
   * libdeflate_deflate_compress_bound() depends on it.
   */
  STATIC_ASSERT(SOFT_MAX_BLOCK_LENGTH >= MIN_BLOCK_LENGTH);
  STATIC_ASSERT(FAST_SOFT_MAX_BLOCK_LENGTH >= MIN_BLOCK_LENGTH);
  STATIC_ASSERT(SEQ_STORE_LENGTH * DEFLATE_MIN_MATCH_LEN >=
          MIN_BLOCK_LENGTH);
  STATIC_ASSERT(FAST_SEQ_STORE_LENGTH * HT_MATCHFINDER_MIN_MATCH_LEN >=
          MIN_BLOCK_LENGTH);
#if SUPPORT_NEAR_OPTIMAL_PARSING
  STATIC_ASSERT(MIN_BLOCK_LENGTH * MAX_MATCHES_PER_POS <=
          MATCH_CACHE_LENGTH);
#endif

  /* The definition of MAX_BLOCK_LENGTH assumes this. */
  STATIC_ASSERT(FAST_SOFT_MAX_BLOCK_LENGTH <= SOFT_MAX_BLOCK_LENGTH);

  /* Verify that the sequence stores aren't uselessly large. */
  STATIC_ASSERT(SEQ_STORE_LENGTH * DEFLATE_MIN_MATCH_LEN <=
          SOFT_MAX_BLOCK_LENGTH + MIN_BLOCK_LENGTH);
  STATIC_ASSERT(FAST_SEQ_STORE_LENGTH * HT_MATCHFINDER_MIN_MATCH_LEN <=
          FAST_SOFT_MAX_BLOCK_LENGTH + MIN_BLOCK_LENGTH);

  /* Verify that the maximum codeword lengths are valid. */
  STATIC_ASSERT(
    MAX_LITLEN_CODEWORD_LEN <= DEFLATE_MAX_LITLEN_CODEWORD_LEN);
  STATIC_ASSERT(
    MAX_OFFSET_CODEWORD_LEN <= DEFLATE_MAX_OFFSET_CODEWORD_LEN);
  STATIC_ASSERT(
    MAX_PRE_CODEWORD_LEN <= DEFLATE_MAX_PRE_CODEWORD_LEN);
  STATIC_ASSERT(
    (1U << MAX_LITLEN_CODEWORD_LEN) >= DEFLATE_NUM_LITLEN_SYMS);
  STATIC_ASSERT(
    (1U << MAX_OFFSET_CODEWORD_LEN) >= DEFLATE_NUM_OFFSET_SYMS);
  STATIC_ASSERT(
    (1U << MAX_PRE_CODEWORD_LEN) >= DEFLATE_NUM_PRECODE_SYMS);
}

/******************************************************************************/

/* Table: length slot => length slot base value */
static const u32 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 u32 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,
};

/*
 * Table: 'offset - 1 => offset_slot' for offset <= 256.
 * This was generated by scripts/gen_offset_slot_map.py.
 */
static const u8 deflate_offset_slot[256] = {
  0, 1, 2, 3, 4, 4, 5, 5, 6, 6, 6, 6, 7, 7, 7, 7,
  8, 8, 8, 8, 8, 8, 8, 8, 9, 9, 9, 9, 9, 9, 9, 9,
  10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
  11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11, 11,
  12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
  12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12,
  13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
  13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13,
  14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14, 14,
  15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
  15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15, 15,
};

/* 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
};

/* Table: precode symbol => number of extra bits */
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
};

/* 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 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.
   */
#define SEQ_LENGTH_SHIFT 23
#define SEQ_LITRUNLEN_MASK (((u32)1 << SEQ_LENGTH_SHIFT) - 1)
  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
   * slot of the match which follows the literals.
   */
  u16 offset_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;
};

struct deflate_output_bitstream;

/* The main DEFLATE compressor structure */
struct libdeflate_compressor {

  /* Pointer to the compress() implementation chosen at allocation time */
  void (*impl)(struct libdeflate_compressor *restrict c, const u8 *in,
         size_t in_nbytes, struct deflate_output_bitstream *os);

  /* The free() function for this struct, chosen at allocation time */
  free_func_t free_func;

  /* The compression level with which this compressor was created */
  unsigned compression_level;

  /* Anything of this size or less we won't bother trying to compress. */
  size_t max_passthrough_size;

  /*
   * The maximum search depth: consider at most this many potential
   * matches at each position
   */
  u32 max_search_depth;

  /*
   * The "nice" match length: if a match of this length is found, choose
   * it immediately without further consideration
   */
  u32 nice_match_length;

  /* Frequency counters for the current block */
  struct deflate_freqs freqs;

  /* Block split statistics for the current block */
  struct block_split_stats split_stats;

  /* Dynamic Huffman codes for the current block */
  struct deflate_codes codes;

  /* The static Huffman codes defined by the DEFLATE format */
  struct deflate_codes static_codes;

  /* Temporary space for block flushing */
  union {
    /* Information about the precode */
    struct {
      u32 freqs[DEFLATE_NUM_PRECODE_SYMS];
      u32 codewords[DEFLATE_NUM_PRECODE_SYMS];
      u8 lens[DEFLATE_NUM_PRECODE_SYMS];
      unsigned items[DEFLATE_NUM_LITLEN_SYMS +
               DEFLATE_NUM_OFFSET_SYMS];
      unsigned num_litlen_syms;
      unsigned num_offset_syms;
      unsigned num_explicit_lens;
      unsigned num_items;
    } precode;
    /*
     * The "full" length codewords.  Used only after the information
     * in 'precode' is no longer needed.
     */
    struct {
      u32 codewords[DEFLATE_MAX_MATCH_LEN + 1];
      u8 lens[DEFLATE_MAX_MATCH_LEN + 1];
    } length;
  } o;

  union {
    /* Data for greedy or lazy parsing */
    struct {
      /* Matches and literals chosen for the current block */
      struct deflate_sequence sequences[SEQ_STORE_LENGTH + 1];

      /*
       * Hash chains matchfinder.  This must be last, since
       * only the part of it that will be used is allocated.
       */
      struct hc_matchfinder hc_mf;

    } g; /* (g)reedy */

    /* Data for fastest parsing */
    struct {
      /* Hash table matchfinder */
      struct ht_matchfinder ht_mf;

      /* Matches and literals chosen for the current block */
      struct deflate_sequence sequences[
            FAST_SEQ_STORE_LENGTH + 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 is MAX_BLOCK_LENGTH
       * plus 1 for the end-of-block node.
       */
      struct deflate_optimum_node optimum_nodes[
        MAX_BLOCK_LENGTH + 1];

      /* The current cost model being used */
      struct deflate_costs costs;

      /* Saved cost model */
      struct deflate_costs costs_saved;

      /*
       * A table that maps match offset to offset slot.  This
       * differs from deflate_offset_slot[] in that this is a
       * full map, not a condensed one.  The full map is more
       * appropriate for the near-optimal parser, since the
       * near-optimal parser does more offset => offset_slot
       * translations, it doesn't intersperse them with
       * matchfinding (so cache evictions are less of a
       * concern), and it uses more memory anyway.
       */
      u8 offset_slot_full[DEFLATE_MAX_MATCH_OFFSET + 1];

      /* 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 new_match_len_freqs[DEFLATE_MAX_MATCH_LEN + 1];
      u32 match_len_freqs[DEFLATE_MAX_MATCH_LEN + 1];

      /*
       * The maximum number of optimization passes
       * (min-cost path searches) per block.
       * Larger values = more compression.
       */
      unsigned max_optim_passes;

      /*
       * If an optimization pass improves the cost by fewer
       * than this number of bits, then optimization will stop
       * early, before max_optim_passes has been reached.
       * Smaller values = more compression.
       */
      u32 min_improvement_to_continue;

      /*
       * The minimum number of bits that would need to be
       * saved for it to be considered worth the time to
       * regenerate and use the min-cost path from a previous
       * optimization pass, in the case where the final
       * optimization pass actually increased the cost.
       * Smaller values = more compression.
       */
      u32 min_bits_to_use_nonfinal_path;

      /*
       * The maximum block length, in uncompressed bytes, at
       * which to find and consider the optimal match/literal
       * list for the static Huffman codes.  This strategy
       * improves the compression ratio produced by static
       * Huffman blocks and can discover more cases in which
       * static blocks are worthwhile.  This helps mostly with
       * small blocks, hence why this parameter is a max_len.
       *
       * Above this block length, static Huffman blocks are
       * only used opportunistically.  I.e. a static Huffman
       * block is only used if a static block using the same
       * match/literal list as the optimized dynamic block
       * happens to be cheaper than the dynamic block itself.
       */
      u32 max_len_to_optimize_static_block;

    } 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;

/*
 * The capacity of the bitbuffer, in bits.  This is 1 less than the real size,
 * in order to avoid undefined behavior when doing bitbuf >>= bitcount & ~7.
 */
#define BITBUF_NBITS  (8 * sizeof(bitbuf_t) - 1)

/*
 * Can the specified number of bits always be added to 'bitbuf' after any
 * pending bytes have been flushed?  There can be up to 7 bits remaining after a
 * flush, so the count must not exceed BITBUF_NBITS after adding 'n' more bits.
 */
#define CAN_BUFFER(n) (7 + (n) <= BITBUF_NBITS)

/*
 * 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.  This can be between 0 and
   * BITBUF_NBITS in general, or between 0 and 7 after a flush.
   */
  unsigned bitcount;

  /*
   * Pointer to the position in the output buffer at which the next byte
   * should be written
   */
  u8 *next;

  /* Pointer to the end of the output buffer */
  u8 *end;

  /* true if the output buffer ran out of space */
  bool overflow;
};

/*
 * Add some bits to the bitbuffer variable of the output bitstream.  The caller
 * must ensure that 'bitcount + n <= BITBUF_NBITS', by calling FLUSH_BITS()
 * frequently enough.
 */
#define ADD_BITS(bits, n)     \
do {           \
  bitbuf |= (bitbuf_t)(bits) << bitcount; \
  bitcount += (n);      \
  ASSERT(bitcount <= BITBUF_NBITS); \
} while (0)

/*
 * Flush bits from the bitbuffer variable to the output buffer.  After this, the
 * bitbuffer will contain at most 7 bits (a partial byte).
 *
 * Since deflate_flush_block() verified ahead of time that there is enough space
 * remaining before actually writing the block, it's guaranteed that out_next
 * won't exceed os->end.  However, there might not be enough space remaining to
 * flush a whole word, even though that's fastest.  Therefore, flush a whole
 * word if there is space for it, otherwise flush a byte at a time.
 */
#define FLUSH_BITS()              \
do {                 \
  if (UNALIGNED_ACCESS_IS_FAST && likely(out_next < out_fast_end)) { \
    /* Flush a whole word (branchlessly). */    \
    put_unaligned_leword(bitbuf, out_next);     \
    bitbuf >>= bitcount & ~7;       \
    out_next += bitcount >> 3;        \
    bitcount &= 7;            \
  } else {             \
    /* Flush a byte at a time. */       \
    while (bitcount >= 8) {         \
      ASSERT(out_next < os->end);     \
      *out_next++ = bitbuf;       \
      bitcount -= 8;          \
      bitbuf >>= 8;         \
    }             \
  }                \
} while (0)

/*
 * 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 or equal to both of its children, so that the maxheap property is
 * satisfied in the subtree rooted at A[subtree_idx].  'A' uses 1-based indices.
 */
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.
 */
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 NUM_FREQ_BITS (32 - NUM_SYMBOL_BITS)
#define SYMBOL_MASK ((1 << NUM_SYMBOL_BITS) - 1)
#define FREQ_MASK (~SYMBOL_MASK)

#define GET_NUM_COUNTERS(num_syms)  (num_syms)

/*
 * 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, at most 1 << NUM_SYMBOL_BITS.
 *
 * @freqs[num_syms]
 *  Frequency of each symbol, summing to at most (1 << NUM_FREQ_BITS) - 1.
 *
 * @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[], u8 lens[], u32 symout[])
{
  unsigned sym;
  unsigned i;
  unsigned num_used_syms;
  unsigned num_counters;
  unsigned counters[GET_NUM_COUNTERS(DEFLATE_MAX_NUM_SYMS)];

  /*
   * We use heapsort, but with an added optimization.  Since often most
   * symbol frequencies are low, we first do a count sort using a limited
   * number of counters.  High frequencies are 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 symbol seems fastest.
   */

  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 a 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)
{
  const unsigned last_idx = sym_count - 1;

  /* Index of the next lowest frequency leaf that still needs a parent */
  unsigned i = 0;

  /*
   * Index of the next lowest frequency non-leaf that still needs a
   * parent, or 'e' if there is currently no such node
   */
  unsigned b = 0;

  /* Index of the next spot for a non-leaf (will overwrite a leaf) */
  unsigned e = 0;

  do {
    u32 new_freq;

    /*
     * Select the next two lowest frequency nodes among the leaves
     * A[i] and non-leaves A[b], and create a new node A[e] to be
     * their parent.  Set the new node's frequency to the sum of the
     * frequencies of its two children.
     *
     * Usually the next two lowest frequency nodes are of the same
     * type (leaf or non-leaf), so check those cases first.
     */
    if (i + 1 <= last_idx &&
        (b == e || (A[i + 1] & FREQ_MASK) <= (A[b] & FREQ_MASK))) {
      /* Two leaves */
      new_freq = (A[i] & FREQ_MASK) + (A[i + 1] & FREQ_MASK);
      i += 2;
    } else if (b + 2 <= e &&
         (i > last_idx ||
          (A[b + 1] & FREQ_MASK) < (A[i] & FREQ_MASK))) {
      /* Two non-leaves */
      new_freq = (A[b] & FREQ_MASK) + (A[b + 1] & FREQ_MASK);
      A[b] = (e << NUM_SYMBOL_BITS) | (A[b] & SYMBOL_MASK);
      A[b + 1] = (e << NUM_SYMBOL_BITS) |
           (A[b + 1] & SYMBOL_MASK);
      b += 2;
    } else {
      /* One leaf and one non-leaf */
      new_freq = (A[i] & FREQ_MASK) + (A[b] & FREQ_MASK);
      A[b] = (e << NUM_SYMBOL_BITS) | (A[b] & SYMBOL_MASK);
      i++;
      b++;
    }
    A[e] = new_freq | (A[e] & SYMBOL_MASK);
    /*
     * A binary tree with 'n' leaves has 'n - 1' non-leaves, so the
     * tree is complete once we've created 'n - 1' non-leaves.
     */
  } while (++e < last_idx);
}

/*
 * 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[], unsigned root_idx, unsigned len_counts[],
          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;

    /*
     * 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 (depth >= max_codeword_len) {
      depth = max_codeword_len;
      do {
        depth--;
      } while (len_counts[depth] == 0);
    }

    /*
     * Account for the fact that we have a non-leaf node at the
     * current depth.
     */
    len_counts[depth]--;
    len_counts[depth + 1] += 2;
  }
}

/*
 * DEFLATE uses bit-reversed codewords, so we must bit-reverse the codewords
 * after generating them.  All codewords have length <= 16 bits.  If the CPU has
 * a bit-reversal instruction, then that is the fastest method.  Otherwise the
 * fastest method is to reverse the bits in each of the two bytes using a table.
 * The table method is slightly faster than using bitwise operations to flip
 * adjacent 1, 2, 4, and then 8-bit fields, even if 2 to 4 codewords are packed
 * into a machine word and processed together using that method.
 */

#ifdef rbit32
static forceinline u32 reverse_codeword(u32 codeword, u8 len)
{
  return rbit32(codeword) >> ((32 - len) & 31);
}
#else
/* Generated by scripts/gen_bitreverse_tab.py */
static const u8 bitreverse_tab[256] = {
  0x00, 0x80, 0x40, 0xc0, 0x20, 0xa0, 0x60, 0xe0,
  0x10, 0x90, 0x50, 0xd0, 0x30, 0xb0, 0x70, 0xf0,
  0x08, 0x88, 0x48, 0xc8, 0x28, 0xa8, 0x68, 0xe8,
  0x18, 0x98, 0x58, 0xd8, 0x38, 0xb8, 0x78, 0xf8,
  0x04, 0x84, 0x44, 0xc4, 0x24, 0xa4, 0x64, 0xe4,
  0x14, 0x94, 0x54, 0xd4, 0x34, 0xb4, 0x74, 0xf4,
  0x0c, 0x8c, 0x4c, 0xcc, 0x2c, 0xac, 0x6c, 0xec,
  0x1c, 0x9c, 0x5c, 0xdc, 0x3c, 0xbc, 0x7c, 0xfc,
  0x02, 0x82, 0x42, 0xc2, 0x22, 0xa2, 0x62, 0xe2,
  0x12, 0x92, 0x52, 0xd2, 0x32, 0xb2, 0x72, 0xf2,
  0x0a, 0x8a, 0x4a, 0xca, 0x2a, 0xaa, 0x6a, 0xea,
  0x1a, 0x9a, 0x5a, 0xda, 0x3a, 0xba, 0x7a, 0xfa,
  0x06, 0x86, 0x46, 0xc6, 0x26, 0xa6, 0x66, 0xe6,
  0x16, 0x96, 0x56, 0xd6, 0x36, 0xb6, 0x76, 0xf6,
  0x0e, 0x8e, 0x4e, 0xce, 0x2e, 0xae, 0x6e, 0xee,
  0x1e, 0x9e, 0x5e, 0xde, 0x3e, 0xbe, 0x7e, 0xfe,
  0x01, 0x81, 0x41, 0xc1, 0x21, 0xa1, 0x61, 0xe1,
  0x11, 0x91, 0x51, 0xd1, 0x31, 0xb1, 0x71, 0xf1,
  0x09, 0x89, 0x49, 0xc9, 0x29, 0xa9, 0x69, 0xe9,
  0x19, 0x99, 0x59, 0xd9, 0x39, 0xb9, 0x79, 0xf9,
  0x05, 0x85, 0x45, 0xc5, 0x25, 0xa5, 0x65, 0xe5,
  0x15, 0x95, 0x55, 0xd5, 0x35, 0xb5, 0x75, 0xf5,
  0x0d, 0x8d, 0x4d, 0xcd, 0x2d, 0xad, 0x6d, 0xed,
  0x1d, 0x9d, 0x5d, 0xdd, 0x3d, 0xbd, 0x7d, 0xfd,
  0x03, 0x83, 0x43, 0xc3, 0x23, 0xa3, 0x63, 0xe3,
  0x13, 0x93, 0x53, 0xd3, 0x33, 0xb3, 0x73, 0xf3,
  0x0b, 0x8b, 0x4b, 0xcb, 0x2b, 0xab, 0x6b, 0xeb,
  0x1b, 0x9b, 0x5b, 0xdb, 0x3b, 0xbb, 0x7b, 0xfb,
  0x07, 0x87, 0x47, 0xc7, 0x27, 0xa7, 0x67, 0xe7,
  0x17, 0x97, 0x57, 0xd7, 0x37, 0xb7, 0x77, 0xf7,
  0x0f, 0x8f, 0x4f, 0xcf, 0x2f, 0xaf, 0x6f, 0xef,
  0x1f, 0x9f, 0x5f, 0xdf, 0x3f, 0xbf, 0x7f, 0xff,
};

static forceinline u32 reverse_codeword(u32 codeword, u8 len)
{
  STATIC_ASSERT(DEFLATE_MAX_CODEWORD_LEN <= 16);
  codeword = ((u32)bitreverse_tab[codeword & 0xff] << 8) |
       bitreverse_tab[codeword >> 8];
  return codeword >> (16 - len);
}
#endif /* !rbit32 */

/*
 * 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[], u8 lens[], const unsigned len_counts[],
        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++) {
    /* DEFLATE requires bit-reversed codewords. */
    A[sym] = reverse_codeword(next_codewords[lens[sym]]++,
            lens[sym]);
  }
}

/*
 * ---------------------------------------------------------------------
 *      deflate_make_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
 *  must not exceed (1 << NUM_SYMBOL_BITS).
 *
 * @max_codeword_len
 *  The maximum permissible codeword length.
 *
 * @freqs
 *  An array of length @num_syms that gives the frequency of each symbol.
 *  It is valid for some, none, or all of the frequencies to be 0.  The sum
 *  of frequencies must not exceed (1 << NUM_FREQ_BITS) - 1.
 *
 * @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 was placed in the public domain by Igor Pavlov.
 */
static void
deflate_make_huffman_code(unsigned num_syms, unsigned max_codeword_len,
        const u32 freqs[], u8 lens[], u32 codewords[])
{
  u32 *A = codewords;
  unsigned num_used_syms;

  STATIC_ASSERT(DEFLATE_MAX_NUM_SYMS <= 1 << NUM_SYMBOL_BITS);
  STATIC_ASSERT(MAX_BLOCK_LENGTH <= ((u32)1 << NUM_FREQ_BITS) - 1);

  /*
   * 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.
   */

  /*
   * A complete Huffman code must contain at least 2 codewords.  Yet, it's
   * possible that fewer than 2 symbols were used.  When this happens,
   * it's usually for the offset code (0-1 symbols used).  But it's also
   * theoretically possible for the litlen and pre codes (1 symbol used).
   *
   * The DEFLATE RFC explicitly allows the offset code to contain just 1
   * codeword, or even be completely empty.  But it's silent about the
   * other codes.  It also doesn't say whether, in the 1-codeword case,
   * the codeword (which it says must be 1 bit) is '0' or '1'.
   *
   * In any case, some DEFLATE decompressors reject these cases.  zlib
   * generally allows them, but it does reject precodes that have just 1
   * codeword.  More problematically, zlib v1.2.1 and earlier rejected
   * empty offset codes, and this behavior can also be seen in Windows
   * Explorer's ZIP unpacker (supposedly even still in Windows 11).
   *
   * Other DEFLATE compressors, including zlib, always send at least 2
   * codewords in order to make a complete Huffman code.  Therefore, this
   * is a case where practice does not entirely match the specification.
   * We follow practice by generating 2 codewords of length 1: codeword
   * '0' for symbol 0, and codeword '1' for another symbol -- the used
   * symbol if it exists and is not symbol 0, otherwise symbol 1.  This
   * does worsen the compression ratio by having to send an unnecessary
   * offset codeword length.  But this only affects rare cases such as
   * blocks containing all literals, and it only makes a tiny difference.
   */
  if (unlikely(num_used_syms < 2)) {
    unsigned sym = num_used_syms ? (A[0] & SYMBOL_MASK) : 0;
    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));
}

/*
 * Build the literal/length and offset Huffman codes for a DEFLATE block.
 *
 * This takes as input the frequency tables for each alphabet 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)
{
  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 given match offset, using the small map. */
static forceinline unsigned
deflate_get_offset_slot(u32 offset)
{
  /*
   * 1 <= offset <= 32768 here.  For 1 <= offset <= 256,
   * deflate_offset_slot[offset - 1] gives the slot.
   *
   * For 257 <= offset <= 32768, we take advantage of the fact that 257 is
   * the beginning of slot 16, and each slot [16..30) is exactly 1 << 7 ==
   * 128 times larger than each slot [2..16) (since the number of extra
   * bits increases by 1 every 2 slots).  Thus, the slot is:
   *
   *  deflate_offset_slot[2 + ((offset - 257) >> 7)] + (16 - 2)
   *   == deflate_offset_slot[((offset - 1) >> 7)] + 14
   *
   * Define 'n = (offset <= 256) ? 0 : 7'.  Then any offset is handled by:
   *
   *      deflate_offset_slot[(offset - 1) >> n] + (n << 1)
   *
   * For better performance, replace 'n = (offset <= 256) ? 0 : 7' with
   * the equivalent (for offset <= 536871168) 'n = (256 - offset) >> 29'.
   */
  unsigned n = (256 - offset) >> 29;

  ASSERT(offset >= 1 && offset <= 32768);

  return deflate_offset_slot[(offset - 1) >> n] + (n << 1);
}

static unsigned
deflate_compute_precode_items(const u8 lens[], const unsigned num_lens,
            u32 precode_freqs[], unsigned precode_items[])
{
  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 dynamic Huffman codes. */
static void
deflate_precompute_huffman_header(struct libdeflate_compressor *c)
{
  /* Compute how many litlen and offset symbols are needed. */

  for (c->o.precode.num_litlen_syms = DEFLATE_NUM_LITLEN_SYMS;
       c->o.precode.num_litlen_syms > 257;
       c->o.precode.num_litlen_syms--)
    if (c->codes.lens.litlen[c->o.precode.num_litlen_syms - 1] != 0)
      break;

  for (c->o.precode.num_offset_syms = DEFLATE_NUM_OFFSET_SYMS;
       c->o.precode.num_offset_syms > 1;
       c->o.precode.num_offset_syms--)
    if (c->codes.lens.offset[c->o.precode.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->o.precode.num_litlen_syms != DEFLATE_NUM_LITLEN_SYMS) {
    memmove((u8 *)&c->codes.lens + c->o.precode.num_litlen_syms,
      (u8 *)&c->codes.lens + DEFLATE_NUM_LITLEN_SYMS,
      c->o.precode.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->o.precode.num_items =
    deflate_compute_precode_items((u8 *)&c->codes.lens,
                c->o.precode.num_litlen_syms +
                c->o.precode.num_offset_syms,
                c->o.precode.freqs,
                c->o.precode.items);

  /* Build the precode. */
  deflate_make_huffman_code(DEFLATE_NUM_PRECODE_SYMS,
          MAX_PRE_CODEWORD_LEN,
          c->o.precode.freqs, c->o.precode.lens,
          c->o.precode.codewords);

  /* Count how many precode lengths we actually need to output. */
  for (c->o.precode.num_explicit_lens = DEFLATE_NUM_PRECODE_SYMS;
       c->o.precode.num_explicit_lens > 4;
       c->o.precode.num_explicit_lens--)
    if (c->o.precode.lens[deflate_precode_lens_permutation[
        c->o.precode.num_explicit_lens - 1]] != 0)
      break;

  /* Restore the offset codeword lengths if needed. */
  if (c->o.precode.num_litlen_syms != DEFLATE_NUM_LITLEN_SYMS) {
    memmove((u8 *)&c->codes.lens + DEFLATE_NUM_LITLEN_SYMS,
      (u8 *)&c->codes.lens + c->o.precode.num_litlen_syms,
      c->o.precode.num_offset_syms);
  }
}

/*
 * To make it faster to output matches, compute the "full" match length
 * codewords, i.e. the concatenation of the litlen codeword and the extra bits
 * for each possible match length.
 */
static void
deflate_compute_full_len_codewords(struct libdeflate_compressor *c,
           const struct deflate_codes *codes)
{
  u32 len;

  STATIC_ASSERT(MAX_LITLEN_CODEWORD_LEN +
          DEFLATE_MAX_EXTRA_LENGTH_BITS <= 32);

  for (len = DEFLATE_MIN_MATCH_LEN; len <= DEFLATE_MAX_MATCH_LEN; len++) {
    unsigned slot = deflate_length_slot[len];
    unsigned litlen_sym = DEFLATE_FIRST_LEN_SYM + slot;
    u32 extra_bits = len - deflate_length_slot_base[slot];

    c->o.length.codewords[len] =
      codes->codewords.litlen[litlen_sym] |
      (extra_bits << codes->lens.litlen[litlen_sym]);
    c->o.length.lens[len] = codes->lens.litlen[litlen_sym] +
          deflate_extra_length_bits[slot];
  }
}

/* Write a match to the output buffer. */
#define WRITE_MATCH(c_, codes_, length_, offset_, offset_slot_)   \
do {                 \
  const struct libdeflate_compressor *c__ = (c_);     \
  const struct deflate_codes *codes__ = (codes_);     \
  u32 length__ = (length_);         \
  u32 offset__ = (offset_);         \
  unsigned offset_slot__ = (offset_slot_);      \
                  \
  /* Litlen symbol and extra length bits */     \
  STATIC_ASSERT(CAN_BUFFER(MAX_LITLEN_CODEWORD_LEN +   \
         DEFLATE_MAX_EXTRA_LENGTH_BITS)); \
  ADD_BITS(c__->o.length.codewords[length__],     \
     c__->o.length.lens[length__]);       \
                  \
  if (!CAN_BUFFER(MAX_LITLEN_CODEWORD_LEN +      \
      DEFLATE_MAX_EXTRA_LENGTH_BITS +     \
      MAX_OFFSET_CODEWORD_LEN +     \
      DEFLATE_MAX_EXTRA_OFFSET_BITS))     \
    FLUSH_BITS();           \
                  \
  /* Offset symbol */           \
  ADD_BITS(codes__->codewords.offset[offset_slot__],    \
     codes__->lens.offset[offset_slot__]);      \
                  \
  if (!CAN_BUFFER(MAX_OFFSET_CODEWORD_LEN +      \
      DEFLATE_MAX_EXTRA_OFFSET_BITS))     \
    FLUSH_BITS();           \
                  \
  /* Extra offset bits */           \
  ADD_BITS(offset__ - deflate_offset_slot_base[offset_slot__],  \
     deflate_extra_offset_bits[offset_slot__]);   \
                  \
  FLUSH_BITS();             \
} while (0)

/*
 * Choose the best type of block to use (dynamic Huffman, static Huffman, or
 * uncompressed), then output it.
 *
 * The uncompressed data of the block is @block_begin[0..@block_length-1].  The
 * sequence of literals and matches that will be used to compress the block (if
 * a compressed block is chosen) is given by @sequences if it's non-NULL, or
 * else @c->p.n.optimum_nodes.  @c->freqs and @c->codes must be already set
 * according to the literals, matches, and end-of-block symbol.
 */
static void
deflate_flush_block(struct libdeflate_compressor *c,
        struct deflate_output_bitstream *os,
        const u8 *block_begin, u32 block_length,
        const struct deflate_sequence *sequences,
        bool is_final_block)
{
  /*
   * It is hard to get compilers to understand that writes to 'os->next'
   * don't alias 'os'.  That hurts performance significantly, as
   * everything in 'os' would keep getting re-loaded.  ('restrict'
   * *should* do the trick, but it's unreliable.)  Therefore, we keep all
   * the output bitstream state in local variables, and output bits using
   * macros.  This is similar to what the decompressor does.
   */
  const u8 *in_next = block_begin;
  const u8 * const in_end = block_begin + block_length;
  bitbuf_t bitbuf = os->bitbuf;
  unsigned bitcount = os->bitcount;
  u8 *out_next = os->next;
  u8 * const out_fast_end =
    os->end - MIN(WORDBYTES - 1, os->end - out_next);
  /*
   * The cost for each block type, in bits.  Start with the cost of the
   * block header which is 3 bits.
   */
  u32 dynamic_cost = 3;
  u32 static_cost = 3;
  u32 uncompressed_cost = 3;
  u32 best_cost;
  struct deflate_codes *codes;
  unsigned sym;

  ASSERT(block_length >= MIN_BLOCK_LENGTH ||
         (is_final_block && block_length > 0));
  ASSERT(block_length <= MAX_BLOCK_LENGTH);
  ASSERT(bitcount <= 7);
  ASSERT((bitbuf & ~(((bitbuf_t)1 << bitcount) - 1)) == 0);
  ASSERT(out_next <= os->end);
  ASSERT(!os->overflow);

  /* Precompute the precode items and build the precode. */
  deflate_precompute_huffman_header(c);

  /* Account for the cost of encoding dynamic Huffman codes. */
  dynamic_cost += 5 + 5 + 4 + (3 * c->o.precode.num_explicit_lens);
  for (sym = 0; sym < DEFLATE_NUM_PRECODE_SYMS; sym++) {
    u32 extra = deflate_extra_precode_bits[sym];

    dynamic_cost += c->o.precode.freqs[sym] *
        (extra + c->o.precode.lens[sym]);
  }

  /* Account for the cost of encoding literals. */
  for (sym = 0; sym < 144; sym++) {
    dynamic_cost += c->freqs.litlen[sym] *
        c->codes.lens.litlen[sym];
    static_cost += c->freqs.litlen[sym] * 8;
  }
  for (; sym < 256; sym++) {
    dynamic_cost += c->freqs.litlen[sym] *
        c->codes.lens.litlen[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 += (-(bitcount + 3) & 7) + 32 +
           (40 * (DIV_ROUND_UP(block_length,
             UINT16_MAX) - 1)) +
           (8 * block_length);

  /*
   * Choose and output the cheapest type of block.  If there is a tie,
   * prefer uncompressed, then static, then dynamic.
   */

  best_cost = MIN(dynamic_cost, MIN(static_cost, uncompressed_cost));

  /* If the block isn't going to fit, then stop early. */
  if (DIV_ROUND_UP(bitcount + best_cost, 8) > os->end - out_next) {
    os->overflow = true;
    return;
  }
  /*
   * Else, now we know that the block fits, so no further bounds checks on
   * the output buffer are required until the next block.
   */

  if (best_cost == uncompressed_cost) {
    /*
     * Uncompressed block(s).  DEFLATE limits the length of
     * uncompressed blocks to UINT16_MAX bytes, so if the length of
     * the "block" we're flushing is over UINT16_MAX, we actually
     * output multiple blocks.
     */
    do {
      u8 bfinal = 0;
      size_t len = UINT16_MAX;

      if (in_end - in_next <= UINT16_MAX) {
        bfinal = is_final_block;
        len = in_end - in_next;
      }
      /* It was already checked that there is enough space. */
      ASSERT(os->end - out_next >=
             DIV_ROUND_UP(bitcount + 3, 8) + 4 + len);
      /*
       * Output BFINAL (1 bit) and BTYPE (2 bits), then align
       * to a byte boundary.
       */
      STATIC_ASSERT(DEFLATE_BLOCKTYPE_UNCOMPRESSED == 0);
      *out_next++ = (bfinal << bitcount) | bitbuf;
      if (bitcount > 5)
        *out_next++ = 0;
      bitbuf = 0;
      bitcount = 0;
      /* Output LEN and NLEN, then the data itself. */
      put_unaligned_le16(len, out_next);
      out_next += 2;
      put_unaligned_le16(~len, out_next);
      out_next += 2;
      memcpy(out_next, in_next, len);
      out_next += len;
      in_next += len;
    } while (in_next != in_end);
    /* Done outputting uncompressed block(s) */
    goto out;
  }

  if (best_cost == static_cost) {
    /* Static Huffman block */
    codes = &c->static_codes;
    ADD_BITS(is_final_block, 1);
    ADD_BITS(DEFLATE_BLOCKTYPE_STATIC_HUFFMAN, 2);
    FLUSH_BITS();
  } else {
    const unsigned num_explicit_lens = c->o.precode.num_explicit_lens;
    const unsigned num_precode_items = c->o.precode.num_items;
    unsigned precode_sym, precode_item;
    unsigned i;

    /* Dynamic Huffman block */

    codes = &c->codes;
    STATIC_ASSERT(CAN_BUFFER(1 + 2 + 5 + 5 + 4 + 3));
    ADD_BITS(is_final_block, 1);
    ADD_BITS(DEFLATE_BLOCKTYPE_DYNAMIC_HUFFMAN, 2);
    ADD_BITS(c->o.precode.num_litlen_syms - 257, 5);
    ADD_BITS(c->o.precode.num_offset_syms - 1, 5);
    ADD_BITS(num_explicit_lens - 4, 4);

    /* Output the lengths of the codewords in the precode. */
    if (CAN_BUFFER(3 * (DEFLATE_NUM_PRECODE_SYMS - 1))) {
      /*
       * A 64-bit bitbuffer is just one bit too small to hold
       * the maximum number of precode lens, so to minimize
       * flushes we merge one len with the previous fields.
       */
      precode_sym = deflate_precode_lens_permutation[0];
      ADD_BITS(c->o.precode.lens[precode_sym], 3);
      FLUSH_BITS();
      i = 1; /* num_explicit_lens >= 4 */
      do {
        precode_sym =
          deflate_precode_lens_permutation[i];
        ADD_BITS(c->o.precode.lens[precode_sym], 3);
      } while (++i < num_explicit_lens);
      FLUSH_BITS();
    } else {
      FLUSH_BITS();
      i = 0;
      do {
        precode_sym =
          deflate_precode_lens_permutation[i];
        ADD_BITS(c->o.precode.lens[precode_sym], 3);
        FLUSH_BITS();
      } while (++i < num_explicit_lens);
    }

    /*
     * Output the lengths of the codewords in the litlen and offset
     * codes, encoded by the precode.
     */
    i = 0;
    do {
      precode_item = c->o.precode.items[i];
      precode_sym = precode_item & 0x1F;
      STATIC_ASSERT(CAN_BUFFER(MAX_PRE_CODEWORD_LEN + 7));
      ADD_BITS(c->o.precode.codewords[precode_sym],
         c->o.precode.lens[precode_sym]);
      ADD_BITS(precode_item >> 5,
         deflate_extra_precode_bits[precode_sym]);
      FLUSH_BITS();
    } while (++i < num_precode_items);
  }

  /* Output the literals and matches for a dynamic or static block. */
  ASSERT(bitcount <= 7);
  deflate_compute_full_len_codewords(c, codes);
#if SUPPORT_NEAR_OPTIMAL_PARSING
  if (sequences == NULL) {
    /* Output the literals and matches from the minimum-cost path */
    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 {
      u32 length = cur_node->item & OPTIMUM_LEN_MASK;
      u32 offset = cur_node->item >> OPTIMUM_OFFSET_SHIFT;

      if (length == 1) {
        /* Literal */
        ADD_BITS(codes->codewords.litlen[offset],
           codes->lens.litlen[offset]);
        FLUSH_BITS();
      } else {
        /* Match */
        WRITE_MATCH(c, codes, length, offset,
              c->p.n.offset_slot_full[offset]);
      }
      cur_node += length;
    } while (cur_node != end_node);
  } else
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
  {
    /* Output the literals and matches from the sequences list. */
    const struct deflate_sequence *seq;

    for (seq = sequences; ; seq++) {
      u32 litrunlen = seq->litrunlen_and_length &
          SEQ_LITRUNLEN_MASK;
      u32 length = seq->litrunlen_and_length >>
             SEQ_LENGTH_SHIFT;
      unsigned lit;

      /* Output a run of literals. */
      if (CAN_BUFFER(4 * MAX_LITLEN_CODEWORD_LEN)) {
        for (; litrunlen >= 4; litrunlen -= 4) {
          lit = *in_next++;
          ADD_BITS(codes->codewords.litlen[lit],
             codes->lens.litlen[lit]);
          lit = *in_next++;
          ADD_BITS(codes->codewords.litlen[lit],
             codes->lens.litlen[lit]);
          lit = *in_next++;
          ADD_BITS(codes->codewords.litlen[lit],
             codes->lens.litlen[lit]);
          lit = *in_next++;
          ADD_BITS(codes->codewords.litlen[lit],
             codes->lens.litlen[lit]);
          FLUSH_BITS();
        }
        if (litrunlen-- != 0) {
          lit = *in_next++;
          ADD_BITS(codes->codewords.litlen[lit],
             codes->lens.litlen[lit]);
          if (litrunlen-- != 0) {
            lit = *in_next++;
            ADD_BITS(codes->codewords.litlen[lit],
               codes->lens.litlen[lit]);
            if (litrunlen-- != 0) {
              lit = *in_next++;
              ADD_BITS(codes->codewords.litlen[lit],
                 codes->lens.litlen[lit]);
            }
          }
          FLUSH_BITS();
        }
      } else {
        while (litrunlen--) {
          lit = *in_next++;
          ADD_BITS(codes->codewords.litlen[lit],
             codes->lens.litlen[lit]);
          FLUSH_BITS();
        }
      }

      if (length == 0) { /* Last sequence? */
        ASSERT(in_next == in_end);
        break;
      }

      /* Output a match. */
      WRITE_MATCH(c, codes, length, seq->offset,
            seq->offset_slot);
      in_next += length;
    }
  }

  /* Output the end-of-block symbol. */
  ASSERT(bitcount <= 7);
  ADD_BITS(codes->codewords.litlen[DEFLATE_END_OF_BLOCK],
     codes->lens.litlen[DEFLATE_END_OF_BLOCK]);
  FLUSH_BITS();
out:
  ASSERT(bitcount <= 7);
  /*
   * Assert that the block cost was computed correctly.  This is relied on
   * above for the bounds check on the output buffer.  Also,
   * libdeflate_deflate_compress_bound() relies on this via the assumption
   * that uncompressed blocks will always be used when cheapest.
   */
  ASSERT(8 * (out_next - os->next) + bitcount - os->bitcount == best_cost);
  os->bitbuf = bitbuf;
  os->bitcount = bitcount;
  os->next = out_next;
}

static void
deflate_finish_block(struct libdeflate_compressor *c,
         struct deflate_output_bitstream *os,
         const u8 *block_begin, u32 block_length,
         const struct deflate_sequence *sequences,
         bool is_final_block)
{
  c->freqs.litlen[DEFLATE_END_OF_BLOCK]++;
  deflate_make_huffman_codes(&c->freqs, &c->codes);
  deflate_flush_block(c, os, block_begin, block_length, sequences,
          is_final_block);
}

/******************************************************************************/

/*
 * 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 probabilities, 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 probability distributions are "different enough"
 * to start a new block, the simple 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, u32 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) {
    /*
     * Compute the sum of absolute differences of probabilities.  To
     * avoid needing to use floating point arithmetic or do slow
     * divisions, we do all arithmetic with the probabilities
     * multiplied by num_observations * num_new_observations.  E.g.,
     * for the "old" observations the probabilities would be
     * (double)observations[i] / num_observations, but since we
     * multiply by both num_observations and num_new_observations we
     * really do observations[i] * num_new_observations.
     */
    u32 total_delta = 0;
    u32 num_items;
    u32 cutoff;
    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;
    }

    num_items = stats->num_observations +
          stats->num_new_observations;
    /*
     * Heuristic: the cutoff is when the sum of absolute differences
     * of probabilities becomes at least 200/512.  As above, the
     * probability is multiplied by both num_new_observations and
     * num_observations.  Be careful to avoid integer overflow.
     */
    cutoff = stats->num_new_observations * 200 / 512 *
       stats->num_observations;
    /*
     * Very short blocks have a lot of overhead for the Huffman
     * codes, so only use them if it clearly seems worthwhile.
     * (This is an additional penalty, which adds to the smaller
     * penalty below which scales more slowly.)
     */
    if (block_length < 10000 && num_items < 8192)
      cutoff += (u64)cutoff * (8192 - num_items) / 8192;

    /* Ready to end the block? */
    if (total_delta +
        (block_length / 4096) * stats->num_observations >= cutoff)
      return true;
  }
  merge_new_observations(stats);
  return false;
}

static forceinline bool
ready_to_check_block(const struct block_split_stats *stats,
         const u8 *in_block_begin, const u8 *in_next,
         const u8 *in_end)
{
  return stats->num_new_observations >= NUM_OBSERVATIONS_PER_BLOCK_CHECK
    && in_next - in_block_begin >= MIN_BLOCK_LENGTH
    && in_end - in_next >= MIN_BLOCK_LENGTH;
}

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 try to end the block (again)? */
  if (!ready_to_check_block(stats, in_block_begin, in_next, in_end))
    return false;

  return do_end_block_check(stats, in_next - in_block_begin);
}

/******************************************************************************/

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,
           bool gather_split_stats, struct deflate_sequence *seq)
{
  c->freqs.litlen[literal]++;

  if (gather_split_stats)
    observe_literal(&c->split_stats, literal);

  STATIC_ASSERT(MAX_BLOCK_LENGTH <= SEQ_LITRUNLEN_MASK);
  seq->litrunlen_and_length++;
}

static forceinline void
deflate_choose_match(struct libdeflate_compressor *c,
         u32 length, u32 offset, bool gather_split_stats,
         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(offset);

  c->freqs.litlen[DEFLATE_FIRST_LEN_SYM + length_slot]++;
  c->freqs.offset[offset_slot]++;
  if (gather_split_stats)
    observe_match(&c->split_stats, length);

  seq->litrunlen_and_length |= length << SEQ_LENGTH_SHIFT;
  seq->offset = offset;
  seq->offset_slot = offset_slot;

  seq++;
  seq->litrunlen_and_length = 0;
  *seq_p = seq;
}

/*
 * 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(u32 *max_len, u32 *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 u32
choose_min_match_len(u32 num_used_literals, u32 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. */
  };
  u32 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 u32
calculate_min_match_len(const u8 *data, size_t data_len, u32 max_search_depth)
{
  u8 used[256] = { 0 };
  u32 num_used_literals = 0;
  size_t i;

  /*
   * For very short inputs, the static Huffman code has a good chance of
   * being best, in which case there is no reason to avoid short matches.
   */
  if (data_len < 512)
    return DEFLATE_MIN_MATCH_LEN;

  /*
   * For an initial approximation, scan the first 4 KiB of data.  The
   * caller may use recalculate_min_match_len() to update 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 u32
recalculate_min_match_len(const struct deflate_freqs *freqs,
        u32 max_search_depth)
{
  u32 literal_freq = 0;
  u32 cutoff;
  u32 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);
}

static forceinline const u8 *
choose_max_block_end(const u8 *in_block_begin, const u8 *in_end,
         size_t soft_max_len)
{
  if (in_end - in_block_begin < soft_max_len + MIN_BLOCK_LENGTH)
    return in_end;
  return in_block_begin + soft_max_len;
}

/*
 * This is the level 0 "compressor".  It always outputs uncompressed blocks.
 */
static size_t
deflate_compress_none(const u8 *in, size_t in_nbytes,
          u8 *out, size_t out_nbytes_avail)
{
  const u8 *in_next = in;
  const u8 * const in_end = in + in_nbytes;
  u8 *out_next = out;
  u8 * const out_end = out + out_nbytes_avail;

  /*
   * If the input is zero-length, we still must output a block in order
   * for the output to be a valid DEFLATE stream.  Handle this case
   * specially to avoid potentially passing NULL to memcpy() below.
   */
  if (unlikely(in_nbytes == 0)) {
    if (out_nbytes_avail < 5)
      return 0;
    /* BFINAL and BTYPE */
    *out_next++ = 1 | (DEFLATE_BLOCKTYPE_UNCOMPRESSED << 1);
    /* LEN and NLEN */
    put_unaligned_le32(0xFFFF0000, out_next);
    return 5;
  }

  do {
    u8 bfinal = 0;
    size_t len = UINT16_MAX;

    if (in_end - in_next <= UINT16_MAX) {
      bfinal = 1;
      len = in_end - in_next;
    }
    if (out_end - out_next < 5 + len)
      return 0;
    /*
     * Output BFINAL and BTYPE.  The stream is already byte-aligned
     * here, so this step always requires outputting exactly 1 byte.
     */
    *out_next++ = bfinal | (DEFLATE_BLOCKTYPE_UNCOMPRESSED << 1);

    /* Output LEN and NLEN, then the data itself. */
    put_unaligned_le16(len, out_next);
    out_next += 2;
    put_unaligned_le16(~len, out_next);
    out_next += 2;
    memcpy(out_next, in_next, len);
    out_next += len;
    in_next += len;
  } while (in_next != in_end);

  return out_next - out;
}

/*
 * 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 void
deflate_compress_fastest(struct libdeflate_compressor * restrict c,
       const u8 *in, size_t in_nbytes,
       struct deflate_output_bitstream *os)
{
  const u8 *in_next = in;
  const u8 *in_end = in_next + in_nbytes;
  const u8 *in_cur_base = in_next;
  u32 max_len = DEFLATE_MAX_MATCH_LEN;
  u32 nice_len = MIN(c->nice_match_length, max_len);
  u32 next_hash = 0;

  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 = choose_max_block_end(
        in_next, in_end, FAST_SOFT_MAX_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++, false, 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, false,
                 &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++, false,
                   seq);
      }

      /* Check if it's time to output another block. */
    } while (in_next < in_max_block_end &&
       seq < &c->p.f.sequences[FAST_SEQ_STORE_LENGTH]);

    deflate_finish_block(c, os, in_block_begin,
             in_next - in_block_begin,
             c->p.f.sequences, in_next == in_end);
  } while (in_next != in_end && !os->overflow);
}

/*
 * Whether the greedy and lazy compressors use multi-hop links in the hash
 * chains matchfinder (see 'next_tab' in hc_matchfinder.h).  Multi-hop links
 * make searching faster but inserting slower, so they pay off only for the
 * deeper searches done by the lazy compressors.  The choice doesn't affect the
 * output.
 */
#define GREEDY_MULTI_HOP  false
#define LAZY_MULTI_HOP    true

/*
 * This is the "greedy" DEFLATE compressor. It always chooses the longest match.
 */
static void
deflate_compress_greedy(struct libdeflate_compressor * restrict c,
      const u8 *in, size_t in_nbytes,
      struct deflate_output_bitstream *os)
{
  const u8 *in_next = in;
  const u8 *in_end = in_next + in_nbytes;
  const u8 *in_cur_base = in_next;
  u32 max_len = DEFLATE_MAX_MATCH_LEN;
  u32 nice_len = MIN(c->nice_match_length, max_len);
  u32 next_hashes[2] = {0, 0};

  hc_matchfinder_init(&c->p.g.hc_mf, GREEDY_MULTI_HOP);

  do {
    /* Starting a new DEFLATE block */

    const u8 * const in_block_begin = in_next;
    const u8 * const in_max_block_end = choose_max_block_end(
        in_next, in_end, SOFT_MAX_BLOCK_LENGTH);
    struct deflate_sequence *seq = c->p.g.sequences;
    u32 min_len;

    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,
            GREEDY_MULTI_HOP);

      if (length >= min_len &&
          (length > DEFLATE_MIN_MATCH_LEN ||
           offset <= 4096)) {
        /* Match found */
        deflate_choose_match(c, length, offset, true,
                 &seq);
        hc_matchfinder_skip_bytes(&c->p.g.hc_mf,
                &in_cur_base,
                in_next + 1,
                in_end,
                length - 1,
                next_hashes,
                GREEDY_MULTI_HOP);
        in_next += length;
      } else {
        /* No match found */
        deflate_choose_literal(c, *in_next++, true,
                   seq);
      }

      /* Check if it's time to output another block. */
    } while (in_next < in_max_block_end &&
       seq < &c->p.g.sequences[SEQ_STORE_LENGTH] &&
       !should_end_block(&c->split_stats,
             in_block_begin, in_next, in_end));

    deflate_finish_block(c, os, in_block_begin,
             in_next - in_block_begin,
             c->p.g.sequences, in_next == in_end);
  } while (in_next != in_end && !os->overflow);
}

static forceinline void
deflate_compress_lazy_generic(struct libdeflate_compressor * restrict c,
            const u8 *in, size_t in_nbytes,
            struct deflate_output_bitstream *os, bool lazy2)
{
  const u8 *in_next = in;
  const u8 *in_end = in_next + in_nbytes;
  const u8 *in_cur_base = in_next;
  u32 max_len = DEFLATE_MAX_MATCH_LEN;
  u32 nice_len = MIN(c->nice_match_length, max_len);
  u32 next_hashes[2] = {0, 0};

  hc_matchfinder_init(&c->p.g.hc_mf, LAZY_MULTI_HOP);

  do {
    /* Starting a new DEFLATE block */

    const u8 * const in_block_begin = in_next;
    const u8 * const in_max_block_end = choose_max_block_end(
        in_next, in_end, SOFT_MAX_BLOCK_LENGTH);
    const u8 *next_recalc_min_len =
      in_next + MIN(in_end - in_next, 10000);
    struct deflate_sequence *seq = c->p.g.sequences;
    u32 min_len;

    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 cur_len;
      u32 cur_offset;
      u32 next_len;
      u32 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,
            LAZY_MULTI_HOP);
      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++, true,
                   seq);
        continue;
      }
      in_next++;

have_cur_match:
      /*
       * 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,
                 true, &seq);
        hc_matchfinder_skip_bytes(&c->p.g.hc_mf,
                &in_cur_base,
                in_next,
                in_end,
                cur_len - 1,
                next_hashes,
                LAZY_MULTI_HOP);
        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,
            LAZY_MULTI_HOP);
      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), true,
                   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,
            LAZY_MULTI_HOP);
        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), true, seq);
          deflate_choose_literal(
            c, *(in_next - 2), true, 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,
                 true, &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,
                  LAZY_MULTI_HOP);
          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,
                 true, &seq);
        hc_matchfinder_skip_bytes(&c->p.g.hc_mf,
                &in_cur_base,
                in_next,
                in_end,
                cur_len - 2,
                next_hashes,
                LAZY_MULTI_HOP);
        in_next += cur_len - 2;
      }
      /* Check if it's time to output another block. */
    } while (in_next < in_max_block_end &&
       seq < &c->p.g.sequences[SEQ_STORE_LENGTH] &&
       !should_end_block(&c->split_stats,
             in_block_begin, in_next, in_end));

    deflate_finish_block(c, os, in_block_begin,
             in_next - in_block_begin,
             c->p.g.sequences, in_next == in_end);
  } while (in_next != in_end && !os->overflow);
}

/*
 * 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 void
deflate_compress_lazy(struct libdeflate_compressor * restrict c,
          const u8 *in, size_t in_nbytes,
          struct deflate_output_bitstream *os)
{
  deflate_compress_lazy_generic(c, in, in_nbytes, os, 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 void
deflate_compress_lazy2(struct libdeflate_compressor * restrict c,
           const u8 *in, size_t in_nbytes,
           struct deflate_output_bitstream *os)
{
  deflate_compress_lazy_generic(c, in, in_nbytes, os, 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 {
    u32 length = cur_node->item & OPTIMUM_LEN_MASK;
    u32 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[c->p.n.offset_slot_full[offset]]++;
    }
    cur_node += length;
  } while (cur_node != end_node);

  /* Tally the end-of-block symbol. */
  c->freqs.litlen[DEFLATE_END_OF_BLOCK]++;
}

static void
deflate_choose_all_literals(struct libdeflate_compressor *c,
          const u8 *block, u32 block_length)
{
  u32 i;

  deflate_reset_symbol_frequencies(c);
  for (i = 0; i < block_length; i++)
    c->freqs.litlen[block[i]]++;
  c->freqs.litlen[DEFLATE_END_OF_BLOCK]++;

  deflate_make_huffman_codes(&c->freqs, &c->codes);
}

/*
 * Compute the exact cost, in bits, that would be required to output the matches
 * and literals described by @c->freqs as a dynamic Huffman block.  The litlen
 * and offset codes are assumed to have already been built in @c->codes.
 */
static u32
deflate_compute_true_cost(struct libdeflate_compressor *c)
{
  u32 cost = 0;
  unsigned sym;

  deflate_precompute_huffman_header(c);

  memset(&c->codes.lens.litlen[c->o.precode.num_litlen_syms], 0,
         DEFLATE_NUM_LITLEN_SYMS - c->o.precode.num_litlen_syms);

  cost += 5 + 5 + 4 + (3 * c->o.precode.num_explicit_lens);
  for (sym = 0; sym < DEFLATE_NUM_PRECODE_SYMS; sym++) {
    cost += c->o.precode.freqs[sym] *
      (c->o.precode.lens[sym] +
       deflate_extra_precode_bits[sym]);
  }

  for (sym = 0; sym < DEFLATE_FIRST_LEN_SYM; sym++)
    cost += c->freqs.litlen[sym] * c->codes.lens.litlen[sym];

  for (; sym < DEFLATE_FIRST_LEN_SYM +
         ARRAY_LEN(deflate_extra_length_bits); sym++)
    cost += c->freqs.litlen[sym] *
      (c->codes.lens.litlen[sym] +
       deflate_extra_length_bits[sym - DEFLATE_FIRST_LEN_SYM]);

  for (sym = 0; sym < ARRAY_LEN(deflate_extra_offset_bits); sym++)
    cost += c->freqs.offset[sym] *
      (c->codes.lens.offset[sym] +
       deflate_extra_offset_bits[sym]);
  return cost;
}

/* 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 in a useful way.
 */
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,
            const u8 *block_begin, u32 block_length,
            u32 *lit_cost, u32 *len_sym_cost)
{
  u32 num_used_literals = 0;
  u32 literal_freq = block_length;
  u32 match_freq = 0;
  u32 cutoff;
  u32 i;

  /* Calculate the number of distinct literals that exist in the data. */
  memset(c->freqs.litlen, 0,
         DEFLATE_NUM_LITERALS * sizeof(c->freqs.litlen[0]));
  cutoff = literal_freq >> 11; /* Ignore literals used very rarely. */
  for (i = 0; i < block_length; i++)
    c->freqs.litlen[block_begin[i]]++;
  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 */

  STATIC_ASSERT(BIT_COST == 16);
  *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(u32 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)
{
  u32 i;

  /*
   * Also zero the entries that are never used, so that cost models can
   * be compared with memcmp().
   */
  memset(&c->p.n.costs, 0, sizeof(c->p.n.costs));

  /* 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)
{
  u32 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 are optimized for the data already, it can be helpful
 * to reuse them instead of starting over with the default costs.  However, this
 * depends on how similar the new block is to the previous block.  Therefore,
 * use a heuristic to decide how similar the blocks are, and mix together the
 * current costs and the default costs accordingly.
 */
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 (total_delta > 3 * cutoff)
    /* Big change in the data; just use the default costs. */
    deflate_set_default_costs(c, lit_cost, len_sym_cost);
  else 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);
}

static void
deflate_set_initial_costs(struct libdeflate_compressor *c,
        const u8 *block_begin, u32 block_length,
        bool is_first_block)
{
  u32 lit_cost, len_sym_cost;

  deflate_choose_default_litlen_costs(c, block_begin, 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);
}

/*
 * 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;
    u32 literal;
    u32 best_cost_to_end;
    u32 best_item;

    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;
    best_item = (literal << OPTIMUM_OFFSET_SHIFT) | 1;

    /* Also consider matches if there are any. */
    if (num_matches) {
      const struct lz_match *match;
      u32 len;
      u32 offset;
      u32 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 {
        u32 offset_bits;

        offset = match->offset;
        offset_slot = c->p.n.offset_slot_full[offset];
        offset_cost =
          c->p.n.costs.offset_slot[offset_slot];
        offset_bits = offset << OPTIMUM_OFFSET_SHIFT;
        do {
          bool better;

          cost_to_end = offset_cost +
            c->p.n.costs.length[len] +
            (cur_node + len)->cost_to_end;
          /*
           * Whether this is the best choice so
           * far is hard to predict, so update the
           * best choice without branching.
           */
          better = cost_to_end < best_cost_to_end;
          best_cost_to_end = better ?
            cost_to_end : best_cost_to_end;
          best_item = better ?
            (len | offset_bits) : best_item;
        } while (++len <= match->length);
      } while (++match != cache_ptr);
      cache_ptr -= num_matches;
    }
    cur_node->cost_to_end = best_cost_to_end;
    cur_node->item = best_item;
  } while (cur_node != &c->p.n.optimum_nodes[0]);

  deflate_reset_symbol_frequencies(c);
  deflate_tally_item_list(c, block_length);
  deflate_make_huffman_codes(&c->freqs, &c->codes);
}

/*
 * Choose the literals and matches for the current block, then output the block.
 *
 * To choose the literal/match sequence, we find the minimum-cost path through
 * the block's graph of literal/match choices, given a cost model.  However, the
 * true 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 when it seems appropriate.  Later passes use the
 * Huffman codeword lengths from the previous pass as the costs.
 *
 * As an alternate strategy, also consider using only literals.  The boolean
 * returned in *used_only_literals indicates whether that strategy was best.
 */
static void
deflate_optimize_and_flush_block(struct libdeflate_compressor *c,
         struct deflate_output_bitstream *os,
         const u8 *block_begin, u32 block_length,
         const struct lz_match *cache_ptr,
         bool is_first_block, bool is_final_block,
         bool *used_only_literals)
{
  unsigned num_passes_remaining = c->p.n.max_optim_passes;
  u32 best_true_cost = UINT32_MAX;
  u32 true_cost;
  u32 only_lits_cost;
  u32 static_cost = UINT32_MAX;
  struct deflate_sequence seq_;
  struct deflate_sequence *seq = NULL;
  u32 i;

  /*
   * On some data, using only literals (no matches) ends up being better
   * than what the iterative optimization algorithm produces.  Therefore,
   * consider using only literals.
   */
  deflate_choose_all_literals(c, block_begin, block_length);
  only_lits_cost = deflate_compute_true_cost(c);

  /*
   * 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;

  /*
   * Sometimes a static Huffman block ends up being cheapest, particularly
   * if the block is small.  So, if the block is sufficiently small, find
   * the optimal static block solution and remember its cost.
   */
  if (block_length <= c->p.n.max_len_to_optimize_static_block) {
    /* Save c->p.n.costs temporarily. */
    c->p.n.costs_saved = c->p.n.costs;

    deflate_set_costs_from_codes(c, &c->static_codes.lens);
    deflate_find_min_cost_path(c, block_length, cache_ptr);
    static_cost = c->p.n.optimum_nodes[0].cost_to_end / BIT_COST;
    static_cost += 7; /* for the end-of-block symbol */

    /* Restore c->p.n.costs. */
    c->p.n.costs = c->p.n.costs_saved;
  }

  /* Initialize c->p.n.costs with default costs. */
  deflate_set_initial_costs(c, block_begin, block_length, is_first_block);

  do {
    /*
     * Find the minimum-cost path for this pass.
     * Also set c->freqs and c->codes to match the path.
     */
    deflate_find_min_cost_path(c, block_length, cache_ptr);

    /*
     * Compute the exact cost of the block if the path were to be
     * used.  Note that this differs from
     * c->p.n.optimum_nodes[0].cost_to_end in that true_cost uses
     * the actual Huffman codes instead of c->p.n.costs.
     */
    true_cost = deflate_compute_true_cost(c);

    /*
     * If the cost didn't improve much from the previous pass, then
     * doing more passes probably won't be helpful, so stop early.
     */
    if (true_cost + c->p.n.min_improvement_to_continue >
        best_true_cost)
      break;

    best_true_cost = true_cost;

    /* Save the cost model that gave 'best_true_cost'. */
    c->p.n.costs_saved = c->p.n.costs;

    /* Update the cost model from the Huffman codes. */
    deflate_set_costs_from_codes(c, &c->codes.lens);

    /*
     * If the cost model didn't change, then another pass would just
     * find the same path again, and then stop because the cost
     * didn't improve.  Skip that pass.
     */
    if (memcmp(&c->p.n.costs, &c->p.n.costs_saved,
         sizeof(c->p.n.costs)) == 0)
      break;
  } while (--num_passes_remaining);

  *used_only_literals = false;
  if (MIN(only_lits_cost, static_cost) < best_true_cost) {
    if (only_lits_cost < static_cost) {
      /* Using only literals ended up being best! */
      deflate_choose_all_literals(c, block_begin, block_length);
      deflate_set_costs_from_codes(c, &c->codes.lens);
      seq_.litrunlen_and_length = block_length;
      seq = &seq_;
      *used_only_literals = true;
    } else {
      /* Static block ended up being best! */
      deflate_set_costs_from_codes(c, &c->static_codes.lens);
      deflate_find_min_cost_path(c, block_length, cache_ptr);
    }
  } else if (true_cost >=
       best_true_cost + c->p.n.min_bits_to_use_nonfinal_path) {
    /*
     * The best solution was actually from a non-final optimization
     * pass, so recover and use the min-cost path from that pass.
     */
    c->p.n.costs = c->p.n.costs_saved;
    deflate_find_min_cost_path(c, block_length, cache_ptr);
    deflate_set_costs_from_codes(c, &c->codes.lens);
  }
  deflate_flush_block(c, os, block_begin, block_length, seq,
          is_final_block);
}

static void
deflate_near_optimal_init_stats(struct libdeflate_compressor *c)
{
  init_block_split_stats(&c->split_stats);
  memset(c->p.n.new_match_len_freqs, 0,
         sizeof(c->p.n.new_match_len_freqs));
  memset(c->p.n.match_len_freqs, 0, sizeof(c->p.n.match_len_freqs));
}

static void
deflate_near_optimal_merge_stats(struct libdeflate_compressor *c)
{
  unsigned i;

  merge_new_observations(&c->split_stats);
  for (i = 0; i < ARRAY_LEN(c->p.n.match_len_freqs); i++) {
    c->p.n.match_len_freqs[i] += c->p.n.new_match_len_freqs[i];
    c->p.n.new_match_len_freqs[i] = 0;
  }
}

/*
 * 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.
 */
static void
deflate_near_optimal_save_stats(struct libdeflate_compressor *c)
{
  int i;

  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;
}

static void
deflate_near_optimal_clear_old_stats(struct libdeflate_compressor *c)
{
  int i;

  for (i = 0; i < NUM_OBSERVATION_TYPES; i++)
    c->split_stats.observations[i] = 0;
  c->split_stats.num_observations = 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 void
deflate_compress_near_optimal(struct libdeflate_compressor * restrict c,
            const u8 *in, size_t in_nbytes,
            struct deflate_output_bitstream *os)
{
  const u8 *in_next = in;
  const u8 *in_block_begin = in_next;
  const u8 *in_end = in_next + in_nbytes;
  const u8 *in_cur_base = in_next;
  const u8 *in_next_slide =
    in_next + MIN(in_end - in_next, MATCHFINDER_WINDOW_SIZE);
  u32 max_len = DEFLATE_MAX_MATCH_LEN;
  u32 nice_len = MIN(c->nice_match_length, max_len);
  struct lz_match *cache_ptr = c->p.n.match_cache;
  u32 next_hashes[2] = {0, 0};
  bool prev_block_used_only_literals = false;

  bt_matchfinder_init(&c->p.n.bt_mf);
  deflate_near_optimal_init_stats(c);

  do {
    /* Starting a new DEFLATE block */
    const u8 * const in_max_block_end = choose_max_block_end(
        in_block_begin, in_end, SOFT_MAX_BLOCK_LENGTH);
    const u8 *prev_end_block_check = NULL;
    bool change_detected = false;
    const u8 *next_observation = in_next;
    u32 min_len;

    /*
     * Use the minimum match length heuristic to improve the
     * literal/match statistics gathered during matchfinding.
     * However, the actual near-optimal parse won't respect min_len,
     * as it can accurately assess the costs of different matches.
     *
     * If the "use only literals" strategy happened to be the best
     * strategy on the previous block, then probably the
     * min_match_len heuristic is still not aggressive enough for
     * the data, so force gathering literal stats only.
     */
    if (prev_block_used_only_literals)
      min_len = DEFLATE_MAX_MATCH_LEN + 1;
    else
      min_len = calculate_min_match_len(
          in_block_begin,
          in_max_block_end - in_block_begin,
          c->max_search_depth);

    /*
     * 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.
     */
    for (;;) {
      struct lz_match *matches;
      u32 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,
            matches);
        if (cache_ptr > matches)
          best_len = cache_ptr[-1].length;
      }
      if (in_next >= next_observation) {
        if (best_len >= min_len) {
          observe_match(&c->split_stats,
                  best_len);
          next_observation = in_next + best_len;
          c->p.n.new_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;
          in_next++;
          cache_ptr++;
        } while (--best_len);
      }
      /* Maximum block length or end of input reached? */
      if (in_next >= in_max_block_end)
        break;
      /* Match cache overflowed? */
      if (cache_ptr >=
          &c->p.n.match_cache[MATCH_CACHE_LENGTH])
        break;
      /* Not ready to try to end the block (again)? */
      if (!ready_to_check_block(&c->split_stats,
              in_block_begin, in_next,
              in_end))
        continue;
      /* Check if it would be worthwhile to end the block. */
      if (do_end_block_check(&c->split_stats,
                 in_next - in_block_begin)) {
        change_detected = true;
        break;
      }
      /* Ending the block doesn't seem worthwhile here. */
      deflate_near_optimal_merge_stats(c);
      prev_end_block_check = in_next;
    }
    /*
     * All the matches for this block have been cached.  Now choose
     * the precise end of the block and the sequence of items to
     * output to represent it, then flush the block.
     */
    if (change_detected && prev_end_block_check != NULL) {
      /*
       * The block is being ended because a recent chunk of
       * data differs from the rest of the block.  We could
       * end the block at 'in_next' like the greedy and lazy
       * compressors do, but that's not ideal since it would
       * include the differing chunk in the block.  The
       * near-optimal compressor has time to do a better job.
       * Therefore, we rewind to just before the chunk, and
       * output a block that only goes up to there.
       *
       * We then set things up to correctly start the next
       * block, considering that some work has already been
       * done on it (some matches found and stats gathered).
       */
      struct lz_match *orig_cache_ptr = cache_ptr;
      const u8 *in_block_end = prev_end_block_check;
      u32 block_length = in_block_end - in_block_begin;
      bool is_first = (in_block_begin == in);
      bool is_final = false;
      u32 num_bytes_to_rewind = in_next - in_block_end;
      size_t cache_len_rewound;

      /* Rewind the match cache. */
      do {
        cache_ptr--;
        cache_ptr -= cache_ptr->length;
      } while (--num_bytes_to_rewind);
      cache_len_rewound = orig_cache_ptr - cache_ptr;

      deflate_optimize_and_flush_block(
            c, os, in_block_begin,
            block_length, cache_ptr,
            is_first, is_final,
            &prev_block_used_only_literals);
      memmove(c->p.n.match_cache, cache_ptr,
        cache_len_rewound * sizeof(*cache_ptr));
      cache_ptr = &c->p.n.match_cache[cache_len_rewound];
      deflate_near_optimal_save_stats(c);
      /*
       * Clear the stats for the just-flushed block, leaving
       * just the stats for the beginning of the next block.
       */
      deflate_near_optimal_clear_old_stats(c);
      in_block_begin = in_block_end;
    } else {
      /*
       * The block is being ended for a reason other than a
       * differing data chunk being detected.  Don't rewind at
       * all; just end the block at the current position.
       */
      u32 block_length = in_next - in_block_begin;
      bool is_first = (in_block_begin == in);
      bool is_final = (in_next == in_end);

      deflate_near_optimal_merge_stats(c);
      deflate_optimize_and_flush_block(
            c, os, in_block_begin,
            block_length, cache_ptr,
            is_first, is_final,
            &prev_block_used_only_literals);
      cache_ptr = &c->p.n.match_cache[0];
      deflate_near_optimal_save_stats(c);
      deflate_near_optimal_init_stats(c);
      in_block_begin = in_next;
    }
  } while (in_next != in_end && !os->overflow);
}

/* Initialize c->p.n.offset_slot_full. */
static void
deflate_init_offset_slot_full(struct libdeflate_compressor *c)
{
  u32 offset_slot;
  u32 offset;
  u32 offset_end;

  for (offset_slot = 0; offset_slot < ARRAY_LEN(deflate_offset_slot_base);
       offset_slot++) {
    offset = deflate_offset_slot_base[offset_slot];
    offset_end = offset +
           (1 << deflate_extra_offset_bits[offset_slot]);
    do {
      c->p.n.offset_slot_full[offset] = offset_slot;
    } while (++offset != offset_end);
  }
}

#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */

LIBDEFLATEAPI struct libdeflate_compressor *
libdeflate_alloc_compressor_ex(int compression_level,
             const struct libdeflate_options *options)
{
  struct libdeflate_compressor *c;
  size_t size = offsetof(struct libdeflate_compressor, p);

  check_buildtime_parameters();

  /*
   * Note: if more fields are added to libdeflate_options, this code will
   * need to be updated to support both the old and new structs.
   */
  if (options->sizeof_options != sizeof(*options))
    return NULL;

  /*
   * Note: For similarity with zlib's API, -1 is accepted as an alias for
   * the default compression level.
   */
  if (compression_level == -1)
    compression_level = 6;

  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) {
      /* Levels 5 and higher use the lazy compressors. */
      size = offsetof(struct libdeflate_compressor,
          p.g.hc_mf) +
             hc_matchfinder_size(compression_level >= 5 ?
               LAZY_MULTI_HOP :
               GREEDY_MULTI_HOP);
    } else if (compression_level == 1) {
      size += sizeof(c->p.f);
    }
  }

  c = libdeflate_aligned_malloc(options->malloc_func ?
              options->malloc_func :
              libdeflate_default_malloc_func,
              MATCHFINDER_MEM_ALIGNMENT, size);
  if (!c)
    return NULL;
  c->free_func = options->free_func ?
           options->free_func : libdeflate_default_free_func;

  c->compression_level = compression_level;

  /*
   * The higher the compression level, the more we should bother trying to
   * compress very small inputs.
   */
  c->max_passthrough_size = 55 - (compression_level * 4);

  switch (compression_level) {
  case 0:
    c->max_passthrough_size = SIZE_MAX;
    c->impl = NULL; /* not used */
    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.max_optim_passes = 2;
    c->p.n.min_improvement_to_continue = 32;
    c->p.n.min_bits_to_use_nonfinal_path = 32;
    c->p.n.max_len_to_optimize_static_block = 0;
    deflate_init_offset_slot_full(c);
    break;
  case 11:
    c->impl = deflate_compress_near_optimal;
    c->max_search_depth = 100;
    c->nice_match_length = 150;
    c->p.n.max_optim_passes = 4;
    c->p.n.min_improvement_to_continue = 16;
    c->p.n.min_bits_to_use_nonfinal_path = 16;
    c->p.n.max_len_to_optimize_static_block = 1000;
    deflate_init_offset_slot_full(c);
    break;
  case 12:
  default:
    c->impl = deflate_compress_near_optimal;
    c->max_search_depth = 300;
    c->nice_match_length = DEFLATE_MAX_MATCH_LEN;
    c->p.n.max_optim_passes = 10;
    c->p.n.min_improvement_to_continue = 1;
    c->p.n.min_bits_to_use_nonfinal_path = 1;
    c->p.n.max_len_to_optimize_static_block = 10000;
    deflate_init_offset_slot_full(c);
    break;
#endif /* SUPPORT_NEAR_OPTIMAL_PARSING */
  }

  deflate_init_static_codes(c);

  return c;
}


LIBDEFLATEAPI struct libdeflate_compressor *
libdeflate_alloc_compressor(int compression_level)
{
  static const struct libdeflate_options defaults = {
    .sizeof_options = sizeof(defaults),
  };
  return libdeflate_alloc_compressor_ex(compression_level, &defaults);
}

LIBDEFLATEAPI size_t
libdeflate_deflate_compress(struct libdeflate_compressor *c,
          const void *in, size_t in_nbytes,
          void *out, size_t out_nbytes_avail)
{
  struct deflate_output_bitstream os;

  /*
   * For extremely short inputs, or for compression level 0, just output
   * uncompressed blocks.
   */
  if (unlikely(in_nbytes <= c->max_passthrough_size))
    return deflate_compress_none(in, in_nbytes,
               out, out_nbytes_avail);

  /* Initialize the output bitstream structure. */
  os.bitbuf = 0;
  os.bitcount = 0;
  os.next = out;
  os.end = os.next + out_nbytes_avail;
  os.overflow = false;

  /* Call the actual compression function. */
  (*c->impl)(c, in, in_nbytes, &os);

  /* Return 0 if the output buffer is too small. */
  if (os.overflow)
    return 0;

  /*
   * Write the final byte if needed.  This can't overflow the output
   * buffer because deflate_flush_block() would have set the overflow flag
   * if there wasn't enough space remaining for the full final block.
   */
  ASSERT(os.bitcount <= 7);
  if (os.bitcount) {
    ASSERT(os.next < os.end);
    *os.next++ = os.bitbuf;
  }

  /* Return the compressed size in bytes. */
  return os.next - (u8 *)out;
}

LIBDEFLATEAPI void
libdeflate_free_compressor(struct libdeflate_compressor *c)
{
  if (c)
    libdeflate_aligned_free(c->free_func, c);
}

unsigned int
libdeflate_get_compression_level(struct libdeflate_compressor *c)
{
  return c->compression_level;
}

LIBDEFLATEAPI size_t
libdeflate_deflate_compress_bound(struct libdeflate_compressor *c,
          size_t in_nbytes)
{
  size_t max_blocks;

  /*
   * Since the compressor never uses a compressed block when an
   * uncompressed block is cheaper, the worst case can be no worse than
   * the case where only uncompressed blocks are used.
   *
   * This is true even though up to 7 bits are "wasted" to byte-align the
   * bitstream when a compressed block is followed by an uncompressed
   * block.  This is because a compressed block wouldn't have been used if
   * it wasn't cheaper than an uncompressed block, and uncompressed blocks
   * always end on a byte boundary.  So the alignment bits will, at worst,
   * go up to the place where the uncompressed block would have ended.
   */

  /*
   * Calculate the maximum number of uncompressed blocks that the
   * compressor can use for 'in_nbytes' of data.
   *
   * The minimum length that is passed to deflate_flush_block() is
   * MIN_BLOCK_LENGTH bytes, except for the final block if needed.  If
   * deflate_flush_block() decides to use an uncompressed block, it
   * actually will (in general) output a series of uncompressed blocks in
   * order to stay within the UINT16_MAX limit of DEFLATE.  But this can
   * be disregarded here as long as '2 * MIN_BLOCK_LENGTH <= UINT16_MAX',
   * as in that case this behavior can't result in more blocks than the
   * case where deflate_flush_block() is called with min-length inputs.
   *
   * So the number of uncompressed blocks needed would be bounded by
   * DIV_ROUND_UP(in_nbytes, MIN_BLOCK_LENGTH).  However, empty inputs
   * need 1 (empty) block, which gives the final expression below.
   */
  STATIC_ASSERT(2 * MIN_BLOCK_LENGTH <= UINT16_MAX);
  max_blocks = MAX(DIV_ROUND_UP(in_nbytes, MIN_BLOCK_LENGTH), 1);

  /*
   * Each uncompressed block has 5 bytes of overhead, for the BFINAL,
   * BTYPE, LEN, and NLEN fields.  (For the reason explained earlier, the
   * alignment bits at the very start of the block can be disregarded;
   * they would otherwise increase the overhead to 6 bytes per block.)
   * Therefore, the maximum number of overhead bytes is '5 * max_blocks'.
   * To get the final bound, add the number of uncompressed bytes.
   */
  return (5 * max_blocks) + in_nbytes;
}

Coverage Report

Created: 2026-10-28 07:26