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 //
 // Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail dot com)
 //
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 // Official repository: https://github.com/boostorg/beast
 //
 // This is a derivative work based on Zlib, copyright below:
 /*
     Copyright (C) 1995-2018 Jean-loup Gailly and Mark Adler
 
     This software is provided 'as-is', without any express or implied
     warranty.  In no event will the authors be held liable for any damages
     arising from the use of this software.
 
     Permission is granted to anyone to use this software for any purpose,
     including commercial applications, and to alter it and redistribute it
     freely, subject to the following restrictions:
 
     1. The origin of this software must not be misrepresented; you must not
        claim that you wrote the original software. If you use this software
        in a product, an acknowledgment in the product documentation would be
        appreciated but is not required.
     2. Altered source versions must be plainly marked as such, and must not be
        misrepresented as being the original software.
     3. This notice may not be removed or altered from any source distribution.
 
     Jean-loup Gailly        Mark Adler
     jloup@gzip.org          madler@alumni.caltech.edu
 
     The data format used by the zlib library is described by RFCs (Request for
     Comments) 1950 to 1952 in the files http://tools.ietf.org/html/rfc1950
     (zlib format), rfc1951 (deflate format) and rfc1952 (gzip format).
 */
 
 #ifndef BOOST_BEAST_ZLIB_DETAIL_DEFLATE_STREAM_IPP
 #define BOOST_BEAST_ZLIB_DETAIL_DEFLATE_STREAM_IPP
 
 #include <boost/beast/zlib/detail/deflate_stream.hpp>
 #include <boost/beast/zlib/detail/ranges.hpp>
 #include <boost/assert.hpp>
 #include <boost/config.hpp>
 #include <boost/make_unique.hpp>
 #include <boost/optional.hpp>
 #include <boost/throw_exception.hpp>
 #include <cstdint>
 #include <cstdlib>
 #include <cstring>
 #include <memory>
 #include <stdexcept>
 #include <type_traits>
 
 namespace boost {
 namespace beast {
 namespace zlib {
 namespace detail {
 
 /*
  *  ALGORITHM
  *
  *      The "deflation" process depends on being able to identify portions
  *      of the input text which are identical to earlier input (within a
  *      sliding window trailing behind the input currently being processed).
  *
  *      Each code tree is stored in a compressed form which is itself
  *      a Huffman encoding of the lengths of all the code strings (in
  *      ascending order by source values).  The actual code strings are
  *      reconstructed from the lengths in the inflate process, as described
  *      in the deflate specification.
  *
  *      The most straightforward technique turns out to be the fastest for
  *      most input files: try all possible matches and select the longest.
  *      The key feature of this algorithm is that insertions into the string
  *      dictionary are very simple and thus fast, and deletions are avoided
  *      completely. Insertions are performed at each input character, whereas
  *      string matches are performed only when the previous match ends. So it
  *      is preferable to spend more time in matches to allow very fast string
  *      insertions and avoid deletions. The matching algorithm for small
  *      strings is inspired from that of Rabin & Karp. A brute force approach
  *      is used to find longer strings when a small match has been found.
  *      A similar algorithm is used in comic (by Jan-Mark Wams) and freeze
  *      (by Leonid Broukhis).
  *         A previous version of this file used a more sophisticated algorithm
  *      (by Fiala and Greene) which is guaranteed to run in linear amortized
  *      time, but has a larger average cost, uses more memory and is patented.
  *      However the F&G algorithm may be faster for some highly redundant
  *      files if the parameter max_chain_length (described below) is too large.
  *
  *  ACKNOWLEDGEMENTS
  *
  *      The idea of lazy evaluation of matches is due to Jan-Mark Wams, and
  *      I found it in 'freeze' written by Leonid Broukhis.
  *      Thanks to many people for bug reports and testing.
  *
  *  REFERENCES
  *
  *      Deutsch, L.P.,"DEFLATE Compressed Data Format Specification".
  *      Available in http://tools.ietf.org/html/rfc1951
  *
  *      A description of the Rabin and Karp algorithm is given in the book
  *         "Algorithms" by R. Sedgewick, Addison-Wesley, p252.
  *
  *      Fiala,E.R., and Greene,D.H.
  *         Data Compression with Finite Windows, Comm.ACM, 32,4 (1989) 490-595
  *
  */
 
 /*  Generate the codes for a given tree and bit counts (which need not be optimal).
     IN assertion: the array bl_count contains the bit length statistics for
        the given tree and the field len is set for all tree elements.
     OUT assertion: the field code is set for all tree elements of non
         zero code length.
 */
 void
 deflate_stream::
 gen_codes(ct_data *tree, int max_code, std::uint16_t *bl_count)
 {
     std::uint16_t next_code[maxBits+1]; /* next code value for each bit length */
     std::uint16_t code = 0;              /* running code value */
     int bits;                  /* bit index */
     int n;                     /* code index */
 
     // The distribution counts are first used to
     // generate the code values without bit reversal.
     for(bits = 1; bits <= maxBits; bits++)
     {
         code = (code + bl_count[bits-1]) << 1;
         next_code[bits] = code;
     }
     // Check that the bit counts in bl_count are consistent.
     // The last code must be all ones.
     BOOST_ASSERT(code + bl_count[maxBits]-1 == (1<<maxBits)-1);
     for(n = 0; n <= max_code; n++)
     {
         int len = tree[n].dl;
         if(len == 0)
             continue;
         tree[n].fc = bi_reverse(next_code[len]++, len);
     }
 }
 
 auto
 deflate_stream::get_lut() ->
     lut_type const&
 {
     struct init
     {
         lut_type tables;
 
         init()
         {
             // number of codes at each bit length for an optimal tree
             //std::uint16_t bl_count[maxBits+1];
 
             // Initialize the mapping length (0..255) -> length code (0..28)
             std::uint8_t length = 0;
             for(std::uint8_t code = 0; code < lengthCodes-1; ++code)
             {
                 tables.base_length[code] = length;
                 auto const run = 1U << tables.extra_lbits[code];
                 for(unsigned n = 0; n < run; ++n)
                     tables.length_code[length++] = code;
             }
             BOOST_ASSERT(length == 0);
             // Note that the length 255 (match length 258) can be represented
             // in two different ways: code 284 + 5 bits or code 285, so we
             // overwrite length_code[255] to use the best encoding:
             tables.length_code[255] = lengthCodes-1;
 
             // Initialize the mapping dist (0..32K) -> dist code (0..29)
             {
                 std::uint8_t code;
                 std::uint16_t dist = 0;
                 for(code = 0; code < 16; code++)
                 {
                     tables.base_dist[code] = dist;
                     auto const run = 1U << tables.extra_dbits[code];
                     for(unsigned n = 0; n < run; ++n)
                         tables.dist_code[dist++] = code;
                 }
                 BOOST_ASSERT(dist == 256);
                 // from now on, all distances are divided by 128
                 dist >>= 7;
                 for(; code < dCodes; ++code)
                 {
                     tables.base_dist[code] = dist << 7;
                     auto const run = 1U << (tables.extra_dbits[code]-7);
                     for(std::size_t n = 0; n < run; ++n)
                         tables.dist_code[256 + dist++] = code;
                 }
                 BOOST_ASSERT(dist == 256);
             }
 
             // Construct the codes of the static literal tree
             std::uint16_t bl_count[maxBits+1];
             std::memset(bl_count, 0, sizeof(bl_count));
             unsigned n = 0;
             while (n <= 143)
                 tables.ltree[n++].dl = 8;
             bl_count[8] += 144;
             while (n <= 255)
                 tables.ltree[n++].dl = 9;
             bl_count[9] += 112;
             while (n <= 279)
                 tables.ltree[n++].dl = 7;
             bl_count[7] += 24;
             while (n <= 287)
                 tables.ltree[n++].dl = 8;
             bl_count[8] += 8;
             // Codes 286 and 287 do not exist, but we must include them in the tree
             // construction to get a canonical Huffman tree (longest code all ones)
             gen_codes(tables.ltree, lCodes+1, bl_count);
 
             for(n = 0; n < dCodes; ++n)
             {
                 tables.dtree[n].dl = 5;
                 tables.dtree[n].fc =
                     static_cast<std::uint16_t>(bi_reverse(n, 5));
             }
         }
     };
     static init const data;
     return data.tables;
 }
 
 void
 deflate_stream::
 doReset(
     int level,
     int windowBits,
     int memLevel,
     Strategy strategy)
 {
     if(level == default_size)
         level = 6;
 
     // VFALCO What do we do about this?
     // until 256-byte window bug fixed
     if(windowBits == 8)
         windowBits = 9;
 
     if(level < 0 || level > 9)
         BOOST_THROW_EXCEPTION(std::invalid_argument{
             "invalid level"});
 
     if(windowBits < 8 || windowBits > 15)
         BOOST_THROW_EXCEPTION(std::invalid_argument{
             "invalid windowBits"});
 
     if(memLevel < 1 || memLevel > max_mem_level)
         BOOST_THROW_EXCEPTION(std::invalid_argument{
             "invalid memLevel"});
 
     w_bits_ = windowBits;
 
     hash_bits_ = memLevel + 7;
 
     // 16K elements by default
     lit_bufsize_ = 1 << (memLevel + 6);
 
     level_ = level;
     strategy_ = strategy;
     inited_ = false;
 }
 
 void
 deflate_stream::
 doReset()
 {
     inited_ = false;
 }
 
 void
 deflate_stream::
 doClear()
 {
     inited_ = false;
     buf_.reset();
 }
 
 std::size_t
 deflate_stream::
 doUpperBound(std::size_t sourceLen) const
 {
     std::size_t complen;
     std::size_t wraplen;
 
     /* conservative upper bound for compressed data */
     complen = sourceLen +
               ((sourceLen + 7) >> 3) + ((sourceLen + 63) >> 6) + 5;
 
     /* compute wrapper length */
     wraplen = 0;
 
     /* if not default parameters, return conservative bound */
     if(w_bits_ != 15 || hash_bits_ != 8 + 7)
         return complen + wraplen;
 
     /* default settings: return tight bound for that case */
     return sourceLen + (sourceLen >> 12) + (sourceLen >> 14) +
            (sourceLen >> 25) + 13 - 6 + wraplen;
 }
 
 void
 deflate_stream::
 doTune(
     int good_length,
     int max_lazy,
     int nice_length,
     int max_chain)
 {
     good_match_ = good_length;
     nice_match_ = nice_length;
     max_lazy_match_ = max_lazy;
     max_chain_length_ = max_chain;
 }
 
 void
 deflate_stream::
 doParams(z_params& zs, int level, Strategy strategy, error_code& ec)
 {
     compress_func func;
 
     if(level == default_size)
         level = 6;
     if(level < 0 || level > 9)
     {
         BOOST_BEAST_ASSIGN_EC(ec, error::stream_error);
         return;
     }
     func = get_config(level_).func;
 
     if((strategy != strategy_ || func != get_config(level).func) &&
         zs.total_in != 0)
     {
         // Flush the last buffer:
         doWrite(zs, Flush::block, ec);
         if(ec == error::need_buffers && pending_ == 0)
             ec = {};
     }
     if(level_ != level)
     {
         level_ = level;
         max_lazy_match_   = get_config(level).max_lazy;
         good_match_       = get_config(level).good_length;
         nice_match_       = get_config(level).nice_length;
         max_chain_length_ = get_config(level).max_chain;
     }
     strategy_ = strategy;
 }
 
 // VFALCO boost::optional param is a workaround for
 //        gcc "maybe uninitialized" warning
 //        https://github.com/boostorg/beast/issues/532
 //
 void
 deflate_stream::
 doWrite(z_params& zs, boost::optional<Flush> flush, error_code& ec)
 {
     maybe_init();
 
     if(zs.next_in == nullptr && zs.avail_in != 0)
         BOOST_THROW_EXCEPTION(std::invalid_argument{"invalid input"});
 
     if(zs.next_out == nullptr ||
         (status_ == FINISH_STATE && flush != Flush::finish))
     {
         BOOST_BEAST_ASSIGN_EC(ec, error::stream_error);
         return;
     }
     if(zs.avail_out == 0)
     {
         BOOST_BEAST_ASSIGN_EC(ec, error::need_buffers);
         return;
     }
 
     // value of flush param for previous deflate call
     auto old_flush = boost::make_optional<Flush>(
         last_flush_.is_initialized(),
         last_flush_ ? *last_flush_ : Flush::none);
 
     last_flush_ = flush;
 
     // Flush as much pending output as possible
     if(pending_ != 0)
     {
         flush_pending(zs);
         if(zs.avail_out == 0)
         {
             /* Since avail_out is 0, deflate will be called again with
              * more output space, but possibly with both pending and
              * avail_in equal to zero. There won't be anything to do,
              * but this is not an error situation so make sure we
              * return OK instead of BUF_ERROR at next call of deflate:
              */
             last_flush_ = boost::none;
             return;
         }
     }
     else if(zs.avail_in == 0 && (
             old_flush && flush <= *old_flush // Caution: depends on enum order
         ) && flush != Flush::finish)
     {
         /* Make sure there is something to do and avoid duplicate consecutive
          * flushes. For repeated and useless calls with Flush::finish, we keep
          * returning Z_STREAM_END instead of Z_BUF_ERROR.
          */
         BOOST_BEAST_ASSIGN_EC(ec, error::need_buffers);
         return;
     }
 
     // User must not provide more input after the first FINISH:
     if(status_ == FINISH_STATE && zs.avail_in != 0)
     {
         BOOST_BEAST_ASSIGN_EC(ec, error::need_buffers);
         return;
     }
 
     /* Start a new block or continue the current one.
      */
     if(zs.avail_in != 0 || lookahead_ != 0 ||
         (flush != Flush::none && status_ != FINISH_STATE))
     {
         block_state bstate;
 
         switch(strategy_)
         {
         case Strategy::huffman:
             bstate = deflate_huff(zs, flush.get());
             break;
         case Strategy::rle:
             bstate = deflate_rle(zs, flush.get());
             break;
         default:
         {
             bstate = (this->*(get_config(level_).func))(zs, flush.get());
             break;
         }
         }
 
         if(bstate == finish_started || bstate == finish_done)
         {
             status_ = FINISH_STATE;
         }
         if(bstate == need_more || bstate == finish_started)
         {
             if(zs.avail_out == 0)
             {
                 last_flush_ = boost::none; /* avoid BUF_ERROR next call, see above */
             }
             return;
             /*  If flush != Flush::none && avail_out == 0, the next call
                 of deflate should use the same flush parameter to make sure
                 that the flush is complete. So we don't have to output an
                 empty block here, this will be done at next call. This also
                 ensures that for a very small output buffer, we emit at most
                 one empty block.
             */
         }
         if(bstate == block_done)
         {
             if(flush == Flush::partial)
             {
                 tr_align();
             }
             else if(flush != Flush::block)
             {
                 /* FULL_FLUSH or SYNC_FLUSH */
                 tr_stored_block(nullptr, 0L, 0);
                 /* For a full flush, this empty block will be recognized
                  * as a special marker by inflate_sync().
                  */
                 if(flush == Flush::full)
                 {
                     clear_hash(); // forget history
                     if(lookahead_ == 0)
                     {
                         strstart_ = 0;
                         block_start_ = 0L;
                         insert_ = 0;
                     }
                 }
             }
             flush_pending(zs);
             if(zs.avail_out == 0)
             {
                 last_flush_ = boost::none; /* avoid BUF_ERROR at next call, see above */
                 return;
             }
         }
     }
 
     if(flush == Flush::finish)
     {
         BOOST_BEAST_ASSIGN_EC(ec, error::end_of_stream);
         return;
     }
 }
 
 // VFALCO Warning: untested
 void
 deflate_stream::
 doDictionary(Byte const* dict, uInt dictLength, error_code& ec)
 {
     if(lookahead_)
     {
         BOOST_BEAST_ASSIGN_EC(ec, error::stream_error);
         return;
     }
 
     maybe_init();
 
     /* if dict would fill window, just replace the history */
     if(dictLength >= w_size_)
     {
         clear_hash();
         strstart_ = 0;
         block_start_ = 0L;
         insert_ = 0;
         dict += dictLength - w_size_;  /* use the tail */
         dictLength = w_size_;
     }
 
     /* insert dict into window and hash */
     z_params zs;
     zs.avail_in = dictLength;
     zs.next_in = (const Byte *)dict;
     zs.avail_out = 0;
     zs.next_out = 0;
     fill_window(zs);
     while(lookahead_ >= minMatch)
     {
         uInt str = strstart_;
         uInt n = lookahead_ - (minMatch-1);
         do
         {
             update_hash(ins_h_, window_[str + minMatch-1]);
             prev_[str & w_mask_] = head_[ins_h_];
             head_[ins_h_] = (std::uint16_t)str;
             str++;
         }
         while(--n);
         strstart_ = str;
         lookahead_ = minMatch-1;
         fill_window(zs);
     }
     strstart_ += lookahead_;
     block_start_ = (long)strstart_;
     insert_ = lookahead_;
     lookahead_ = 0;
     match_length_ = prev_length_ = minMatch-1;
     match_available_ = 0;
 }
 
 void
 deflate_stream::
 doPrime(int bits, int value, error_code& ec)
 {
     maybe_init();
 
     if((Byte *)(sym_buf_) < pending_out_ + ((Buf_size + 7) >> 3))
     {
         BOOST_BEAST_ASSIGN_EC(ec, error::need_buffers);
         return;
     }
 
     do
     {
         int put = Buf_size - bi_valid_;
         if(put > bits)
             put = bits;
         bi_buf_ |= (std::uint16_t)((value & ((1 << put) - 1)) << bi_valid_);
         bi_valid_ += put;
         tr_flush_bits();
         value >>= put;
         bits -= put;
     }
     while(bits);
 }
 
 void
 deflate_stream::
 doPending(unsigned* value, int* bits)
 {
     if(value != 0)
         *value = pending_;
     if(bits != 0)
         *bits = bi_valid_;
 }
 
 //--------------------------------------------------------------------------
 
 // Do lazy initialization
 void
 deflate_stream::
 init()
 {
     //  Caller must set these:
     //      w_bits_
     //      hash_bits_
     //      lit_bufsize_
     //      level_
     //      strategy_
 
     w_size_ = 1 << w_bits_;
     w_mask_ = w_size_ - 1;
 
     hash_size_ = 1 << hash_bits_;
     hash_mask_ = hash_size_ - 1;
     hash_shift_ =  ((hash_bits_+minMatch-1)/minMatch);
 
     auto const nwindow  = w_size_ * 2*sizeof(Byte);
     auto const nprev    = w_size_ * sizeof(std::uint16_t);
     auto const nhead    = hash_size_ * sizeof(std::uint16_t);
     auto const noverlay = lit_bufsize_ * (sizeof(std::uint16_t)+2);
     auto const needed   = nwindow + nprev + nhead + noverlay;
 
     if(! buf_ || buf_size_ != needed)
     {
         buf_ = boost::make_unique_noinit<
             std::uint8_t[]>(needed);
         buf_size_ = needed;
     }
 
     window_ = reinterpret_cast<Byte*>(buf_.get());
     prev_   = reinterpret_cast<std::uint16_t*>(buf_.get() + nwindow);
     std::memset(prev_, 0, nprev);
     head_   = reinterpret_cast<std::uint16_t*>(buf_.get() + nwindow + nprev);
 
     // nothing written to window_ yet
     high_water_ = 0;
 
 
     /* We overlay pending_buf and sym_buf. This works since the average size
        for length/distance pairs over any compressed block is assured to be 31
        bits or less.
 
        Analysis: The longest fixed codes are a length code of 8 bits plus 5
        extra bits, for lengths 131 to 257. The longest fixed distance codes are
        5 bits plus 13 extra bits, for distances 16385 to 32768. The longest
        possible fixed-codes length/distance pair is then 31 bits total.
 
        sym_buf starts one-fourth of the way into pending_buf. So there are
        three bytes in sym_buf for every four bytes in pending_buf. Each symbol
        in sym_buf is three bytes -- two for the distance and one for the
        literal/length. As each symbol is consumed, the pointer to the next
        sym_buf value to read moves forward three bytes. From that symbol, up to
        31 bits are written to pending_buf. The closest the written pending_buf
        bits gets to the next sym_buf symbol to read is just before the last
        code is written. At that time, 31*(n-2) bits have been written, just
        after 24*(n-2) bits have been consumed from sym_buf. sym_buf starts at
        8*n bits into pending_buf. (Note that the symbol buffer fills when n-1
        symbols are written.) The closest the writing gets to what is unread is
        then n+14 bits. Here n is lit_bufsize, which is 16384 by default, and
        can range from 128 to 32768.
 
        Therefore, at a minimum, there are 142 bits of space between what is
        written and what is read in the overlain buffers, so the symbols cannot
        be overwritten by the compressed data. That space is actually 139 bits,
        due to the three-bit fixed-code block header.
 
        That covers the case where either Z_FIXED is specified, forcing fixed
        codes, or when the use of fixed codes is chosen, because that choice
        results in a smaller compressed block than dynamic codes. That latter
        condition then assures that the above analysis also covers all dynamic
        blocks. A dynamic-code block will only be chosen to be emitted if it has
        fewer bits than a fixed-code block would for the same set of symbols.
        Therefore its average symbol length is assured to be less than 31. So
        the compressed data for a dynamic block also cannot overwrite the
        symbols from which it is being constructed.
      */
     pending_buf_ =
         buf_.get() + nwindow + nprev + nhead;
     pending_buf_size_ =
         static_cast<std::uint32_t>(lit_bufsize_) * 4;
 
     sym_buf_ = pending_buf_ + lit_bufsize_;
     sym_end_ = (lit_bufsize_ - 1) * 3;
 
     pending_ = 0;
     pending_out_ = pending_buf_;
 
     status_ = BUSY_STATE;
     last_flush_ = Flush::none;
 
     tr_init();
     lm_init();
 
     inited_ = true;
 }
 
 /*  Initialize the "longest match" routines for a new zlib stream
 */
 void
 deflate_stream::
 lm_init()
 {
     window_size_ = (std::uint32_t)2L*w_size_;
 
     clear_hash();
 
     /* Set the default configuration parameters:
      */
     // VFALCO TODO just copy the config struct
     max_lazy_match_   = get_config(level_).max_lazy;
     good_match_       = get_config(level_).good_length;
     nice_match_       = get_config(level_).nice_length;
     max_chain_length_ = get_config(level_).max_chain;
 
     strstart_ = 0;
     block_start_ = 0L;
     lookahead_ = 0;
     insert_ = 0;
     match_length_ = prev_length_ = minMatch-1;
     match_available_ = 0;
     ins_h_ = 0;
 }
 
 // Initialize a new block.
 //
 void
 deflate_stream::
 init_block()
 {
     for(int n = 0; n < lCodes;  n++)
         dyn_ltree_[n].fc = 0;
     for(int n = 0; n < dCodes;  n++)
         dyn_dtree_[n].fc = 0;
     for(int n = 0; n < blCodes; n++)
         bl_tree_[n].fc = 0;
     dyn_ltree_[END_BLOCK].fc = 1;
     opt_len_ = 0L;
     static_len_ = 0L;
     sym_next_ = 0;
     matches_ = 0;
 }
 
 /*  Restore the heap property by moving down the tree starting at node k,
     exchanging a node with the smallest of its two sons if necessary,
     stopping when the heap property is re-established (each father smaller
     than its two sons).
 */
 void
 deflate_stream::
 pqdownheap(
     ct_data const* tree,    // the tree to restore
     int k)                          // node to move down
 {
     int v = heap_[k];
     int j = k << 1;  // left son of k
     while(j <= heap_len_)
     {
         // Set j to the smallest of the two sons:
         if(j < heap_len_ &&
                 smaller(tree, heap_[j+1], heap_[j]))
             j++;
         // Exit if v is smaller than both sons
         if(smaller(tree, v, heap_[j]))
             break;
 
         // Exchange v with the smallest son
         heap_[k] = heap_[j];
         k = j;
 
         // And continue down the tree,
         // setting j to the left son of k
         j <<= 1;
     }
     heap_[k] = v;
 }
 
 /*  Remove the smallest element from the heap and recreate the heap
     with one less element. Updates heap and heap_len.
 */
 void
 deflate_stream::
 pqremove(ct_data const* tree, int& top)
 {
     top = heap_[kSmallest];
     heap_[kSmallest] = heap_[heap_len_--];
     pqdownheap(tree, kSmallest);
 }
 
 /*  Compute the optimal bit lengths for a tree and update the total bit length
     for the current block.
     IN assertion: the fields freq and dad are set, heap[heap_max] and
        above are the tree nodes sorted by increasing frequency.
     OUT assertions: the field len is set to the optimal bit length, the
         array bl_count contains the frequencies for each bit length.
         The length opt_len is updated; static_len is also updated if stree is
         not null.
 */
 void
 deflate_stream::
 gen_bitlen(tree_desc *desc)
 {
     ct_data *tree           = desc->dyn_tree;
     int max_code                    = desc->max_code;
     ct_data const* stree    = desc->stat_desc->static_tree;
     std::uint8_t const *extra       = desc->stat_desc->extra_bits;
     int base                        = desc->stat_desc->extra_base;
     int max_length                  = desc->stat_desc->max_length;
     int h;                          // heap index
     int n, m;                       // iterate over the tree elements
     int bits;                       // bit length
     int xbits;                      // extra bits
     std::uint16_t f;                // frequency
     int overflow = 0;               // number of elements with bit length too large
 
     std::fill(&bl_count_[0], &bl_count_[maxBits+1], std::uint16_t{0});
 
     /* In a first pass, compute the optimal bit lengths (which may
      * overflow in the case of the bit length tree).
      */
     tree[heap_[heap_max_]].dl = 0; // root of the heap
 
     for(h = heap_max_+1; h < HEAP_SIZE; h++) {
         n = heap_[h];
         bits = tree[tree[n].dl].dl + 1;
         if(bits > max_length) bits = max_length, overflow++;
         // We overwrite tree[n].dl which is no longer needed
         tree[n].dl = (std::uint16_t)bits;
 
         if(n > max_code)
             continue; // not a leaf node
 
         bl_count_[bits]++;
         xbits = 0;
         if(n >= base)
             xbits = extra[n-base];
         f = tree[n].fc;
         opt_len_ += (std::uint32_t)f * (bits + xbits);
         if(stree)
             static_len_ += (std::uint32_t)f * (stree[n].dl + xbits);
     }
     if(overflow == 0)
         return;
 
     // Find the first bit length which could increase:
     do
     {
         bits = max_length-1;
         while(bl_count_[bits] == 0)
             bits--;
         bl_count_[bits]--;      // move one leaf down the tree
         bl_count_[bits+1] += 2; // move one overflow item as its brother
         bl_count_[max_length]--;
         /* The brother of the overflow item also moves one step up,
          * but this does not affect bl_count[max_length]
          */
         overflow -= 2;
     }
     while(overflow > 0);
 
     /* Now recompute all bit lengths, scanning in increasing frequency.
      * h is still equal to HEAP_SIZE. (It is simpler to reconstruct all
      * lengths instead of fixing only the wrong ones. This idea is taken
      * from 'ar' written by Haruhiko Okumura.)
      */
     for(bits = max_length; bits != 0; bits--)
     {
         n = bl_count_[bits];
         while(n != 0)
         {
             m = heap_[--h];
             if(m > max_code)
                 continue;
             if((unsigned) tree[m].dl != (unsigned) bits)
             {
                 opt_len_ += ((long)bits - (long)tree[m].dl) *(long)tree[m].fc;
                 tree[m].dl = (std::uint16_t)bits;
             }
             n--;
         }
     }
 }
 
 /*  Construct one Huffman tree and assigns the code bit strings and lengths.
     Update the total bit length for the current block.
     IN assertion: the field freq is set for all tree elements.
     OUT assertions: the fields len and code are set to the optimal bit length
         and corresponding code. The length opt_len is updated; static_len is
         also updated if stree is not null. The field max_code is set.
 */
 void
 deflate_stream::
 build_tree(tree_desc *desc)
 {
     ct_data *tree         = desc->dyn_tree;
     ct_data const* stree  = desc->stat_desc->static_tree;
     int elems                     = desc->stat_desc->elems;
     int n, m;          // iterate over heap elements
     int max_code = -1; // largest code with non zero frequency
     int node;          // new node being created
 
     /* Construct the initial heap, with least frequent element in
      * heap[kSmallest]. The sons of heap[n] are heap[2*n] and heap[2*n+1].
      * heap[0] is not used.
      */
     heap_len_ = 0;
     heap_max_ = HEAP_SIZE;
 
     for(n = 0; n < elems; n++)
     {
         if(tree[n].fc != 0)
         {
             heap_[++(heap_len_)] = max_code = n;
             depth_[n] = 0;
         }
         else
         {
             tree[n].dl = 0;
         }
     }
 
     /* The pkzip format requires that at least one distance code exists,
      * and that at least one bit should be sent even if there is only one
      * possible code. So to avoid special checks later on we force at least
      * two codes of non zero frequency.
      */
     while(heap_len_ < 2)
     {
         node = heap_[++(heap_len_)] = (max_code < 2 ? ++max_code : 0);
         tree[node].fc = 1;
         depth_[node] = 0;
         opt_len_--;
         if(stree)
             static_len_ -= stree[node].dl;
         // node is 0 or 1 so it does not have extra bits
     }
     desc->max_code = max_code;
 
     /* The elements heap[heap_len/2+1 .. heap_len] are leaves of the tree,
      * establish sub-heaps of increasing lengths:
      */
     for(n = heap_len_/2; n >= 1; n--)
         pqdownheap(tree, n);
 
     /* Construct the Huffman tree by repeatedly combining the least two
      * frequent nodes.
      */
     node = elems;              /* next internal node of the tree */
     do
     {
         pqremove(tree, n);  /* n = node of least frequency */
         m = heap_[kSmallest]; /* m = node of next least frequency */
 
         heap_[--(heap_max_)] = n; /* keep the nodes sorted by frequency */
         heap_[--(heap_max_)] = m;
 
         /* Create a new node father of n and m */
         tree[node].fc = tree[n].fc + tree[m].fc;
         depth_[node] = (std::uint8_t)((depth_[n] >= depth_[m] ?
                                 depth_[n] : depth_[m]) + 1);
         tree[n].dl = tree[m].dl = (std::uint16_t)node;
         /* and insert the new node in the heap */
         heap_[kSmallest] = node++;
         pqdownheap(tree, kSmallest);
 
     }
     while(heap_len_ >= 2);
 
     heap_[--(heap_max_)] = heap_[kSmallest];
 
     /* At this point, the fields freq and dad are set. We can now
      * generate the bit lengths.
      */
     gen_bitlen((tree_desc *)desc);
 
     /* The field len is now set, we can generate the bit codes */
     gen_codes(tree, max_code, bl_count_);
 }
 
 /*  Scan a literal or distance tree to determine the frequencies
     of the codes in the bit length tree.
 */
 void
 deflate_stream::
 scan_tree(
     ct_data *tree,      // the tree to be scanned
     int max_code)               // and its largest code of non zero frequency
 {
     int n;                      // iterates over all tree elements
     int prevlen = -1;           // last emitted length
     int curlen;                 // length of current code
     int nextlen = tree[0].dl;   // length of next code
     std::uint16_t count = 0;    // repeat count of the current code
     int max_count = 7;          // max repeat count
     int min_count = 4;          // min repeat count
 
     if(nextlen == 0)
     {
         max_count = 138;
         min_count = 3;
     }
     tree[max_code+1].dl = (std::uint16_t)0xffff; // guard
 
     for(n = 0; n <= max_code; n++)
     {
         curlen = nextlen; nextlen = tree[n+1].dl;
         if(++count < max_count && curlen == nextlen)
         {
             continue;
         }
         else if(count < min_count)
         {
             bl_tree_[curlen].fc += count;
         }
         else if(curlen != 0)
         {
             if(curlen != prevlen) bl_tree_[curlen].fc++;
                 bl_tree_[REP_3_6].fc++;
         }
         else if(count <= 10)
         {
             bl_tree_[REPZ_3_10].fc++;
         }
         else
         {
             bl_tree_[REPZ_11_138].fc++;
         }
         count = 0;
         prevlen = curlen;
         if(nextlen == 0)
         {
             max_count = 138;
             min_count = 3;
         }
         else if(curlen == nextlen)
         {
             max_count = 6;
             min_count = 3;
         }
         else
         {
             max_count = 7;
             min_count = 4;
         }
     }
 }
 
 /*  Send a literal or distance tree in compressed form,
     using the codes in bl_tree.
 */
 void
 deflate_stream::
 send_tree(
     ct_data *tree,      // the tree to be scanned
     int max_code)               // and its largest code of non zero frequency
 {
     int n;                      // iterates over all tree elements
     int prevlen = -1;           // last emitted length
     int curlen;                 // length of current code
     int nextlen = tree[0].dl;   // length of next code
     int count = 0;              // repeat count of the current code
     int max_count = 7;          // max repeat count
     int min_count = 4;          // min repeat count
 
     // tree[max_code+1].dl = -1; // guard already set
     if(nextlen == 0)
     {
         max_count = 138;
         min_count = 3;
     }
 
     for(n = 0; n <= max_code; n++)
     {
         curlen = nextlen;
         nextlen = tree[n+1].dl;
         if(++count < max_count && curlen == nextlen)
         {
             continue;
         }
         else if(count < min_count)
         {
             do
             {
                 send_code(curlen, bl_tree_);
             }
             while (--count != 0);
         }
         else if(curlen != 0)
         {
             if(curlen != prevlen)
             {
                 send_code(curlen, bl_tree_);
                 count--;
             }
             BOOST_ASSERT(count >= 3 && count <= 6);
             send_code(REP_3_6, bl_tree_);
             send_bits(count-3, 2);
         }
         else if(count <= 10)
         {
             send_code(REPZ_3_10, bl_tree_);
             send_bits(count-3, 3);
         }
         else
         {
             send_code(REPZ_11_138, bl_tree_);
             send_bits(count-11, 7);
         }
         count = 0;
         prevlen = curlen;
         if(nextlen == 0)
         {
             max_count = 138;
             min_count = 3;
         }
         else if(curlen == nextlen)
         {
             max_count = 6;
             min_count = 3;
         }
         else
         {
             max_count = 7;
             min_count = 4;
         }
     }
 }
 
 /*  Construct the Huffman tree for the bit lengths and return
     the index in bl_order of the last bit length code to send.
 */
 int
 deflate_stream::
 build_bl_tree()
 {
     int max_blindex;  // index of last bit length code of non zero freq
 
     // Determine the bit length frequencies for literal and distance trees
     scan_tree((ct_data *)dyn_ltree_, l_desc_.max_code);
     scan_tree((ct_data *)dyn_dtree_, d_desc_.max_code);
 
     // Build the bit length tree:
     build_tree((tree_desc *)(&(bl_desc_)));
     /* opt_len now includes the length of the tree representations, except
      * the lengths of the bit lengths codes and the 5+5+4 bits for the counts.
      */
 
     /* Determine the number of bit length codes to send. The pkzip format
      * requires that at least 4 bit length codes be sent. (appnote.txt says
      * 3 but the actual value used is 4.)
      */
     for(max_blindex = blCodes-1; max_blindex >= 3; max_blindex--)
     {
         if(bl_tree_[lut_.bl_order[max_blindex]].dl != 0)
             break;
     }
     // Update opt_len to include the bit length tree and counts
     opt_len_ += 3*(max_blindex+1) + 5+5+4;
     return max_blindex;
 }
 
 /*  Send the header for a block using dynamic Huffman trees: the counts,
     the lengths of the bit length codes, the literal tree and the distance
     tree.
     IN assertion: lcodes >= 257, dcodes >= 1, blcodes >= 4.
 */
 void
 deflate_stream::
 send_all_trees(
     int lcodes,
     int dcodes,
     int blcodes)    // number of codes for each tree
 {
     int rank;       // index in bl_order
 
     BOOST_ASSERT(lcodes >= 257 && dcodes >= 1 && blcodes >= 4);
     BOOST_ASSERT(lcodes <= lCodes && dcodes <= dCodes && blcodes <= blCodes);
     send_bits(lcodes-257, 5); // not +255 as stated in appnote.txt
     send_bits(dcodes-1,   5);
     send_bits(blcodes-4,  4); // not -3 as stated in appnote.txt
     for(rank = 0; rank < blcodes; rank++)
         send_bits(bl_tree_[lut_.bl_order[rank]].dl, 3);
     send_tree((ct_data *)dyn_ltree_, lcodes-1); // literal tree
     send_tree((ct_data *)dyn_dtree_, dcodes-1); // distance tree
 }
 
 /*  Send the block data compressed using the given Huffman trees
 */
 void
 deflate_stream::
 compress_block(
     ct_data const* ltree, // literal tree
     ct_data const* dtree) // distance tree
 {
     unsigned dist;      /* distance of matched string */
     int lc;             /* match length or unmatched char (if dist == 0) */
     unsigned sx = 0;    /* running index in sym_buf */
     unsigned code;      /* the code to send */
     int extra;          /* number of extra bits to send */
 
     if(sym_next_ != 0)
     {
         do
         {
             dist = sym_buf_[sx++] & 0xff;
             dist += (unsigned)(sym_buf_[sx++] & 0xff) << 8;
             lc = sym_buf_[sx++];
             if(dist == 0)
             {
                 send_code(lc, ltree); /* send a literal byte */
             }
             else
             {
                 /* Here, lc is the match length - minMatch */
                 code = lut_.length_code[lc];
                 send_code(code+literals+1, ltree); /* send the length code */
                 extra = lut_.extra_lbits[code];
                 if(extra != 0)
                 {
                     lc -= lut_.base_length[code];
                     send_bits(lc, extra);       /* send the extra length bits */
                 }
                 dist--; /* dist is now the match distance - 1 */
                 code = d_code(dist);
                 BOOST_ASSERT(code < dCodes);
 
                 send_code(code, dtree);       /* send the distance code */
                 extra = lut_.extra_dbits[code];
                 if(extra != 0)
                 {
                     dist -= lut_.base_dist[code];
                     send_bits(dist, extra);   /* send the extra distance bits */
                 }
             } /* literal or match pair ? */
 
             /* Check that the overlay between pending_buf and d_buf+l_buf is ok: */
             BOOST_ASSERT((uInt)(pending_) < lit_bufsize_ + sx);
         }
         while(sx < sym_next_);
     }
 
     send_code(END_BLOCK, ltree);
 }
 
 /*  Check if the data type is TEXT or BINARY, using the following algorithm:
     - TEXT if the two conditions below are satisfied:
         a) There are no non-portable control characters belonging to the
             "block list" (0..6, 14..25, 28..31).
         b) There is at least one printable character belonging to the
             "allow list" (9 {TAB}, 10 {LF}, 13 {CR}, 32..255).
     - BINARY otherwise.
     - The following partially-portable control characters form a
         "gray list" that is ignored in this detection algorithm:
         (7 {BEL}, 8 {BS}, 11 {VT}, 12 {FF}, 26 {SUB}, 27 {ESC}).
     IN assertion: the fields fc of dyn_ltree are set.
 */
 int
 deflate_stream::
 detect_data_type()
 {
     /* block_mask is the bit mask of block-listed bytes
      * set bits 0..6, 14..25, and 28..31
      * 0xf3ffc07f = binary 11110011111111111100000001111111
      */
     unsigned long block_mask = 0xf3ffc07fUL;
     int n;
 
     // Check for non-textual ("block-listed") bytes.
     for(n = 0; n <= 31; n++, block_mask >>= 1)
         if((block_mask & 1) && (dyn_ltree_[n].fc != 0))
             return binary;
 
     // Check for textual ("allow-listed") bytes. */
     if(dyn_ltree_[9].fc != 0 || dyn_ltree_[10].fc != 0
             || dyn_ltree_[13].fc != 0)
         return text;
     for(n = 32; n < literals; n++)
         if(dyn_ltree_[n].fc != 0)
             return text;
 
     /* There are no "block-listed" or "white-listed" bytes:
      * this stream either is empty or has tolerated ("gray-listed") bytes only.
      */
     return binary;
 }
 
 /*  Flush the bit buffer and align the output on a byte boundary
 */
 void
 deflate_stream::
 bi_windup()
 {
     if(bi_valid_ > 8)
         put_short(bi_buf_);
     else if(bi_valid_ > 0)
         put_byte((Byte)bi_buf_);
     bi_buf_ = 0;
     bi_valid_ = 0;
 }
 
 /*  Flush the bit buffer, keeping at most 7 bits in it.
 */
 void
 deflate_stream::
 bi_flush()
 {
     if(bi_valid_ == 16)
     {
         put_short(bi_buf_);
         bi_buf_ = 0;
         bi_valid_ = 0;
     }
     else if(bi_valid_ >= 8)
     {
         put_byte((Byte)bi_buf_);
         bi_buf_ >>= 8;
         bi_valid_ -= 8;
     }
 }
 
 /*  Copy a stored block, storing first the length and its
     one's complement if requested.
 */
 void
 deflate_stream::
 copy_block(
     char    *buf,       // the input data
     unsigned len,       // its length
     int      header)    // true if block header must be written
 {
     bi_windup();        // align on byte boundary
 
     if(header)
     {
         put_short((std::uint16_t)len);
         put_short((std::uint16_t)~len);
     }
     if(buf)
         std::memcpy(&pending_buf_[pending_], buf, len);
     pending_ += len;
 }
 
 //------------------------------------------------------------------------------
 
 /* Initialize the tree data structures for a new zlib stream.
 */
 void
 deflate_stream::
 tr_init()
 {
     l_desc_.dyn_tree = dyn_ltree_;
     l_desc_.stat_desc = &lut_.l_desc;
 
     d_desc_.dyn_tree = dyn_dtree_;
     d_desc_.stat_desc = &lut_.d_desc;
 
     bl_desc_.dyn_tree = bl_tree_;
     bl_desc_.stat_desc = &lut_.bl_desc;
 
     bi_buf_ = 0;
     bi_valid_ = 0;
 
     // Initialize the first block of the first file:
     init_block();
 }
 
 /*  Send one empty static block to give enough lookahead for inflate.
     This takes 10 bits, of which 7 may remain in the bit buffer.
 */
 void
 deflate_stream::
 tr_align()
 {
     send_bits(STATIC_TREES<<1, 3);
     send_code(END_BLOCK, lut_.ltree);
     bi_flush();
 }
 
 /* Flush the bits in the bit buffer to pending output (leaves at most 7 bits)
 */
 void
 deflate_stream::
 tr_flush_bits()
 {
     bi_flush();
 }
 
 /* Send a stored block
 */
 void
 deflate_stream::
 tr_stored_block(
     char *buf,                  // input block
     std::uint32_t stored_len,   // length of input block
     int last)                   // one if this is the last block for a file
 {
     send_bits((STORED_BLOCK<<1)+last, 3);       // send block type
     copy_block(buf, (unsigned)stored_len, 1);   // with header
 }
 
 void
 deflate_stream::
 tr_tally_dist(std::uint16_t dist, std::uint8_t len, bool& flush)
 {
     sym_buf_[sym_next_++] = dist & 0xFF;
     sym_buf_[sym_next_++] = dist >> 8;
     sym_buf_[sym_next_++] = len;
     dist--;
     dyn_ltree_[lut_.length_code[len]+literals+1].fc++;
     dyn_dtree_[d_code(dist)].fc++;
     flush = (sym_next_ == sym_end_);
 }
 
 void
 deflate_stream::
 tr_tally_lit(std::uint8_t c, bool& flush)
 {
     sym_buf_[sym_next_++] = 0;
     sym_buf_[sym_next_++] = 0;
     sym_buf_[sym_next_++] = c;
     dyn_ltree_[c].fc++;
     flush = (sym_next_ == sym_end_);
 }
 
 //------------------------------------------------------------------------------
 
 /*  Determine the best encoding for the current block: dynamic trees,
     static trees or store, and output the encoded block to the zip file.
 */
 void
 deflate_stream::
 tr_flush_block(
     z_params& zs,
     char *buf,                  // input block, or NULL if too old
     std::uint32_t stored_len,   // length of input block
     int last)                   // one if this is the last block for a file
 {
     std::uint32_t opt_lenb;
     std::uint32_t static_lenb;  // opt_len and static_len in bytes
     int max_blindex = 0;        // index of last bit length code of non zero freq
 
     // Build the Huffman trees unless a stored block is forced
     if(level_ > 0)
     {
         // Check if the file is binary or text
         if(zs.data_type == unknown)
             zs.data_type = detect_data_type();
 
         // Construct the literal and distance trees
         build_tree((tree_desc *)(&(l_desc_)));
 
         build_tree((tree_desc *)(&(d_desc_)));
         /* At this point, opt_len and static_len are the total bit lengths of
          * the compressed block data, excluding the tree representations.
          */
 
         /* Build the bit length tree for the above two trees, and get the index
          * in bl_order of the last bit length code to send.
          */
         max_blindex = build_bl_tree();
 
         /* Determine the best encoding. Compute the block lengths in bytes. */
         opt_lenb = (opt_len_+3+7)>>3;
         static_lenb = (static_len_+3+7)>>3;
 
         if(static_lenb <= opt_lenb)
             opt_lenb = static_lenb;
     }
     else
     {
         // VFALCO This assertion fails even in the original ZLib,
         //        happens with strategy == Z_HUFFMAN_ONLY, see:
         //        https://github.com/madler/zlib/issues/172
 
     #if 0
         BOOST_ASSERT(buf);
     #endif
         opt_lenb = static_lenb = stored_len + 5; // force a stored block
     }
 
 #ifdef FORCE_STORED
     if(buf != (char*)0) { /* force stored block */
 #else
     if(stored_len+4 <= opt_lenb && buf != (char*)0) {
                        /* 4: two words for the lengths */
 #endif
         /* The test buf != NULL is only necessary if LIT_BUFSIZE > WSIZE.
          * Otherwise we can't have processed more than WSIZE input bytes since
          * the last block flush, because compression would have been
          * successful. If LIT_BUFSIZE <= WSIZE, it is never too late to
          * transform a block into a stored block.
          */
         tr_stored_block(buf, stored_len, last);
 
 #ifdef FORCE_STATIC
     }
     else if(static_lenb >= 0)
     {
         // force static trees
 #else
     }
     else if(strategy_ == Strategy::fixed || static_lenb == opt_lenb)
     {
 #endif
         send_bits((STATIC_TREES<<1)+last, 3);
         compress_block(lut_.ltree, lut_.dtree);
     }
     else
     {
         send_bits((DYN_TREES<<1)+last, 3);
         send_all_trees(l_desc_.max_code+1, d_desc_.max_code+1,
                        max_blindex+1);
         compress_block((const ct_data *)dyn_ltree_,
                        (const ct_data *)dyn_dtree_);
     }
     /* The above check is made mod 2^32, for files larger than 512 MB
      * and std::size_t implemented on 32 bits.
      */
     init_block();
 
     if(last)
         bi_windup();
 }
 
 void
 deflate_stream::
 fill_window(z_params& zs)
 {
     unsigned n, m;
     unsigned more;    // Amount of free space at the end of the window.
     std::uint16_t *p;
     uInt wsize = w_size_;
 
     do
     {
         more = (unsigned)(window_size_ -
             (std::uint32_t)lookahead_ -(std::uint32_t)strstart_);
 
         // VFALCO We don't support systems below 32-bit
     #if 0
         // Deal with !@#$% 64K limit:
         if(sizeof(int) <= 2)
         {
             if(more == 0 && strstart_ == 0 && lookahead_ == 0)
             {
                 more = wsize;
             }
             else if(more == (unsigned)(-1))
             {
                 /* Very unlikely, but possible on 16 bit machine if
                  * strstart == 0 && lookahead == 1 (input done a byte at time)
                  */
                 more--;
             }
         }
     #endif
 
         /*  If the window is almost full and there is insufficient lookahead,
             move the upper half to the lower one to make room in the upper half.
         */
         if(strstart_ >= wsize+max_dist())
         {
             std::memcpy(window_, window_+wsize, (unsigned)wsize);
             match_start_ -= wsize;
             strstart_    -= wsize; // we now have strstart >= max_dist
             block_start_ -= (long) wsize;
             if (insert_ > strstart_)
                 insert_ = strstart_;
 
             /* Slide the hash table (could be avoided with 32 bit values
                at the expense of memory usage). We slide even when level == 0
                to keep the hash table consistent if we switch back to level > 0
                later. (Using level 0 permanently is not an optimal usage of
                zlib, so we don't care about this pathological case.)
             */
             n = hash_size_;
             p = &head_[n];
             do
             {
                 m = *--p;
                 *p = (std::uint16_t)(m >= wsize ? m-wsize : 0);
             }
             while(--n);
 
             n = wsize;
             p = &prev_[n];
             do
             {
                 m = *--p;
                 *p = (std::uint16_t)(m >= wsize ? m-wsize : 0);
                 /*  If n is not on any hash chain, prev[n] is garbage but
                     its value will never be used.
                 */
             }
             while(--n);
             more += wsize;
         }
         if(zs.avail_in == 0)
             break;
 
         /*  If there was no sliding:
                strstart <= WSIZE+max_dist-1 && lookahead <= kMinLookahead - 1 &&
                more == window_size - lookahead - strstart
             => more >= window_size - (kMinLookahead-1 + WSIZE + max_dist-1)
             => more >= window_size - 2*WSIZE + 2
             In the BIG_MEM or MMAP case (not yet supported),
               window_size == input_size + kMinLookahead  &&
               strstart + lookahead_ <= input_size => more >= kMinLookahead.
             Otherwise, window_size == 2*WSIZE so more >= 2.
             If there was sliding, more >= WSIZE. So in all cases, more >= 2.
         */
         n = read_buf(zs, window_ + strstart_ + lookahead_, more);
         lookahead_ += n;
 
         // Initialize the hash value now that we have some input:
         if(lookahead_ + insert_ >= minMatch)
         {
             uInt str = strstart_ - insert_;
             ins_h_ = window_[str];
             update_hash(ins_h_, window_[str + 1]);
             while(insert_)
             {
                 update_hash(ins_h_, window_[str + minMatch-1]);
                 prev_[str & w_mask_] = head_[ins_h_];
                 head_[ins_h_] = (std::uint16_t)str;
                 str++;
                 insert_--;
                 if(lookahead_ + insert_ < minMatch)
                     break;
             }
         }
         /*  If the whole input has less than minMatch bytes, ins_h is garbage,
             but this is not important since only literal bytes will be emitted.
         */
     }
     while(lookahead_ < kMinLookahead && zs.avail_in != 0);
 
     /*  If the kWinInit bytes after the end of the current data have never been
         written, then zero those bytes in order to avoid memory check reports of
         the use of uninitialized (or uninitialised as Julian writes) bytes by
         the longest match routines.  Update the high water mark for the next
         time through here.  kWinInit is set to maxMatch since the longest match
         routines allow scanning to strstart + maxMatch, ignoring lookahead.
     */
     if(high_water_ < window_size_)
     {
         std::uint32_t curr = strstart_ + (std::uint32_t)(lookahead_);
         std::uint32_t winit;
 
         if(high_water_ < curr)
         {
             /*  Previous high water mark below current data -- zero kWinInit
                 bytes or up to end of window, whichever is less.
             */
             winit = window_size_ - curr;
             if(winit > kWinInit)
                 winit = kWinInit;
             std::memset(window_ + curr, 0, (unsigned)winit);
             high_water_ = curr + winit;
         }
         else if(high_water_ < (std::uint32_t)curr + kWinInit)
         {
             /*  High water mark at or above current data, but below current data
                 plus kWinInit -- zero out to current data plus kWinInit, or up
                 to end of window, whichever is less.
             */
             winit = (std::uint32_t)curr + kWinInit - high_water_;
             if(winit > window_size_ - high_water_)
                 winit = window_size_ - high_water_;
             std::memset(window_ + high_water_, 0, (unsigned)winit);
             high_water_ += winit;
         }
     }
 }
 
 /*  Flush as much pending output as possible. All write() output goes
     through this function so some applications may wish to modify it
     to avoid allocating a large strm->next_out buffer and copying into it.
     (See also read_buf()).
 */
 void
 deflate_stream::
 flush_pending(z_params& zs)
 {
     tr_flush_bits();
     auto len = clamp(pending_, zs.avail_out);
     if(len == 0)
         return;
 
     std::memcpy(zs.next_out, pending_out_, len);
     zs.next_out =
         static_cast<std::uint8_t*>(zs.next_out) + len;
     pending_out_  += len;
     zs.total_out += len;
     zs.avail_out  -= len;
     pending_ -= len;
     if(pending_ == 0)
         pending_out_ = pending_buf_;
 }
 
 /*  Flush the current block, with given end-of-file flag.
     IN assertion: strstart is set to the end of the current match.
 */
 void
 deflate_stream::
 flush_block(z_params& zs, bool last)
 {
     tr_flush_block(zs,
         (block_start_ >= 0L ?
             (char *)&window_[(unsigned)block_start_] :
             (char *)0),
         (std::uint32_t)((long)strstart_ - block_start_),
         last);
    block_start_ = strstart_;
    flush_pending(zs);
 }
 
 /*  Read a new buffer from the current input stream, update the adler32
     and total number of bytes read.  All write() input goes through
     this function so some applications may wish to modify it to avoid
     allocating a large strm->next_in buffer and copying from it.
     (See also flush_pending()).
 */
 int
 deflate_stream::
 read_buf(z_params& zs, Byte *buf, unsigned size)
 {
     auto len = clamp(zs.avail_in, size);
     if(len == 0)
         return 0;
 
     zs.avail_in  -= len;
 
     std::memcpy(buf, zs.next_in, len);
     zs.next_in = static_cast<
         std::uint8_t const*>(zs.next_in) + len;
     zs.total_in += len;
     return (int)len;
 }
 
 /*  Set match_start to the longest match starting at the given string and
     return its length. Matches shorter or equal to prev_length are discarded,
     in which case the result is equal to prev_length and match_start is
     garbage.
     IN assertions: cur_match is the head of the hash chain for the current
         string (strstart) and its distance is <= max_dist, and prev_length >= 1
     OUT assertion: the match length is not greater than s->lookahead_.
 
     For 80x86 and 680x0, an optimized version will be provided in match.asm or
     match.S. The code will be functionally equivalent.
 */
 uInt
 deflate_stream::
 longest_match(IPos cur_match)
 {
     unsigned chain_length = max_chain_length_;/* max hash chain length */
     Byte *scan = window_ + strstart_; /* current string */
     Byte *match;                       /* matched string */
     int len;                           /* length of current match */
     int best_len = prev_length_;              /* best match length so far */
     int nice_match = nice_match_;             /* stop if match long enough */
     IPos limit = strstart_ > (IPos)max_dist() ?
         strstart_ - (IPos)max_dist() : 0;
     /* Stop when cur_match becomes <= limit. To simplify the code,
      * we prevent matches with the string of window index 0.
      */
     std::uint16_t *prev = prev_;
     uInt wmask = w_mask_;
 
     Byte *strend = window_ + strstart_ + maxMatch;
     Byte scan_end1  = scan[best_len-1];
     Byte scan_end   = scan[best_len];
 
     /* The code is optimized for HASH_BITS >= 8 and maxMatch-2 multiple of 16.
      * It is easy to get rid of this optimization if necessary.
      */
     BOOST_ASSERT(hash_bits_ >= 8 && maxMatch == 258);
 
     /* Do not waste too much time if we already have a good match: */
     if(prev_length_ >= good_match_) {
         chain_length >>= 2;
     }
     /* Do not look for matches beyond the end of the input. This is necessary
      * to make deflate deterministic.
      */
     if((uInt)nice_match > lookahead_)
         nice_match = lookahead_;
 
     BOOST_ASSERT((std::uint32_t)strstart_ <= window_size_-kMinLookahead);
 
     do {
         BOOST_ASSERT(cur_match < strstart_);
         match = window_ + cur_match;
 
         /* Skip to next match if the match length cannot increase
          * or if the match length is less than 2.  Note that the checks below
          * for insufficient lookahead only occur occasionally for performance
          * reasons.  Therefore uninitialized memory will be accessed, and
          * conditional jumps will be made that depend on those values.
          * However the length of the match is limited to the lookahead, so
          * the output of deflate is not affected by the uninitialized values.
          */
         if(     match[best_len]   != scan_end  ||
                 match[best_len-1] != scan_end1 ||
                 *match            != *scan     ||
                 *++match          != scan[1])
             continue;
 
         /* The check at best_len-1 can be removed because it will be made
          * again later. (This heuristic is not always a win.)
          * It is not necessary to compare scan[2] and match[2] since they
          * are always equal when the other bytes match, given that
          * the hash keys are equal and that HASH_BITS >= 8.
          */
         scan += 2, match++;
         BOOST_ASSERT(*scan == *match);
 
         /* We check for insufficient lookahead only every 8th comparison;
          * the 256th check will be made at strstart+258.
          */
         do
         {
         }
         while(  *++scan == *++match && *++scan == *++match &&
                 *++scan == *++match && *++scan == *++match &&
                 *++scan == *++match && *++scan == *++match &&
                 *++scan == *++match && *++scan == *++match &&
                 scan < strend);
 
         BOOST_ASSERT(scan <= window_+(unsigned)(window_size_-1));
 
         len = maxMatch - (int)(strend - scan);
         scan = strend - maxMatch;
 
         if(len > best_len) {
             match_start_ = cur_match;
             best_len = len;
             if(len >= nice_match) break;
             scan_end1  = scan[best_len-1];
             scan_end   = scan[best_len];
         }
     }
     while((cur_match = prev[cur_match & wmask]) > limit
         && --chain_length != 0);
 
     if((uInt)best_len <= lookahead_)
         return (uInt)best_len;
     return lookahead_;
 }
 
 //------------------------------------------------------------------------------
 
 /*  Copy without compression as much as possible from the input stream, return
     the current block state.
     This function does not insert new strings in the dictionary since
     uncompressible data is probably not useful. This function is used
     only for the level=0 compression option.
     NOTE: this function should be optimized to avoid extra copying from
     window to pending_buf.
 */
 auto
 deflate_stream::
 f_stored(z_params& zs, Flush flush) ->
     block_state
 {
     /* Stored blocks are limited to 0xffff bytes, pending_buf is limited
      * to pending_buf_size, and each stored block has a 5 byte header:
      */
     std::uint32_t max_block_size = 0xffff;
     std::uint32_t max_start;
 
     if(max_block_size > pending_buf_size_ - 5) {
         max_block_size = pending_buf_size_ - 5;
     }
 
     /* Copy as much as possible from input to output: */
     for(;;) {
         /* Fill the window as much as possible: */
         if(lookahead_ <= 1) {
 
             BOOST_ASSERT(strstart_ < w_size_+max_dist() ||
                    block_start_ >= (long)w_size_);
 
             fill_window(zs);
             if(lookahead_ == 0 && flush == Flush::none)
                 return need_more;
 
             if(lookahead_ == 0) break; /* flush the current block */
         }
         BOOST_ASSERT(block_start_ >= 0L);
 
         strstart_ += lookahead_;
         lookahead_ = 0;
 
         /* Emit a stored block if pending_buf will be full: */
         max_start = block_start_ + max_block_size;
         if(strstart_ == 0 || (std::uint32_t)strstart_ >= max_start) {
             /* strstart == 0 is possible when wraparound on 16-bit machine */
             lookahead_ = (uInt)(strstart_ - max_start);
             strstart_ = (uInt)max_start;
             flush_block(zs, false);
             if(zs.avail_out == 0)
                 return need_more;
         }
         /* Flush if we may have to slide, otherwise block_start may become
          * negative and the data will be gone:
          */
         if(strstart_ - (uInt)block_start_ >= max_dist()) {
             flush_block(zs, false);
             if(zs.avail_out == 0)
                 return need_more;
         }
     }
     insert_ = 0;
     if(flush == Flush::finish)
     {
         flush_block(zs, true);
         if(zs.avail_out == 0)
             return finish_started;
         return finish_done;
     }
     if((long)strstart_ > block_start_)
     {
         flush_block(zs, false);
         if(zs.avail_out == 0)
             return need_more;
     }
     return block_done;
 }
 
 /*  Compress as much as possible from the input stream, return the current
     block state.
     This function does not perform lazy evaluation of matches and inserts
     new strings in the dictionary only for unmatched strings or for short
     matches. It is used only for the fast compression options.
 */
 auto
 deflate_stream::
 f_fast(z_params& zs, Flush flush) ->
     block_state
 {
     IPos hash_head;       /* head of the hash chain */
     bool bflush;           /* set if current block must be flushed */
 
     for(;;)
     {
         /* Make sure that we always have enough lookahead, except
          * at the end of the input file. We need maxMatch bytes
          * for the next match, plus minMatch bytes to insert the
          * string following the next match.
          */
         if(lookahead_ < kMinLookahead)
         {
             fill_window(zs);
             if(lookahead_ < kMinLookahead && flush == Flush::none)
                 return need_more;
             if(lookahead_ == 0)
                 break; /* flush the current block */
         }
 
         /* Insert the string window[strstart .. strstart+2] in the
          * dictionary, and set hash_head to the head of the hash chain:
          */
         hash_head = 0;
         if(lookahead_ >= minMatch) {
             insert_string(hash_head);
         }
 
         /* Find the longest match, discarding those <= prev_length.
          * At this point we have always match_length < minMatch
          */
         if(hash_head != 0 && strstart_ - hash_head <= max_dist()) {
             /* To simplify the code, we prevent matches with the string
              * of window index 0 (in particular we have to avoid a match
              * of the string with itself at the start of the input file).
              */
             match_length_ = longest_match (hash_head);
             /* longest_match() sets match_start */
         }
         if(match_length_ >= minMatch)
         {
             tr_tally_dist(static_cast<std::uint16_t>(strstart_ - match_start_),
                 static_cast<std::uint8_t>(match_length_ - minMatch), bflush);
 
             lookahead_ -= match_length_;
 
             /* Insert new strings in the hash table only if the match length
              * is not too large. This saves time but degrades compression.
              */
             if(match_length_ <= max_lazy_match_ &&
                 lookahead_ >= minMatch) {
                 match_length_--; /* string at strstart already in table */
                 do
                 {
                     strstart_++;
                     insert_string(hash_head);
                     /* strstart never exceeds WSIZE-maxMatch, so there are
                      * always minMatch bytes ahead.
                      */
                 }
                 while(--match_length_ != 0);
                 strstart_++;
             }
             else
             {
                 strstart_ += match_length_;
                 match_length_ = 0;
                 ins_h_ = window_[strstart_];
                 update_hash(ins_h_, window_[strstart_+1]);
                 /* If lookahead < minMatch, ins_h is garbage, but it does not
                  * matter since it will be recomputed at next deflate call.
                  */
             }
         }
         else
         {
             /* No match, output a literal byte */
             tr_tally_lit(window_[strstart_], bflush);
             lookahead_--;
             strstart_++;
         }
         if(bflush)
         {
             flush_block(zs, false);
             if(zs.avail_out == 0)
                 return need_more;
         }
     }
     insert_ = strstart_ < minMatch-1 ? strstart_ : minMatch-1;
     if(flush == Flush::finish)
     {
         flush_block(zs, true);
         if(zs.avail_out == 0)
             return finish_started;
         return finish_done;
     }
     if(sym_next_)
     {
         flush_block(zs, false);
         if(zs.avail_out == 0)
             return need_more;
     }
     return block_done;
 }
 
 /*  Same as above, but achieves better compression. We use a lazy
     evaluation for matches: a match is finally adopted only if there is
     no better match at the next window position.
 */
 auto
 deflate_stream::
 f_slow(z_params& zs, Flush flush) ->
     block_state
 {
     IPos hash_head;          /* head of hash chain */
     bool bflush;              /* set if current block must be flushed */
 
     /* Process the input block. */
     for(;;)
     {
         /* Make sure that we always have enough lookahead, except
          * at the end of the input file. We need maxMatch bytes
          * for the next match, plus minMatch bytes to insert the
          * string following the next match.
          */
         if(lookahead_ < kMinLookahead)
         {
             fill_window(zs);
             if(lookahead_ < kMinLookahead && flush == Flush::none)
                 return need_more;
             if(lookahead_ == 0)
                 break; /* flush the current block */
         }
 
         /* Insert the string window[strstart .. strstart+2] in the
          * dictionary, and set hash_head to the head of the hash chain:
          */
         hash_head = 0;
         if(lookahead_ >= minMatch)
             insert_string(hash_head);
 
         /* Find the longest match, discarding those <= prev_length.
          */
         prev_length_ = match_length_, prev_match_ = match_start_;
         match_length_ = minMatch-1;
 
         if(hash_head != 0 && prev_length_ < max_lazy_match_ &&
             strstart_ - hash_head <= max_dist())
         {
             /* To simplify the code, we prevent matches with the string
              * of window index 0 (in particular we have to avoid a match
              * of the string with itself at the start of the input file).
              */
             match_length_ = longest_match(hash_head);
             /* longest_match() sets match_start */
 
             if(match_length_ <= 5 && (strategy_ == Strategy::filtered
                 || (match_length_ == minMatch &&
                     strstart_ - match_start_ > kTooFar)
                 ))
             {
                 /* If prev_match is also minMatch, match_start is garbage
                  * but we will ignore the current match anyway.
                  */
                 match_length_ = minMatch-1;
             }
         }
         /* If there was a match at the previous step and the current
          * match is not better, output the previous match:
          */
         if(prev_length_ >= minMatch && match_length_ <= prev_length_)
         {
             /* Do not insert strings in hash table beyond this. */
             uInt max_insert = strstart_ + lookahead_ - minMatch;
 
             tr_tally_dist(
                 static_cast<std::uint16_t>(strstart_ -1 - prev_match_),
                 static_cast<std::uint8_t>(prev_length_ - minMatch), bflush);
 
             /* Insert in hash table all strings up to the end of the match.
              * strstart-1 and strstart are already inserted. If there is not
              * enough lookahead, the last two strings are not inserted in
              * the hash table.
              */
             lookahead_ -= prev_length_-1;
             prev_length_ -= 2;
             do {
                 if(++strstart_ <= max_insert)
                     insert_string(hash_head);
             }
             while(--prev_length_ != 0);
             match_available_ = 0;
             match_length_ = minMatch-1;
             strstart_++;
 
             if(bflush)
             {
                 flush_block(zs, false);
                 if(zs.avail_out == 0)
                     return need_more;
             }
 
         }
         else if(match_available_)
         {
             /* If there was no match at the previous position, output a
              * single literal. If there was a match but the current match
              * is longer, truncate the previous match to a single literal.
              */
             tr_tally_lit(window_[strstart_-1], bflush);
             if(bflush)
                 flush_block(zs, false);
             strstart_++;
             lookahead_--;
             if(zs.avail_out == 0)
                 return need_more;
         }
         else
         {
             /* There is no previous match to compare with, wait for
              * the next step to decide.
              */
             match_available_ = 1;
             strstart_++;
             lookahead_--;
         }
     }
     BOOST_ASSERT(flush != Flush::none);
     if(match_available_)
     {
         tr_tally_lit(window_[strstart_-1], bflush);
         match_available_ = 0;
     }
     insert_ = strstart_ < minMatch-1 ? strstart_ : minMatch-1;
     if(flush == Flush::finish)
     {
         flush_block(zs, true);
         if(zs.avail_out == 0)
             return finish_started;
         return finish_done;
     }
     if(sym_next_)
     {
         flush_block(zs, false);
         if(zs.avail_out == 0)
             return need_more;
     }
     return block_done;
 }
 
 /*  For Strategy::rle, simply look for runs of bytes, generate matches only of distance
     one.  Do not maintain a hash table.  (It will be regenerated if this run of
     deflate switches away from Strategy::rle.)
 */
 auto
 deflate_stream::
 f_rle(z_params& zs, Flush flush) ->
     block_state
 {
     bool bflush;            // set if current block must be flushed
     uInt prev;              // byte at distance one to match
     Byte *scan, *strend;    // scan goes up to strend for length of run
 
     for(;;)
     {
         /* Make sure that we always have enough lookahead, except
          * at the end of the input file. We need maxMatch bytes
          * for the longest run, plus one for the unrolled loop.
          */
         if(lookahead_ <= maxMatch) {
             fill_window(zs);
             if(lookahead_ <= maxMatch && flush == Flush::none) {
                 return need_more;
             }
             if(lookahead_ == 0) break; /* flush the current block */
         }
 
         /* See how many times the previous byte repeats */
         match_length_ = 0;
         if(lookahead_ >= minMatch && strstart_ > 0) {
             scan = window_ + strstart_ - 1;
             prev = *scan;
             if(prev == *++scan && prev == *++scan && prev == *++scan) {
                 strend = window_ + strstart_ + maxMatch;
                 do {
                 } while(prev == *++scan && prev == *++scan &&
                          prev == *++scan && prev == *++scan &&
                          prev == *++scan && prev == *++scan &&
                          prev == *++scan && prev == *++scan &&
                          scan < strend);
                 match_length_ = maxMatch - (int)(strend - scan);
                 if(match_length_ > lookahead_)
                     match_length_ = lookahead_;
             }
             BOOST_ASSERT(scan <= window_+(uInt)(window_size_-1));
         }
 
         /* Emit match if have run of minMatch or longer, else emit literal */
         if(match_length_ >= minMatch) {
             tr_tally_dist(std::uint16_t{1},
                 static_cast<std::uint8_t>(match_length_ - minMatch),
                 bflush);
 
             lookahead_ -= match_length_;
             strstart_ += match_length_;
             match_length_ = 0;
         } else {
             /* No match, output a literal byte */
             tr_tally_lit(window_[strstart_], bflush);
             lookahead_--;
             strstart_++;
         }
         if(bflush)
         {
             flush_block(zs, false);
             if(zs.avail_out == 0)
                 return need_more;
         }
     }
     insert_ = 0;
     if(flush == Flush::finish)
     {
         flush_block(zs, true);
         if(zs.avail_out == 0)
             return finish_started;
         return finish_done;
     }
     if(sym_next_)
     {
         flush_block(zs, false);
         if(zs.avail_out == 0)
             return need_more;
     }
     return block_done;
 }
 
 /* ===========================================================================
  * For Strategy::huffman, do not look for matches.  Do not maintain a hash table.
  * (It will be regenerated if this run of deflate switches away from Huffman.)
  */
 auto
 deflate_stream::
 f_huff(z_params& zs, Flush flush) ->
     block_state
 {
     bool bflush;             // set if current block must be flushed
 
     for(;;)
     {
         // Make sure that we have a literal to write.
         if(lookahead_ == 0)
         {
             fill_window(zs);
             if(lookahead_ == 0)
             {
                 if(flush == Flush::none)
                     return need_more;
                 break;      // flush the current block
             }
         }
 
         // Output a literal byte
         match_length_ = 0;
         tr_tally_lit(window_[strstart_], bflush);
         lookahead_--;
         strstart_++;
         if(bflush)
         {
             flush_block(zs, false);
             if(zs.avail_out == 0)
                 return need_more;
         }
     }
     insert_ = 0;
     if(flush == Flush::finish)
     {
         flush_block(zs, true);
         if(zs.avail_out == 0)
             return finish_started;
         return finish_done;
     }
     if(sym_next_)
     {
         flush_block(zs, false);
         if(zs.avail_out == 0)
             return need_more;
     }
     return block_done;
 }
 
 } // detail
 } // zlib
 } // beast
 } // boost
 
 #endif

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