There have been many reports of bugs in the assembler codes
intended to speed up deflate and inflate. They are third-party
contributions in contrib, and so are not supported by the zlib
maintainers.
Limit read() and write() requests to sizes that fit in an int.
This allows storing the return value in an int, and avoiding the
need to use or construct an ssize_t type. This is required for
Microsoft C, whose _read and _write functions take an unsigned
request and return an int.
Normally these are set to size_t and ssize_t. But if they do not
exist, then they are set to the smallest integer type that can
contain a pointer. size_t is unsigned and ssize_t is signed.
gzsetparams() was using Z_PARTIAL_FLUSH when it could use Z_BLOCK
instead. This commit uses Z_BLOCK, which avoids emitting an
unnecessary ten bits into the stream.
In some cases the return values did not match the documentation,
or the documentation did not document all of the return values.
gzprintf() now consistently returns negative values on error,
which matches the behavior of the stdio fprintf() function.
The previous code slid the window and the hash table and copied
every input byte three times in order to just write the data as
stored blocks with no compression. This commit minimizes sliding
and copying, especially for large input and output buffers.
Level 0 compression is now more than 20 times faster than before
the commit.
Most of the speedup is due to deferring hash table slides until
deflateParams() is called to change the compression level away
from 0. More speedup is due to copying directly from next_in to
next_out when the amounts of available input data and output space
permit it, avoiding the intermediate pending buffer. Additionally,
only the last 32K of the used input data is copied back to the
sliding window when large input buffers are provided.
This alters the specification in zlib.h, so that deflateParams()
will not change any parameters if there is not enough output space
in the event that a block is emitted in order to allow switching
the compression function.
When debugging the Huffman coding would warn about resulting codes
greater than 15 bits in length. This is handled properly, and is
not uncommon. This increases the verbosity of the warning by one,
so that it is not displayed by default.
This speeds up level 0 by about a factor of three, as compared to
the previous byte-at-a-time loop. We can do much better though. A
later commit avoids this copy for level 0 with large buffers,
instead copying directly from the input to the output. This commit
still speeds up storing incompressible data found when compressing
normally.
Compression level 0 requests no compression, using only stored
blocks. When Z_HUFFMAN or Z_RLE was used with level 0 (granted,
an odd choice, but permitted), the resulting blocks were mostly
fixed or dynamic. The reason is that deflate_stored() was not
being called in that case. The compressed data was valid, but it
was not what the application requested. This commit assures that
only stored blocks are emitted for compression level 0, regardless
of the strategy selected.
This updates the OS_CODE determination at compile time to match as
closely as possible the operating system mappings documented in
the PKWare APPNOTE.TXT version 6.3.4, section 4.4.2.2. That byte
in the gzip header is used by nobody for anything, as far as I can
tell. However we might as well try to set it appropriately.
