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% XXX The module has been extended (by Jeremy) but this documentation
% hasn't been updated completely.
\section{\module{zlib} ---
Compression and decompression compatible with \program{gzip}.}
\declaremodule{builtin}{zlib}
\modulesynopsis{Low-level interface to compression and decompression
routines compatible with \program{gzip}.}
For applications that require data compression, the functions in this
module allow compression and decompression, using the zlib library.
The zlib library has its own home page at
\url{http://www.cdrom.com/pub/infozip/zlib/}. Version 1.0.4 is the
most recent version as of December, 1997; use a later version if one
is available.
The available exception and functions in this module are:
\begin{excdesc}{error}
Exception raised on compression and decompression errors.
\end{excdesc}
\begin{funcdesc}{adler32}{string\optional{, value}}
Computes a Adler-32 checksum of \var{string}. (An Adler-32
checksum is almost as reliable as a CRC32 but can be computed much
more quickly.) If \var{value} is present, it is used as the
starting value of the checksum; otherwise, a fixed default value is
used. This allows computing a running checksum over the
concatenation of several input strings. The algorithm is not
cryptographically strong, and should not be used for
authentication or digital signatures.
\end{funcdesc}
\begin{funcdesc}{compress}{string\optional{, level}}
Compresses the data in \var{string}, returning a string contained
compressed data. \var{level} is an integer from \code{1} to
\code{9} controlling the level of compression; \code{1} is fastest
and produces the least compression, \code{9} is slowest and produces
the most. The default value is \code{6}. Raises the
\exception{error} exception if any error occurs.
\end{funcdesc}
\begin{funcdesc}{compressobj}{\optional{level}}
Returns a compression object, to be used for compressing data streams
that won't fit into memory at once. \var{level} is an integer from
\code{1} to \code{9} controlling the level of compression; \code{1} is
fastest and produces the least compression, \code{9} is slowest and
produces the most. The default value is \code{6}.
\end{funcdesc}
\begin{funcdesc}{crc32}{string\optional{, value}}
Computes a CRC (Cyclic Redundancy Check)%
\index{Cyclic Redundancy Check}
\index{checksum!Cyclic Redundancy Check}
checksum of \var{string}. If
\var{value} is present, it is used as the starting value of the
checksum; otherwise, a fixed default value is used. This allows
computing a running checksum over the concatenation of several
input strings. The algorithm is not cryptographically strong, and
should not be used for authentication or digital signatures.
\end{funcdesc}
\begin{funcdesc}{decompress}{string\optional{, wbits\optional{, buffsize}}}
Decompresses the data in \var{string}, returning a string containing
the uncompressed data. The \var{wbits} parameter controls the size of
the window buffer. If \var{buffsize} is given, it is used as the
initial size of the output buffer. Raises the \exception{error}
exception if any error occurs.
\end{funcdesc}
\begin{funcdesc}{decompressobj}{\optional{wbits}}
Returns a compression object, to be used for decompressing data
streams that won't fit into memory at once. The \var{wbits}
parameter controls the size of the window buffer.
\end{funcdesc}
Compression objects support the following methods:
\begin{methoddesc}[Compress]{compress}{string}
Compress \var{string}, returning a string containing compressed data
for at least part of the data in \var{string}. This data should be
concatenated to the output produced by any preceding calls to the
\method{compress()} method. Some input may be kept in internal buffers
for later processing.
\end{methoddesc}
\begin{methoddesc}[Compress]{flush}{\optional{mode}}
All pending input is processed, and a string containing the remaining
compressed output is returned. \var{mode} can be selected from the
constants \constant{Z_SYNC_FLUSH}, \constant{Z_FULL_FLUSH}, or
\constant{Z_FINISH}, defaulting to \constant{Z_FINISH}. \constant{Z_SYNC_FLUSH} and
\constant{Z_FULL_FLUSH} allow compressing further strings of data and
are used to allow partial error recovery on decompression, while
\constant{Z_FINISH} finishes the compressed stream and
prevents compressing any more data. After calling
\method{flush()} with \var{mode} set to \constant{Z_FINISH}, the
\method{compress()} method cannot be called again; the only realistic
action is to delete the object.
\end{methoddesc}
Decompression objects support the following methods:
\begin{methoddesc}[Decompress]{decompress}{string}
Decompress \var{string}, returning a string containing the
uncompressed data corresponding to at least part of the data in
\var{string}. This data should be concatenated to the output produced
by any preceding calls to the
\method{decompress()} method. Some of the input data may be preserved
in internal buffers for later processing.
\end{methoddesc}
\begin{methoddesc}[Decompress]{flush}{}
All pending input is processed, and a string containing the remaining
uncompressed output is returned. After calling \method{flush()}, the
\method{decompress()} method cannot be called again; the only realistic
action is to delete the object.
\end{methoddesc}
\begin{seealso}
\seemodule{gzip}{reading and writing \program{gzip}-format files}
\end{seealso}
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