<namesection> <name>bz2</name> <desc>Data compression "bz2"</desc> </namesection> <synopsis> <syntax> package require <package>Tcl</package> <o>8.2</o> package require <package>Trf</package> <o>2.1.3</o> <cmd>bz2</cmd> <o><m>options...</m></o> <o><m>data</m></o> </syntax> </synopsis>
The command <cmd>bz2</cmd> is one of several data compressions provided by the package <package>trf</package>. See <syscmd>trf-intro</syscmd> for an overview of the whole package.

The command is based on the Burroughs-Wheeler transformation as implemented by the bzip2 compression library (<url>http://sources.redhat.com/bzip2/</url>).

<dle>
<cmd>bz2</cmd> <o><m>options...</m></o> <o><m>data</m></o>
  <dle>
  -mode <l>compress</l>|<l>decompress</l>
    This option has to be present and is always understood by the compression.
    For <term>immediate</term> mode the argument value specifies the operation to use. For an <term>attached</term> compress it specifies the operation to use for <emph>writing</emph>. Reading will automatically use the reverse operation. See section IMMEDIATE versus ATTACHED for explanations of these two terms.
    Beyond the argument values listed above all unique abbreviations are recognized too.
    <l>Compress</l> causes the compression of arbitrary (most likely binary) data. <l>Decompression</l> does the reverse .
  </dle> <dle>
  -level <m>integer</m>
    Specifies the compression level. Is either the string <l>default</l> or an integer number in the range <l>1</l> (minimal compression) to <l>9</l> (maximal compression).
  </dle> <dle>
  -attach <m>channel</m>
    The presence/absence of this option determines the main operation mode of the transformation.
    If present the transformation will be stacked onto the <m>channel</m> whose handle was given to the option and run in <term>attached</term> mode. More about this in section IMMEDIATE versus ATTACHED.
    If the option is absent the transformation is used in <term>immediate</term> mode and the options listed below are recognized. More about this in section IMMEDIATE versus ATTACHED.
  </dle> <dle>
  -in <m>channel</m>
    This options is legal if and only if the transformation is used in <term>immediate</term> mode. It provides the handle of the channel the data to transform has to be read from.
    If the transformation is in <term>immediate</term> mode and this option is absent the data to transform is expected as the last argument to the transformation.
  </dle> <dle>
  -out <m>channel</m>
    This options is legal if and only if the transformation is used in <term>immediate</term> mode. It provides the handle of the channel the generated transformation result is written to.
    If the transformation is in <term>immediate</term> mode and this option is absent the generated data is returned as the result of the command itself.
  </dle>
</dle>

IMMEDIATE VERSUS ATTACHED The transformation distinguishes between two main ways of using it. These are the <term>immediate</term> and <term>attached</term> operation modes.

For the <term>attached</term> mode the option -attach is used to associate the transformation with an existing channel. During the execution of the command no transformation is performed, instead the channel is changed in such a way, that from then on all data written to or read from it passes through the transformation and is modified by it according to the definition above. This attachment can be revoked by executing the command <cmd>unstack</cmd> for the chosen channel. This is the only way to do this at the Tcl level.

In the second mode, which can be detected by the absence of option -attach, the transformation immediately takes data from either its commandline or a channel, transforms it, and returns the result either as result of the command, or writes it into a channel. The mode is named after the immediate nature of its execution.

Where the data is taken from, and delivered to, is governed by the presence and absence of the options -in and -out. It should be noted that this ability to immediately read from and/or write to a channel is an historic artifact which was introduced at the beginning of Trf's life when Tcl version 7.6 was current as this and earlier versions have trouble to deal with \0 characters embedded into either input or output.

<seealso> <ref>trf-intro <ref>bz2 <ref>zip </seealso> <keywords> <keyword>Burroughs-Wheeler</keyword> <keyword>bz2</keyword> <keyword>compression</keyword> <keyword>decompression</keyword> <keyword>data compression</keyword> </keywords>