/ 
XML-Filter-Digest-0.06
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/ XML::Filter::Digest

NAME

XML::Filter::Digest

SYNOPSIS

    use strict;
    use XML::Filter::Digest;
    use XML::Handler::YAWriter;
    use IO::File;

    my $digest = new XML::Filter::Digest(
	'Handler'=>
	    new XML::Handler::YAWriter(
	    'Output' => new IO::File( ">-" ),
	    'Pretty' => {
		'AddHiddenNewLine' => 1
		}
	    ),

	'Script' =>
	    new XML::Script::Digest(
	    'Source' => { 'SystemId' => $ARGV[0] }
	    )->parse(),

	'Source' => { 'SystemId' => $ARGV[1] }
	)->parse();

    0;

DESCRIPTION

Most XML tools aim to parse some simple XML and to produce some formatted output. XML::Filter::Digest aims to do the opposite.

Many formats can now be parsed by a SAX Driver. XPath offers a smart way to write queries to XML. XML::Filter::Digest is a PerlSAX Filter to query XML and to provide a simpler digest as a result.

XML::Filter::Digest uses its own script language that can be parsed by XML::Script::Digest to formulate these digest queries.

In fact, a digest script is well-formed XML.

The following script defines that the result XML should have a root element called extract, containing several elements called section starting from the 4th HTML header. Those section elements contain id, title and intro elements, which in turn contain the XPath string-value of their nodes as character data.

<digest name="extract">
<collect
    name="section"
    node="//html//h2[position()&gt;3]"
    >
    <collect
        name="id"
    node="child::a/attribute::name"
    />
    <collect
        name="title"
    node="."
    />
    <collect
        name="intro"
    node="following-sibling::p[position()=1]"
    />
</collect>
</digest>

The digest script parser silently ignores anything other than digest elements and collect elements. The digest element needs a name attribute defining the name of the root element, while the collect element needs an additional node attribute defining XPath queries for nested elements.

Only a single digest element should exist within a script document, but there is no need that the digest script be the root element of the document. Nested within the digest element should be collect elements. They may contain several other collect elements recursivly.

METHODS

The XML::Filter::Digest object may act as a Filter to receive SAX events, or directly as a Driver if you provide a Source option to the parse method. The filter is reusable, if you arrange that the chain of Handlers is also reusable to handle multiple documents in batches. The filter requires a Handler and a Script option before the start_document method is called.

The XML::Script::Digest object may act as a Handler to receive SAX events, or directly if you provide a Source option to the parse method. The script object is reusable and a single script object can be used for several filter objects.

new

Creates a new XML::Driver::HTML object. Default options for parsing, described below, are passed as key-value pairs or as a single hash. Options may be changed directly in the object.

parse

Parses a document by embedding XML::Parser::PerlSAX. This allows you to use XML::Filter::Digest directly as a Driver and simplifies generating a ready-to-use XML::Script::Object.

Options, described below, are passed as key-value pairs or as a single hash. Options passed to parse() override the default options in the object for the duration of the parse.

start_document

Notifies the object about the start of a new document. The object will do its cleanup if it's reused.

end_document

Notifies the object about the end of the document. Return value of XML::Script::Digest is $self, to be used as the return value of the parse method.

XML::Filter::Digest will walk through the script object to generate a stream of SAX events for its Handler. Return value of XML::Filter::Digest is the return value of the end_document method of the Handler object.

OPTIONS

Script

XML::Script::Digest objects can be used for several XML::Filter::Digest objects.

Handler

Default SAX Handler to receive events from XML::Filter::Digest objects.

Source

XML::Filter::Digest and XML::Script can be used on raw XML directly, by calling the parse() method. To do this, the Source option is required for embedding the PerlSAX parser.

The `Source' hash may contain the following parameters:

ByteStream

The raw byte stream (file handle) containing the document.

String

A string containing the document.

SystemId

The system identifier (URI) of the document.

Encoding

A string describing the character encoding.

If more than one of `ByteStream', `String', or `SystemId' are present, preference is given first to `ByteStream', then `String', then `SystemId'.

NOTES

The XML::Filter::Digest is not a streaming filter, but a buffering filter, as any processing is done by the end_document method. This could cause the Perl interpreter to run out of memory on large XML files. Ideally, define a ulimit, to prevent the system going offline for several minutes, till it detects that there is really no memory to seize somewhere in the network. Adding network swapspace ad infinitum only make things worse, so I have the following line in my .bashrc. Other operating systems offer similar constraints.

ulimit -v 98304 -d 98304 -m 98304

This line is ok on a single user machine with 32M ram and 128MB swap. I can raise this value, if I know that I wanna walk the dog.

BUGS

not yet implemented:

reuse of XML::Filter::Digest objects.

XML::XPath::Builder bug:

XML::Filter::Digest 0.06 has been tested with XML::XPath version 1.10.
Prior versions of XML::XPath::Builder wont work.

AUTHOR

Michael Koehne, Kraehe@Copyleft.De
(c) 2001 GNU General Public License

SEE ALSO

XML::Parser::PerlSAX and XML::XPath