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=head1 NAME

Lucy::Index::Segment - Warehouse for information about one segment of an inverted index.

=head1 SYNOPSIS

    # Index-time.
    package MyDataWriter;
    use base qw( Lucy::Index::DataWriter );

    sub finish {
        my $self     = shift;
        my $segment  = $self->get_segment;
        my $metadata = $self->SUPER::metadata();
        $metadata->{foo} = $self->get_foo;
        $segment->store_metadata(
            key       => 'my_component',
            metadata  => $metadata
        );
    }

    # Search-time.
    package MyDataReader;
    use base qw( Lucy::Index::DataReader );

    sub new {
        my $self     = shift->SUPER::new(@_);
        my $segment  = $self->get_segment;
        my $metadata = $segment->fetch_metadata('my_component');
        if ($metadata) {
            $self->set_foo( $metadata->{foo} );
            ...
        }
        return $self;
    }



=head1 DESCRIPTION

Apache Lucy's indexes are made up of individual "segments", each of which is
is an independent inverted index.  On the file system, each segment is a
directory within the main index directory whose name starts with "seg_":
"seg_2", "seg_5a", etc.

Each Segment object keeps track of information about an index segment: its
fields, document count, and so on.  The Segment object itself writes one
file, C<< segmeta.json >>; besides storing info needed by Segment
itself, the "segmeta" file serves as a central repository for metadata
generated by other index components -- relieving them of the burden of
storing metadata themselves.



=head1 METHODS

=head2 add_field(field)

Register a new field and assign it a field number.  If the field was
already known, nothing happens.

=over

=item *

B<field> - Field name.

=back

Returns: the field's field number, which is a positive integer.

=head2 store_metadata( I<[labeled params]> )

Store arbitrary information in the segment's metadata Hash, to be
serialized later.  Throws an error if C<< key >> is used twice.

=over

=item *

B<key> - String identifying an index component.

=item *

B<metadata> - JSON-izable data structure.

=back

=head2 fetch_metadata(key)

Fetch a value from the Segment's metadata hash.

=head2 field_num(field)

Given a field name, return its field number for this segment (which
may differ from its number in other segments).  Return 0 (an invalid
field number) if the field name can't be found.

=over

=item *

B<field> - Field name.

=back

=head2 field_name(field_num)

Given a field number, return the name of its field, or undef if the
field name can't be found.

=head2 get_name()

Getter for the object's seg name.

=head2 get_number()

Getter for the segment number.

=head2 set_count(count)

Setter for the object's document count.

=head2 get_count()

Getter for the object's document count.



=head1 INHERITANCE

Lucy::Index::Segment isa Clownfish::Obj.


=cut