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=head1 NAME

Lucy::Search::Compiler - Query-to-Matcher compiler.

=head1 SYNOPSIS

    # (Compiler is an abstract base class.)
    package MyCompiler;
    use base qw( Lucy::Search::Compiler );

    sub make_matcher {
        my $self = shift;
        return MyMatcher->new( @_, compiler => $self );
    }



=head1 DESCRIPTION

The purpose of the Compiler class is to take a specification in the form of
a L<Query|Lucy::Search::Query> object and compile a
L<Matcher|Lucy::Search::Matcher> object that can do real work.

The simplest Compiler subclasses -- such as those associated with
constant-scoring Query types -- might simply implement a make_matcher()
method which passes along information verbatim from the Query to the
Matcher's constructor.

However it is common for the Compiler to perform some calculations which
affect it's "weight" -- a floating point multiplier that the Matcher will
factor into each document's score.  If that is the case, then the Compiler
subclass may wish to override get_weight(), sum_of_squared_weights(), and
apply_norm_factor().

Compiling a Matcher is a two stage process.

The first stage takes place during the Compiler's construction, which is
where the Query object meets a L<Searcher|Lucy::Search::Searcher>
object for the first time.  Searchers operate on a specific document
collection and they can tell you certain statistical information about the
collection -- such as how many total documents are in the collection, or
how many documents in the collection a particular term is present in.
Lucy's core Compiler classes plug this information into the classic
TF/IDF weighting algorithm to adjust the Compiler's weight; custom
subclasses might do something similar.

The second stage of compilation is make_matcher(), method, which is where
the Compiler meets a L<SegReader|Lucy::Index::SegReader> object.
SegReaders are associated with a single segment within a single index on a
single machine, and are thus lower-level than Searchers, which may
represent a document collection spread out over a search cluster
(comprising several indexes and many segments).  The Compiler object can
use new information supplied by the SegReader -- such as whether a term is
missing from the local index even though it is present within the larger
collection represented by the Searcher -- when figuring out what to feed to
the Matchers's constructor, or whether make_matcher() should return a
Matcher at all.

=head1 CONSTRUCTORS

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

    my $compiler = MyCompiler->SUPER::new(
        parent     => $my_query,
        searcher   => $searcher,
        similarity => $sim,        # default: undef
        boost      => undef,       # default: see below
    );

Abstract constructor.

=over

=item *

B<parent> - The parent Query.

=item *

B<searcher> - A Lucy::Search::Searcher, such as an
IndexSearcher.

=item *

B<similarity> - A Similarity.

=item *

B<boost> - An arbitrary scoring multiplier.  Defaults to the boost of
the parent Query.

=back



=head1 ABSTRACT METHODS

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

Factory method returning a Matcher.

=over

=item *

B<reader> - A SegReader.

=item *

B<need_score> - Indicate whether the Matcher must implement score().

=back

Returns: a Matcher, or undef if the Matcher would have matched no
documents.



=head1 METHODS

=head2 get_weight()

Return the Compiler's numerical weight, a scoring multiplier.  By
default, returns the object's boost.

=head2 sum_of_squared_weights()

Compute and return a raw weighting factor.  (This quantity is used by
normalize()).  By default, simply returns 1.0.

=head2 apply_norm_factor(factor)

Apply a floating point normalization multiplier.  For a TermCompiler,
this involves multiplying its own weight by the supplied factor;
combining classes such as ORCompiler would apply the factor recursively
to their children.

The default implementation is a no-op; subclasses may wish to multiply
their internal weight by the supplied factor.

=over

=item *

B<factor> - The multiplier.

=back

=head2 normalize()

Take a newly minted Compiler object and apply query-specific
normalization factors.  Should be invoked by Query subclasses during
make_compiler() for top-level nodes.

For a TermQuery, the scoring formula is approximately:

    (tf_d * idf_t / norm_d) * (tf_q * idf_t / norm_q)

normalize() is theoretically concerned with applying the second half of
that formula to a the Compiler's weight. What actually happens depends
on how the Compiler and Similarity methods called internally are
implemented.

=head2 get_parent()

Accessor for the Compiler's parent Query object.

=head2 get_similarity()

Accessor for the Compiler's Similarity object.

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

Return an array of Span objects, indicating where in the given field
the text that matches the parent Query occurs and how well each snippet
matches.  The Span's offset and length are measured in Unicode code
points.

The default implementation returns an empty array.

=over

=item *

B<searcher> - A Searcher.

=item *

B<doc_vec> - A DocVector.

=item *

B<field> - The name of the field.

=back



=head1 INHERITANCE

Lucy::Search::Compiler isa L<Lucy::Search::Query> isa L<Lucy::Object::Obj>.


=cut