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=encoding utf8

=head1 NAME

Lucy::Docs::Cookbook::CustomQuery - Sample subclass of Query

=head1 DESCRIPTION

Explore Apache Lucy’s support for custom query types by creating a
“PrefixQuery” class to handle trailing wildcards.

    my $prefix_query = PrefixQuery->new(
        field        => 'content',
        query_string => 'foo*',
    );
    my $hits = $searcher->hits( query => $prefix_query );
    ...

=head2 Query, Compiler, and Matcher

To add support for a new query type, we need three classes: a Query, a
Compiler, and a Matcher.

=over

=item *

PrefixQuery - a subclass of L<Query|Lucy::Search::Query>, and the only class
that client code will deal with directly.

=item *

PrefixCompiler - a subclass of L<Compiler|Lucy::Search::Compiler>, whose primary
role is to compile a PrefixQuery to a PrefixMatcher.

=item *

PrefixMatcher - a subclass of L<Matcher|Lucy::Search::Matcher>, which does the
heavy lifting: it applies the query to individual documents and assigns a
score to each match.

=back

The PrefixQuery class on its own isn’t enough because a Query object’s role is
limited to expressing an abstract specification for the search.  A Query is
basically nothing but metadata; execution is left to the Query’s companion
Compiler and Matcher.

Here’s a simplified sketch illustrating how a Searcher’s hits() method ties
together the three classes.

    sub hits {
        my ( $self, $query ) = @_;
        my $compiler = $query->make_compiler(
            searcher => $self,
            boost    => $query->get_boost,
        );
        my $matcher = $compiler->make_matcher(
            reader     => $self->get_reader,
            need_score => 1,
        );
        my @hits = $matcher->capture_hits;
        return \@hits;
    }

=head3 PrefixQuery

Our PrefixQuery class will have two attributes: a query string and a field
name.

    package PrefixQuery;
    use base qw( Lucy::Search::Query );
    use Carp;
    use Scalar::Util qw( blessed );
    
    # Inside-out member vars and hand-rolled accessors.
    my %query_string;
    my %field;
    sub get_query_string { my $self = shift; return $query_string{$$self} }
    sub get_field        { my $self = shift; return $field{$$self} }

PrefixQuery’s constructor collects and validates the attributes.

    sub new {
        my ( $class, %args ) = @_;
        my $query_string = delete $args{query_string};
        my $field        = delete $args{field};
        my $self         = $class->SUPER::new(%args);
        confess("'query_string' param is required")
            unless defined $query_string;
        confess("Invalid query_string: '$query_string'")
            unless $query_string =~ /\*\s*$/;
        confess("'field' param is required")
            unless defined $field;
        $query_string{$$self} = $query_string;
        $field{$$self}        = $field;
        return $self;
    }

Since this is an inside-out class, we’ll need a destructor:

    sub DESTROY {
        my $self = shift;
        delete $query_string{$$self};
        delete $field{$$self};
        $self->SUPER::DESTROY;
    }

The equals() method determines whether two Queries are logically equivalent:

    sub equals {
        my ( $self, $other ) = @_;
        return 0 unless blessed($other);
        return 0 unless $other->isa("PrefixQuery");
        return 0 unless $field{$$self} eq $field{$$other};
        return 0 unless $query_string{$$self} eq $query_string{$$other};
        return 1;
    }

The last thing we’ll need is a make_compiler() factory method which kicks out
a subclass of L<Compiler|Lucy::Search::Compiler>.

    sub make_compiler {
        my ( $self, %args ) = @_;
        my $subordinate = delete $args{subordinate};
        my $compiler = PrefixCompiler->new( %args, parent => $self );
        $compiler->normalize unless $subordinate;
        return $compiler;
    }

=head3 PrefixCompiler

PrefixQuery’s make_compiler() method will be called internally at search-time
by objects which subclass L<Searcher|Lucy::Search::Searcher> – such as
L<IndexSearchers|Lucy::Search::IndexSearcher>.

A Searcher is associated with a particular collection of documents.   These
documents may all reside in one index, as with IndexSearcher, or they may be
spread out across multiple indexes on one or more machines, as with
LucyX::Remote::ClusterSearcher.

Searcher objects have access to certain statistical information about the
collections they represent; for instance, a Searcher can tell you how many
documents are in the collection…

    my $maximum_number_of_docs_in_collection = $searcher->doc_max;

… or how many documents a specific term appears in:

    my $term_appears_in_this_many_docs = $searcher->doc_freq(
        field => 'content',
        term  => 'foo',
    );

Such information can be used by sophisticated Compiler implementations to
assign more or less heft to individual queries or sub-queries.  However, we’re
not going to bother with weighting for this demo; we’ll just assign a fixed
score of 1.0 to each matching document.

We don’t need to write a constructor, as it will suffice to inherit new() from
Lucy::Search::Compiler.  The only method we need to implement for
PrefixCompiler is make_matcher().

    package PrefixCompiler;
    use base qw( Lucy::Search::Compiler );
    
    sub make_matcher {
        my ( $self, %args ) = @_;
        my $seg_reader = $args{reader};
    
        # Retrieve low-level components LexiconReader and PostingListReader.
        my $lex_reader
            = $seg_reader->obtain("Lucy::Index::LexiconReader");
        my $plist_reader
            = $seg_reader->obtain("Lucy::Index::PostingListReader");
        
        # Acquire a Lexicon and seek it to our query string.
        my $substring = $self->get_parent->get_query_string;
        $substring =~ s/\*.\s*$//;
        my $field = $self->get_parent->get_field;
        my $lexicon = $lex_reader->lexicon( field => $field );
        return unless $lexicon;
        $lexicon->seek($substring);
        
        # Accumulate PostingLists for each matching term.
        my @posting_lists;
        while ( defined( my $term = $lexicon->get_term ) ) {
            last unless $term =~ /^\Q$substring/;
            my $posting_list = $plist_reader->posting_list(
                field => $field,
                term  => $term,
            );
            if ($posting_list) {
                push @posting_lists, $posting_list;
            }
            last unless $lexicon->next;
        }
        return unless @posting_lists;
        
        return PrefixMatcher->new( posting_lists => \@posting_lists );
    }

PrefixCompiler gets access to a L<SegReader|Lucy::Index::SegReader>
object when make_matcher() gets called.  From the SegReader and its
sub-components L<LexiconReader|Lucy::Index::LexiconReader> and
L<PostingListReader|Lucy::Index::PostingListReader>, we acquire a
L<Lexicon|Lucy::Index::Lexicon>, scan through the Lexicon’s unique
terms, and acquire a L<PostingList|Lucy::Index::PostingList> for each
term that matches our prefix.

Each of these PostingList objects represents a set of documents which match
the query.

=head3 PrefixMatcher

The Matcher subclass is the most involved.

    package PrefixMatcher;
    use base qw( Lucy::Search::Matcher );
    
    # Inside-out member vars.
    my %doc_ids;
    my %tick;
    
    sub new {
        my ( $class, %args ) = @_;
        my $posting_lists = delete $args{posting_lists};
        my $self          = $class->SUPER::new(%args);
        
        # Cheesy but simple way of interleaving PostingList doc sets.
        my %all_doc_ids;
        for my $posting_list (@$posting_lists) {
            while ( my $doc_id = $posting_list->next ) {
                $all_doc_ids{$doc_id} = undef;
            }
        }
        my @doc_ids = sort { $a <=> $b } keys %all_doc_ids;
        $doc_ids{$$self} = \@doc_ids;
        
        # Track our position within the array of doc ids.
        $tick{$$self} = -1;
        
        return $self;
    }
    
    sub DESTROY {
        my $self = shift;
        delete $doc_ids{$$self};
        delete $tick{$$self};
        $self->SUPER::DESTROY;
    }

The doc ids must be in order, or some will be ignored; hence the C<sort>
above.

In addition to the constructor and destructor, there are three methods that
must be overridden.

next() advances the Matcher to the next valid matching doc.

    sub next {
        my $self    = shift;
        my $doc_ids = $doc_ids{$$self};
        my $tick    = ++$tick{$$self};
        return 0 if $tick >= scalar @$doc_ids;
        return $doc_ids->[$tick];
    }

get_doc_id() returns the current document id, or 0 if the Matcher is
exhausted.  (L<Document numbers|Lucy::Docs::DocIDs> start at 1, so 0 is
a sentinel.)

    sub get_doc_id {
        my $self    = shift;
        my $tick    = $tick{$$self};
        my $doc_ids = $doc_ids{$$self};
        return $tick < scalar @$doc_ids ? $doc_ids->[$tick] : 0;
    }

score() conveys the relevance score of the current match.  We’ll just return a
fixed score of 1.0:

    sub score { 1.0 }

=head2 Usage

To get a basic feel for PrefixQuery, insert the FlatQueryParser module
described in L<CustomQueryParser|Lucy::Docs::Cookbook::CustomQueryParser> (which supports
PrefixQuery) into the search.cgi sample app.

    my $parser = FlatQueryParser->new( schema => $searcher->get_schema );
    my $query  = $parser->parse($q);

If you’re planning on using PrefixQuery in earnest, though, you may want to
change up analyzers to avoid stemming, because stemming – another approach to
prefix conflation – is not perfectly compatible with prefix searches.

    # Polyanalyzer with no SnowballStemmer.
    my $analyzer = Lucy::Analysis::PolyAnalyzer->new(
        analyzers => [
            Lucy::Analysis::StandardTokenizer->new,
            Lucy::Analysis::Normalizer->new,
        ],
    );

