Peter Karman > Lucy-0.3.0 > Lucy::Search::Compiler

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NAME ^

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

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 );
    }

DESCRIPTION ^

The purpose of the Compiler class is to take a specification in the form of a Query object and compile a 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 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 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.

CONSTRUCTORS ^

new( [labeled params] )

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

Abstract constructor.

ABSTRACT METHODS ^

make_matcher( [labeled params] )

Factory method returning a Matcher.

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

METHODS ^

get_weight()

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

sum_of_squared_weights()

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

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.

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.

get_parent()

Accessor for the Compiler's parent Query object.

get_similarity()

Accessor for the Compiler's Similarity object.

highlight_spans( [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.

INHERITANCE ^

Lucy::Search::Compiler isa Lucy::Search::Query isa Lucy::Object::Obj.

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