Abstract base class for weighting schemes. More...

Inheritance diagram for Xapian::Weight:

Public Types
enum	type_smoothing { TWO_STAGE_SMOOTHING = 1, DIRICHLET_SMOOTHING = 2, ABSOLUTE_DISCOUNT_SMOOTHING = 3, JELINEK_MERCER_SMOOTHING = 4, DIRICHLET_PLUS_SMOOTHING = 5 }
	Type of smoothing to use with the Language Model Weighting scheme. More...

Public Member Functions
	Weight ()
	Default constructor, needed by subclass constructors.

virtual	~Weight ()
	Virtual destructor, because we have virtual methods. More...

virtual Weight *	clone () const =0
	Clone this object. More...

virtual std::string	name () const
	Return the name of this weighting scheme. More...

virtual std::string	serialise () const
	Return this object's parameters serialised as a single string. More...

virtual Weight *	unserialise (const std::string &serialised) const
	Unserialise parameters. More...

virtual double	get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen, Xapian::termcount uniqterms) const =0
	Calculate the weight contribution for this object's term to a document. More...

virtual double	get_maxpart () const =0
	Return an upper bound on what get_sumpart() can return for any document. More...

virtual double	get_sumextra (Xapian::termcount doclen, Xapian::termcount uniqterms) const =0
	Calculate the term-independent weight component for a document. More...

virtual double	get_maxextra () const =0
	Return an upper bound on what get_sumextra() can return for any document. More...

Protected Types
enum	stat_flags { COLLECTION_SIZE = 1, RSET_SIZE = 2, AVERAGE_LENGTH = 4, TERMFREQ = 8, RELTERMFREQ = 16, QUERY_LENGTH = 32, WQF = 64, WDF = 128, DOC_LENGTH = 256, DOC_LENGTH_MIN = 512, DOC_LENGTH_MAX = 1024, WDF_MAX = 2048, COLLECTION_FREQ = 4096, UNIQUE_TERMS = 8192 }
	Stats which the weighting scheme can use (see need_stat()). More...

Protected Member Functions
void	need_stat (stat_flags flag)
	Tell Xapian that your subclass will want a particular statistic. More...

virtual void	init (double factor)=0
	Allow the subclass to perform any initialisation it needs to. More...

	Weight (const Weight &)
	Don't allow copying. More...

Xapian::doccount	get_collection_size () const
	The number of documents in the collection.

Xapian::doccount	get_rset_size () const
	The number of documents marked as relevant.

Xapian::doclength	get_average_length () const
	The average length of a document in the collection.

Xapian::doccount	get_termfreq () const
	The number of documents which this term indexes.

Xapian::doccount	get_reltermfreq () const
	The number of relevant documents which this term indexes.

Xapian::termcount	get_collection_freq () const

Xapian::termcount	get_query_length () const
	The length of the query.

Xapian::termcount	get_wqf () const
	The within-query-frequency of this term.

Xapian::termcount	get_doclength_upper_bound () const
	An upper bound on the maximum length of any document in the database. More...

Xapian::termcount	get_doclength_lower_bound () const
	A lower bound on the minimum length of any document in the database. More...

Xapian::termcount	get_wdf_upper_bound () const
	An upper bound on the wdf of this term. More...

Detailed Description

Abstract base class for weighting schemes.

Member Enumeration Documentation

enum Xapian::Weight::stat_flags

protected

Stats which the weighting scheme can use (see need_stat()).

Enumerator
COLLECTION_SIZE	Number of documents in the collection.
RSET_SIZE	Number of documents in the RSet.
AVERAGE_LENGTH	Average length of documents in the collection.
TERMFREQ	How many documents the current term is in.
RELTERMFREQ	How many documents in the RSet the current term is in.
QUERY_LENGTH	Sum of wqf for terms in the query.
WQF	Within-query-frequency of the current term.
WDF	Within-document-frequency of the current term in the current document.
DOC_LENGTH	Length of the current document (sum wdf).
DOC_LENGTH_MIN	Lower bound on (non-zero) document lengths.
DOC_LENGTH_MAX	Upper bound on document lengths.
WDF_MAX	Upper bound on wdf.
COLLECTION_FREQ	Sum of wdf over the whole collection for the current term.
UNIQUE_TERMS	Number of unique terms in the current document.

enum Xapian::Weight::type_smoothing

Type of smoothing to use with the Language Model Weighting scheme.

Default is TWO_STAGE_SMOOTHING.

Constructor & Destructor Documentation

virtual Xapian::Weight::~Weight ( )

virtual

Virtual destructor, because we have virtual methods.

Xapian::Weight::Weight ( const Weight & )

protected

Don't allow copying.

This would ideally be private, but that causes a compilation error with GCC 4.1 (which appears to be a bug).

Member Function Documentation

virtual Weight* Xapian::Weight::clone ( ) const

pure virtual

Clone this object.

This method allocates and returns a copy of the object it is called on.

If your subclass is called FooWeight and has parameters a and b, then you would implement FooWeight::clone() like so:

FooWeight * FooWeight::clone() const { return new FooWeight(a, b); }

Note that the returned object will be deallocated by Xapian after use with "delete". If you want to handle the deletion in a special way (for example when wrapping the Xapian API for use from another language) then you can define a static operator delete method in your subclass as shown here: https://trac.xapian.org/ticket/554#comment:1

Implemented in Xapian::CoordWeight.

Xapian::termcount Xapian::Weight::get_doclength_lower_bound ( ) const

inlineprotected

A lower bound on the minimum length of any document in the database.

This bound does not include any zero-length documents.

This should only be used by get_maxpart() and get_maxextra().

Xapian::termcount Xapian::Weight::get_doclength_upper_bound ( ) const

inlineprotected

An upper bound on the maximum length of any document in the database.

This should only be used by get_maxpart() and get_maxextra().

virtual double Xapian::Weight::get_maxextra ( ) const

pure virtual

Return an upper bound on what get_sumextra() can return for any document.

This information is used by the matcher to perform various optimisations, so strive to make the bound as tight as possible.

Implemented in Xapian::CoordWeight, Xapian::LMWeight, Xapian::DPHWeight, Xapian::PL2PlusWeight, Xapian::PL2Weight, Xapian::DLHWeight, Xapian::BB2Weight, Xapian::IneB2Weight, Xapian::IfB2Weight, Xapian::InL2Weight, Xapian::TradWeight, Xapian::BM25PlusWeight, Xapian::BM25Weight, Xapian::TfIdfWeight, and Xapian::BoolWeight.

virtual double Xapian::Weight::get_maxpart ( ) const

pure virtual

Return an upper bound on what get_sumpart() can return for any document.

This information is used by the matcher to perform various optimisations, so strive to make the bound as tight as possible.

Implemented in Xapian::CoordWeight, Xapian::LMWeight, Xapian::DPHWeight, Xapian::PL2PlusWeight, Xapian::PL2Weight, Xapian::DLHWeight, Xapian::BB2Weight, Xapian::IneB2Weight, Xapian::IfB2Weight, Xapian::InL2Weight, Xapian::TradWeight, Xapian::BM25PlusWeight, Xapian::BM25Weight, Xapian::TfIdfWeight, and Xapian::BoolWeight.

virtual double Xapian::Weight::get_sumextra	(	Xapian::termcount	doclen,
		Xapian::termcount	uniqterms
	)		const

pure virtual

Calculate the term-independent weight component for a document.

The parameter gives information about the document which may be used in the calculations:

Parameters

doclen	The document's length (unnormalised).
uniqterms	The number of unique terms in the document.

Implemented in Xapian::CoordWeight, Xapian::LMWeight, Xapian::DPHWeight, Xapian::PL2PlusWeight, Xapian::PL2Weight, Xapian::DLHWeight, Xapian::BB2Weight, Xapian::IneB2Weight, Xapian::IfB2Weight, Xapian::InL2Weight, Xapian::TradWeight, Xapian::BM25PlusWeight, Xapian::BM25Weight, Xapian::TfIdfWeight, and Xapian::BoolWeight.

virtual double Xapian::Weight::get_sumpart	(	Xapian::termcount	wdf,
		Xapian::termcount	doclen,
		Xapian::termcount	uniqterms
	)		const

pure virtual

Calculate the weight contribution for this object's term to a document.

The parameters give information about the document which may be used in the calculations:

Parameters

wdf	The within document frequency of the term in the document.
doclen	The document's length (unnormalised).
uniqterms	Number of unique terms in the document (used for absolute smoothing).

Implemented in Xapian::CoordWeight, Xapian::LMWeight, Xapian::DPHWeight, Xapian::PL2PlusWeight, Xapian::PL2Weight, Xapian::DLHWeight, Xapian::BB2Weight, Xapian::IneB2Weight, Xapian::IfB2Weight, Xapian::InL2Weight, Xapian::TradWeight, Xapian::BM25PlusWeight, Xapian::BM25Weight, Xapian::TfIdfWeight, and Xapian::BoolWeight.

Xapian::termcount Xapian::Weight::get_wdf_upper_bound ( ) const

inlineprotected

An upper bound on the wdf of this term.

This should only be used by get_maxpart() and get_maxextra().

virtual void Xapian::Weight::init ( double factor )

protectedpure virtual

Allow the subclass to perform any initialisation it needs to.

Parameters

factor Any scaling factor (e.g. from OP_SCALE_WEIGHT). If the Weight object is for the term-independent weight supplied by get_sumextra()/get_maxextra(), then init(0.0) is called (starting from Xapian 1.2.11 and 1.3.1 - earlier versions failed to call init() for such Weight objects).

Implemented in Xapian::CoordWeight.

virtual std::string Xapian::Weight::name ( ) const

virtual

Return the name of this weighting scheme.

This name is used by the remote backend. It is passed along with the serialised parameters to the remote server so that it knows which class to create.

Return the full namespace-qualified name of your class here - if your class is called FooWeight, return "FooWeight" from this method (Xapian::BM25Weight returns "Xapian::BM25Weight" here).

If you don't want to support the remote backend, you can use the default implementation which simply returns an empty string.

Reimplemented in Xapian::CoordWeight, Xapian::LMWeight, Xapian::DPHWeight, Xapian::PL2PlusWeight, Xapian::PL2Weight, Xapian::DLHWeight, Xapian::BB2Weight, Xapian::IneB2Weight, Xapian::IfB2Weight, Xapian::InL2Weight, Xapian::TradWeight, Xapian::BM25PlusWeight, Xapian::BM25Weight, Xapian::TfIdfWeight, and Xapian::BoolWeight.

void Xapian::Weight::need_stat ( stat_flags flag )

inlineprotected

Tell Xapian that your subclass will want a particular statistic.

Some of the statistics can be costly to fetch or calculate, so Xapian needs to know which are actually going to be used. You should call need_stat() from your constructor for each such statistic.

Parameters

flag	The stat_flags value for a required statistic.

virtual std::string Xapian::Weight::serialise ( ) const

virtual

Return this object's parameters serialised as a single string.

If you don't want to support the remote backend, you can use the default implementation which simply throws Xapian::UnimplementedError.

Reimplemented in Xapian::CoordWeight, Xapian::LMWeight, Xapian::DPHWeight, Xapian::PL2PlusWeight, Xapian::PL2Weight, Xapian::DLHWeight, Xapian::BB2Weight, Xapian::IneB2Weight, Xapian::IfB2Weight, Xapian::InL2Weight, Xapian::TradWeight, Xapian::BM25PlusWeight, Xapian::BM25Weight, Xapian::TfIdfWeight, and Xapian::BoolWeight.

virtual Weight* Xapian::Weight::unserialise ( const std::string & serialised ) const

virtual

Unserialise parameters.

This method unserialises parameters serialised by the serialise() method and allocates and returns a new object initialised with them.

If you don't want to support the remote backend, you can use the default implementation which simply throws Xapian::UnimplementedError.

Note that the returned object will be deallocated by Xapian after use with "delete". If you want to handle the deletion in a special way (for example when wrapping the Xapian API for use from another language) then you can define a static operator delete method in your subclass as shown here: https://trac.xapian.org/ticket/554#comment:1

Parameters

serialised A string containing the serialised parameters.

Reimplemented in Xapian::CoordWeight, Xapian::LMWeight, Xapian::DPHWeight, Xapian::PL2PlusWeight, Xapian::PL2Weight, Xapian::DLHWeight, Xapian::BB2Weight, Xapian::IneB2Weight, Xapian::IfB2Weight, Xapian::InL2Weight, Xapian::TradWeight, Xapian::BM25PlusWeight, Xapian::BM25Weight, Xapian::TfIdfWeight, and Xapian::BoolWeight.

The documentation for this class was generated from the following file:

xapian/weight.h

Public Types

Public Member Functions

Protected Types

Protected Member Functions

Detailed Description

Member Enumeration Documentation

Constructor & Destructor Documentation

Member Function Documentation