Public Types | Public Member Functions | Protected Types | Protected Member Functions | List of all members
Xapian::Weight Class Referenceabstract

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 Weightclone () 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 Weightunserialise (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
 The collection frequency of the term.
 
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

◆ stat_flags

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.

◆ type_smoothing

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

Default is TWO_STAGE_SMOOTHING.

Constructor & Destructor Documentation

◆ ~Weight()

virtual Xapian::Weight::~Weight ( )
virtual

Virtual destructor, because we have virtual methods.

◆ Weight()

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

◆ clone()

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.

◆ get_doclength_lower_bound()

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().

◆ get_doclength_upper_bound()

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().

◆ 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.

◆ get_maxpart()

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.

◆ get_sumextra()

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
doclenThe document's length (unnormalised).
uniqtermsThe 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.

◆ get_sumpart()

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
wdfThe within document frequency of the term in the document.
doclenThe document's length (unnormalised).
uniqtermsNumber 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.

◆ get_wdf_upper_bound()

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().

◆ init()

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

Allow the subclass to perform any initialisation it needs to.

Parameters
factorAny 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.

◆ name()

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.

◆ need_stat()

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
flagThe stat_flags value for a required statistic.

◆ serialise()

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.

◆ unserialise()

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
serialisedA 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:

Documentation for Xapian (version 1.4.9).
Generated on Sat Nov 3 2018 by Doxygen 1.8.13.