Xapian::BM25Weight Class Reference

Xapian::Weight subclass implementing the BM25 probabilistic formula. More...

#include <weight.h>

Inheritance diagram for Xapian::BM25Weight:

Collaboration diagram for Xapian::BM25Weight:

List of all members.

Public Member Functions

BM25Weight (double k1, double k2, double k3, double b, double min_normlen)

Construct a BM25Weight.

BM25Weight ()

std::string name () const

Return the name of this weighting scheme.

std::string serialise () const

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

BM25Weight * unserialise (const std::string &s) const

Unserialise parameters.

Xapian::weight get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen) const

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

Xapian::weight get_maxpart () const

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

Xapian::weight get_sumextra (Xapian::termcount doclen) const

Calculate the term-independent weight component for a document.

Xapian::weight get_maxextra () const

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

Private Member Functions

BM25Weight * clone () const

Clone this object.

void init (double factor)

Allow the subclass to perform any initialisation it needs to.

Private Attributes

Xapian::doclength len_factor

Factor to multiply the document length by.

Xapian::weight termweight

Factor combining all the document independent factors.

double param_k1

The BM25 parameters.

double param_k2

double param_k3

double param_b

Xapian::doclength param_min_normlen

The minimum normalised document length value.

Detailed Description

Xapian::Weight subclass implementing the BM25 probabilistic formula.

Definition at line 336 of file weight.h.

Constructor & Destructor Documentation

Xapian::BM25Weight::BM25Weight	(	double	k1,
		double	k2,
		double	k3,
		double	b,
		double	min_normlen
	)			`[inline]`

Construct a BM25Weight.

Parameters:

	k1	A non-negative parameter controlling how influential within-document-frequency (wdf) is. k1=0 means that wdf doesn't affect the weights. The larger k1 is, the more wdf influences the weights. (default 1)
	k2	A non-negative parameter which controls the strength of a correction factor which depends upon query length and normalised document length. k2=0 disable this factor; larger k2 makes it stronger. (default 0)
	k3	A non-negative parameter controlling how influential within-query-frequency (wqf) is. k3=0 means that wqf doesn't affect the weights. The larger k3 is, the more wqf influences the weights. (default 1)
	b	A parameter between 0 and 1, controlling how strong the document length normalisation of wdf is. 0 means no normalisation; 1 means full normalisation. (default 0.5)
	min_normlen	A parameter specifying a minimum value for normalised document length. Normalised document length values less than this will be clamped to this value, helping to prevent very short documents getting large weights. (default 0.5)

Definition at line 381 of file weight.h.

Xapian::BM25Weight::BM25Weight ( ) [inline]

Definition at line 409 of file weight.h.

Referenced by clone(), and unserialise().

Member Function Documentation

BM25Weight * Xapian::BM25Weight::clone ( ) const [private, 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". It must therefore have been allocated with "new".

Implements Xapian::Weight.

Definition at line 39 of file bm25weight.cc.

References BM25Weight(), param_b, param_k1, param_k2, param_k3, and param_min_normlen.

Xapian::weight Xapian::BM25Weight::get_maxextra ( ) const [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.

Implements Xapian::Weight.

Definition at line 204 of file bm25weight.cc.

References Xapian::Weight::get_doclength_lower_bound(), Xapian::Weight::get_query_length(), LOGCALL, param_k2, param_min_normlen, and RETURN.

Xapian::weight Xapian::BM25Weight::get_maxpart ( ) const [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.

Implements Xapian::Weight.

Definition at line 175 of file bm25weight.cc.

References AssertRel, Xapian::Weight::get_doclength_lower_bound(), Xapian::Weight::get_wdf_upper_bound(), len_factor, LOGCALL, param_b, param_k1, param_min_normlen, RETURN, and termweight.

Xapian::weight Xapian::BM25Weight::get_sumextra ( Xapian::termcount doclen ) const [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).

Implements Xapian::Weight.

Definition at line 196 of file bm25weight.cc.

References Xapian::Weight::get_query_length(), len_factor, LOGCALL, param_k2, param_min_normlen, and RETURN.

Xapian::weight Xapian::BM25Weight::get_sumpart	(	Xapian::termcount	wdf,
		Xapian::termcount	doclen
	)			const `[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).

Implements Xapian::Weight.

Definition at line 163 of file bm25weight.cc.

References AssertRel, len_factor, LOGCALL, param_b, param_k1, param_min_normlen, RETURN, and termweight.

void Xapian::BM25Weight::init ( double factor ) [private, virtual]

Allow the subclass to perform any initialisation it needs to.

Parameters:

factor

Any scaling factor (e.g. from OP_SCALE_WEIGHT).

Implements Xapian::Weight.

Definition at line 46 of file bm25weight.cc.

References AssertRel, Xapian::Weight::get_average_length(), Xapian::Weight::get_collection_size(), Xapian::Weight::get_reltermfreq(), Xapian::Weight::get_rset_size(), Xapian::Weight::get_termfreq(), Xapian::Weight::get_wqf(), len_factor, LOGVALUE, param_b, param_k1, param_k3, rare, and termweight.

string Xapian::BM25Weight::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 from Xapian::Weight.

Definition at line 131 of file bm25weight.cc.

Referenced by DEFINE_TESTCASE().

string Xapian::BM25Weight::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 from Xapian::Weight.

Definition at line 137 of file bm25weight.cc.

References param_b, param_k1, param_k2, param_k3, param_min_normlen, and serialise_double().

Referenced by DEFINE_TESTCASE().

BM25Weight * Xapian::BM25Weight::unserialise ( const std::string & s ) 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". It must therefore have been allocated with "new".

Parameters:

s

A string containing the serialised parameters.

Reimplemented from Xapian::Weight.

Definition at line 148 of file bm25weight.cc.

References BM25Weight(), rare, and unserialise_double().

Member Data Documentation

Xapian::doclength Xapian::BM25Weight::len_factor [mutable, private]

Factor to multiply the document length by.

Definition at line 338 of file weight.h.

Referenced by get_maxpart(), get_sumextra(), get_sumpart(), and init().

double Xapian::BM25Weight::param_b [private]

Definition at line 344 of file weight.h.

Referenced by clone(), get_maxpart(), get_sumpart(), init(), and serialise().

double Xapian::BM25Weight::param_k1 [private]

The BM25 parameters.

Definition at line 344 of file weight.h.

Referenced by clone(), get_maxpart(), get_sumpart(), init(), and serialise().

double Xapian::BM25Weight::param_k2 [private]

Definition at line 344 of file weight.h.

Referenced by clone(), get_maxextra(), get_sumextra(), and serialise().

double Xapian::BM25Weight::param_k3 [private]

Definition at line 344 of file weight.h.

Referenced by clone(), init(), and serialise().

Xapian::doclength Xapian::BM25Weight::param_min_normlen [private]

The minimum normalised document length value.

Definition at line 347 of file weight.h.

Referenced by clone(), get_maxextra(), get_maxpart(), get_sumextra(), get_sumpart(), and serialise().

Xapian::weight Xapian::BM25Weight::termweight [mutable, private]

Factor combining all the document independent factors.

Definition at line 341 of file weight.h.

Referenced by get_maxpart(), get_sumpart(), and init().

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

include/xapian/weight.h
weight/bm25weight.cc


Public Member Functions
	BM25Weight (double k1, double k2, double k3, double b, double min_normlen)
	Construct a BM25Weight.
	BM25Weight ()
std::string	name () const
	Return the name of this weighting scheme.
std::string	serialise () const
	Return this object's parameters serialised as a single string.
BM25Weight *	unserialise (const std::string &s) const
	Unserialise parameters.
Xapian::weight	get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen) const
	Calculate the weight contribution for this object's term to a document.
Xapian::weight	get_maxpart () const
	Return an upper bound on what get_sumpart() can return for any document.
Xapian::weight	get_sumextra (Xapian::termcount doclen) const
	Calculate the term-independent weight component for a document.
Xapian::weight	get_maxextra () const
	Return an upper bound on what get_sumextra() can return for any document.
Private Member Functions
BM25Weight *	clone () const
	Clone this object.
void	init (double factor)
	Allow the subclass to perform any initialisation it needs to.
Private Attributes
Xapian::doclength	len_factor
	Factor to multiply the document length by.
Xapian::weight	termweight
	Factor combining all the document independent factors.
double	param_k1
	The BM25 parameters.
double	param_k2
double	param_k3
double	param_b
Xapian::doclength	param_min_normlen
	The minimum normalised document length value.