xapian-core  1.4.19
Public Member Functions | Private Member Functions | Private Attributes | List of all members
Xapian::LMWeight Class Reference

Xapian::Weight subclass implementing the Language Model formula. More...

#include <weight.h>

+ Inheritance diagram for Xapian::LMWeight:
+ Collaboration diagram for Xapian::LMWeight:

Public Member Functions

 LMWeight (double param_log_=0.0, type_smoothing select_smoothing_=TWO_STAGE_SMOOTHING, double param_smoothing1_=-1.0, double param_smoothing2_=-1.0)
 Construct a LMWeight. More...
 
std::string name () const
 Return the name of this weighting scheme. More...
 
std::string serialise () const
 Return this object's parameters serialised as a single string. More...
 
LMWeightunserialise (const std::string &serialised) const
 Unserialise parameters. More...
 
double get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen, Xapian::termcount uniqterm) const
 Calculate the weight contribution for this object's term to a document. More...
 
double get_maxpart () const
 Return an upper bound on what get_sumpart() can return for any document. More...
 
double get_sumextra (Xapian::termcount doclen, Xapian::termcount) const
 Calculate the term-independent weight component for a document. More...
 
double get_maxextra () const
 Return an upper bound on what get_sumextra() can return for any document. More...
 
- Public Member Functions inherited from Xapian::Weight
 Weight ()
 Default constructor, needed by subclass constructors. More...
 
virtual ~Weight ()
 Virtual destructor, because we have virtual methods. More...
 

Private Member Functions

LMWeightclone () const
 Clone this object. More...
 
void init (double factor)
 Allow the subclass to perform any initialisation it needs to. More...
 

Private Attributes

type_smoothing select_smoothing
 The type of smoothing to use. More...
 
double param_log
 
double param_smoothing1
 
double param_smoothing2
 
double weight_collection
 The factor to multiply weights by. More...
 

Additional Inherited Members

- Public Types inherited from Xapian::Weight
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...
 
- Protected Types inherited from Xapian::Weight
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, TOTAL_LENGTH = COLLECTION_SIZE | AVERAGE_LENGTH
}
 Stats which the weighting scheme can use (see need_stat()). More...
 
- Protected Member Functions inherited from Xapian::Weight
void need_stat (stat_flags flag)
 Tell Xapian that your subclass will want a particular statistic. More...
 
 Weight (const Weight &)
 Don't allow copying. More...
 
Xapian::doccount get_collection_size () const
 The number of documents in the collection. More...
 
Xapian::doccount get_rset_size () const
 The number of documents marked as relevant. More...
 
Xapian::doclength get_average_length () const
 The average length of a document in the collection. More...
 
Xapian::doccount get_termfreq () const
 The number of documents which this term indexes. More...
 
Xapian::doccount get_reltermfreq () const
 The number of relevant documents which this term indexes. More...
 
Xapian::termcount get_collection_freq () const
 The collection frequency of the term. More...
 
Xapian::termcount get_query_length () const
 The length of the query. More...
 
Xapian::termcount get_wqf () const
 The within-query-frequency of this term. More...
 
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...
 
Xapian::totallength get_total_length () const
 Total length of all documents in the collection. More...
 

Detailed Description

Xapian::Weight subclass implementing the Language Model formula.

This class implements the "Language Model" Weighting scheme, as described by the early papers on LM by Bruce Croft.

LM works by comparing the query to a Language Model of the document. The language model itself is parameter-free, though LMWeight takes parameters which specify the smoothing used.

Definition at line 1406 of file weight.h.

Constructor & Destructor Documentation

◆ LMWeight()

Xapian::LMWeight::LMWeight ( double  param_log_ = 0.0,
type_smoothing  select_smoothing_ = TWO_STAGE_SMOOTHING,
double  param_smoothing1_ = -1.0,
double  param_smoothing2_ = -1.0 
)
inlineexplicit

Construct a LMWeight.

Parameters
param_log_A non-negative parameter controlling how much to clamp negative values returned by the log. The log is calculated by multiplying the actual weight by param_log. If param_log is 0.0, then the document length upper bound will be used (default: document length upper bound)
select_smoothing_A parameter of type enum type_smoothing. This parameter controls which smoothing type to use. (default: TWO_STAGE_SMOOTHING)
param_smoothing1_A non-negative parameter for smoothing whose meaning depends on select_smoothing_. In JELINEK_MERCER_SMOOTHING, it plays the role of estimation and in DIRICHLET_SMOOTHING the role of query modelling. (default JELINEK_MERCER, ABSOLUTE, TWOSTAGE(0.7), DIRCHLET(2000))
param_smoothing2_A non-negative parameter which is used with TWO_STAGE_SMOOTHING as parameter for Dirichlet's smoothing (default: 2000) and as parameter delta to control the scale of the tf lower bound in the DIRICHLET_PLUS_SMOOTHING (default 0.05).

Definition at line 1461 of file weight.h.

References name.

Member Function Documentation

◆ clone()

LMWeight * Xapian::LMWeight::clone ( ) const
privatevirtual

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

Implements Xapian::Weight.

Definition at line 40 of file lmweight.cc.

◆ get_maxextra()

double Xapian::LMWeight::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 237 of file lmweight.cc.

◆ get_maxpart()

double Xapian::LMWeight::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 182 of file lmweight.cc.

◆ get_sumextra()

double Xapian::LMWeight::get_sumextra ( Xapian::termcount  doclen,
Xapian::termcount  uniqterms 
) 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
doclenThe document's length (unnormalised).
uniqtermsThe number of unique terms in the document.

Implements Xapian::Weight.

Definition at line 227 of file lmweight.cc.

◆ get_sumpart()

double Xapian::LMWeight::get_sumpart ( Xapian::termcount  wdf,
Xapian::termcount  doclen,
Xapian::termcount  uniqterms 
) 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
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).

Implements Xapian::Weight.

Definition at line 124 of file lmweight.cc.

◆ init()

void Xapian::LMWeight::init ( double  factor)
privatevirtual

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).

Implements Xapian::Weight.

Definition at line 45 of file lmweight.cc.

◆ name()

string Xapian::LMWeight::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 94 of file lmweight.cc.

Referenced by DEFINE_TESTCASE().

◆ serialise()

string Xapian::LMWeight::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 100 of file lmweight.cc.

References serialise_double().

Referenced by DEFINE_TESTCASE().

◆ unserialise()

LMWeight * Xapian::LMWeight::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 from Xapian::Weight.

Definition at line 110 of file lmweight.cc.

References rare, and unserialise_double().

Referenced by DEFINE_TESTCASE().

Member Data Documentation

◆ param_log

double Xapian::LMWeight::param_log
private

Definition at line 1411 of file weight.h.

◆ param_smoothing1

double Xapian::LMWeight::param_smoothing1
private

Definition at line 1411 of file weight.h.

◆ param_smoothing2

double Xapian::LMWeight::param_smoothing2
private

Definition at line 1411 of file weight.h.

◆ select_smoothing

type_smoothing Xapian::LMWeight::select_smoothing
private

The type of smoothing to use.

Definition at line 1408 of file weight.h.

◆ weight_collection

double Xapian::LMWeight::weight_collection
private

The factor to multiply weights by.

The misleading name is due to this having been used to store some other value in 1.4.0. However, that value only takes one multiplication and one division to calculate, so for 1.4.x we can just recalculate it each time we need it, and so this member has been repurposed in 1.4.1 and later (but the name left the same to ensure ABI compatibility with 1.4.0).

Definition at line 1422 of file weight.h.


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