xapiancore
1.4.26

Xapian::Weight subclass implementing the Language Model formula. More...
#include <weight.h>
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...  
LMWeight *  unserialise (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 termindependent 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  
LMWeight *  clone () 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 withinqueryfrequency of this term. More...  
Xapian::termcount  get_doclength_upper_bound () const 
An upper bound on the maximum length of any document in the shard. More...  
Xapian::termcount  get_doclength_lower_bound () const 
A lower bound on the minimum length of any document in the shard. More...  
Xapian::termcount  get_wdf_upper_bound () const 
An upper bound on the wdf of this term in the shard. More...  
Xapian::totallength  get_total_length () const 
Total length of all documents in the collection. More...  
Xapian::Weight subclass implementing the Language Model formula.
We do not recommend using this class. It was meant to implement the "Language Model" Weighting scheme, but we discovered the implementation was incorrect and fixing it requires ABIincompatible changes.
For 1.4.x we've left it in place so as not to break existing code. It will be removed in the next release series and replaced with new separate classes implementing Language Model weighting with each smoothing.

inlineexplicit 
Construct a LMWeight.
param_log_  A nonnegative 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 nonnegative 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 nonnegative 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 1468 of file weight.h.
References name.

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.

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.

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.

virtual 
Calculate the termindependent weight component for a document.
The parameter gives information about the document which may be used in the calculations:
doclen  The document's length (unnormalised). 
uniqterms  The number of unique terms in the document. 
Implements Xapian::Weight.
Definition at line 227 of file lmweight.cc.

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:
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). 
Implements Xapian::Weight.
Definition at line 124 of file lmweight.cc.

privatevirtual 
Allow the subclass to perform any initialisation it needs to.
factor  Any scaling factor (e.g. from OP_SCALE_WEIGHT). If the Weight object is for the termindependent 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.

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 namespacequalified 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.

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().

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
serialised  A string containing the serialised parameters. 
Reimplemented from Xapian::Weight.
Definition at line 110 of file lmweight.cc.
References rare, and unserialise_double().

private 

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).