Xapian::Weight subclass implementing the Language Model formula. More...
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.  
virtual  ~Weight () 
Virtual destructor, because we have virtual methods. 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 } 
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.  
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 withinqueryfrequency 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...  
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 parameterfree, though LMWeight takes parameters which specify the smoothing used.

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). 

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.

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.

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.

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.

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.

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.

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.