Xapian::Weight subclass implementing the tf-idf weighting scheme. More...
Public Member Functions | |
TfIdfWeight (const std::string &normalizations) | |
Construct a TfIdfWeight. | |
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. | |
TfIdfWeight * | unserialise (const std::string &s) const |
Unserialise parameters. | |
double | get_sumpart (Xapian::termcount wdf, Xapian::termcount doclen) const |
Calculate the weight contribution for this object's term to a document. | |
double | get_maxpart () const |
Return an upper bound on what get_sumpart() can return for any document. | |
double | get_sumextra (Xapian::termcount doclen) const |
Calculate the term-independent weight component for a document. | |
double | get_maxextra () const |
Return an upper bound on what get_sumextra() can return for any document. |
Xapian::Weight subclass implementing the tf-idf weighting scheme.
Xapian::TfIdfWeight::TfIdfWeight | ( | const std::string & | normalizations | ) | [explicit] |
Construct a TfIdfWeight.
normalizations | A three character string indicating the normalizations to be used for the tf(wdf), idf and document weight respectively. |
The first character specifies the normalization for the wdf for which the following normalizations are currently available:
'n':None. wdfn=wdf 'b':Boolean wdfn=1 if term in document else wdfn=0 's':Square wdfn=wdf*wdf 'l':Logarithmic wdfn=1+loge(wdf)
The Max-wdf and Augmented Max wdf normalizations aren't yet implemented.
The second character indicates the normalization for the idf, the following of which are currently available:
'n':None idfn=1 't':TfIdf idfn=log(N/Termfreq) where N is the number of documents in collection and Termfreq is the number of documents which are indexed by the term t. 'p':Prob idfn=log((N-Termfreq)/Termfreq)
The third and the final character indicates the normalization for the document weight of which the following are currently available:
'n':None wtn=tfn*idfn Implementing more normalizations for the weight requires access to statistics such as the weight of all terms in the document indexed by the term in the query. This is not available from the current backend.
More normalizations for all components can be implemented by changing the backend to acquire the statistics required for the normalizations which are not currently available from Xapian::Weight.
The default string is "ntn".
double Xapian::TfIdfWeight::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.
double Xapian::TfIdfWeight::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.
double Xapian::TfIdfWeight::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:
doclen | The document's length (unnormalised). |
Implements Xapian::Weight.
double Xapian::TfIdfWeight::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:
wdf | The within document frequency of the term in the document. |
doclen | The document's length (unnormalised). |
Implements Xapian::Weight.
std::string Xapian::TfIdfWeight::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.
std::string Xapian::TfIdfWeight::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.
TfIdfWeight* Xapian::TfIdfWeight::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". 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: http://trac.xapian.org/ticket/554#comment:1
s | A string containing the serialised parameters. |
Reimplemented from Xapian::Weight.